Abgabe Übungstest 5

angabe/Aufgabenblatt5.md

@@ -0,0 +1,102 @@
+# Aufgabenblatt 5
+
+## Allgemeine Anmerkungen
+
+Ihre Lösung für dieses Aufgabenblatt ist bis Montag, 9.5. 11h durch `git commit` und `git push`
+abzugeben. Mit der Angabe werden die Dateien `Massive.java`, `NamedBody.java`, `MassiveLinkedList.java`,
+`MassiveForceHashMap.java`, `Simulation5.java` und `Aufgabe5Test.java` mitgeliefert.
+
+Wenn Sie zusätzlich zu den gefragten Klassen weitere Klassen definieren, achten Sie darauf, dass
+die Klassennamen mit `My` beginnen, um Konflikte mit späteren Aufgabenblättern zu vermeiden.
+
+## Ziel
+
+Ziel der Aufgabe ist die Anwendung der Konzepte: Gleichheit und Hash-Werte, Hash-Tabelle
+(siehe Skriptum Seite 85-91).
+
+## Beschreibung der gegebenen Dateien
+
+- [Massive](../src/Massive.java) ist ein Interface, das Himmelskörper (als kohärente Massen)
+beschreibt. `Massive` ist der gemeinsame Obertyp für verschiedene Klassen von Himmelkörpern. Die
+meisten spezifizierten Methoden sind mit einer `default`-Implementierung definiert. Dieser
+Programmcode wird ausgeführt, falls die entsprechende Klasse (`Body` oder `NamedBody`) über keine
+eigene Definition der Methode verfügt. Verändern Sie diese Datei bitte nicht.
+- [NamedBody](../src/NamedBody.java) ist das Gerüst einer Klassendefinition. Die Klasse
+repräsentiert Himmelskörper, die einen Namen haben.
+- [MassiveLinkedList](../src/MassiveLinkedList.java) ist das Gerüst für eine Implementierung einer
+verketteten Liste von `Massive`-Objekten. Die Liste unterscheidet sich von `BodyLinkedList`
+dadurch, dass der Elementtyp statt `Body` der Obertyp `Massive` ist.
+- [MassiveForceHashMap](../src/MassiveForceHashMap.java) ist das Gerüst für eine Implementierung
+einer assoziativen Datenstruktur, die ein `Massive`-Objekt mit der auf das Objekt wirkenden Kraft
+assoziiert.
+- [Simulation5](../src/Simulation5.java) ist ein Gerüst für eine ausführbare Klasse. Hier soll
+die Simulation analog zur Klasse `Simulation` implementiert werden (damit Sie Ihre [ursprüngliche
+ Datei](../src/Simulation.java) nicht überschreiben müssen).
+- [Aufgabe5Test](../src/Aufgabe5Test.java) ist eine vorgegebene Klasse, die Sie zum Testen Ihrer
+Implementierung verwenden sollten. Bei einer fehlerfreien Implementierung sollten bei der
+Ausführung dieser Klasse keine Exceptions geworfen werden und alle Tests als erfolgreich ("successful")
+ausgegeben werden. Entfernen Sie die Kommentarzeichen, um diese Klasse verwenden zu können. Sie
+müssen diese Klasse nicht weiter verändern, können aber eigene Testfälle hinzufügen.
+
+## Aufgaben
+
+Ihre Aufgaben sind folgende:
+
+**1. Implementieren Sie `Massive` in den Klassen `Body` und `NamedBody`.**
+
+ Passen Sie die bestehende Definition von `Body` so an, dass die Klasse `Massive` implementiert wird.
+ Vervollständigen Sie auch `NamedBody` so, dass sie `Massive` implementiert und die vorgegebene
+ Spezifikationen der Methoden erfüllt.
+
+**2. Überschreiben von `equals` und `hashCode` in `NamedBody`:**
+
+ Überschreiben Sie in `NamedBody` die Methoden `equals` und `hashCode` gemäß der dort angeführten
+ Spezifikation. Achten Sie bei der Implementierung darauf, dass die in der Klasse `Object`
+ beschriebenen Bedingungen für `equals` und `hashCode` eingehalten werden. `equals` und `hashCode`
+ müssen zusammen passen.
+
+**3. Vervollständigen von `MassiveLinkedList`:**
+
+ Definieren Sie `MassiveLinkedList`. Die Klasse ist wie `BodyLinkedList` aufgebaut, mit dem
+ Unterschied, dass der Elementtyp statt `Body` nun der Typ `Massive` ist. Die Methode `indexOf` 
+ vergleicht Objekte mittels `equals`.
+
+**4. Implementierung von `MassiveForceHashMap`:**
+
+ Vervollständigen Sie die Definition der Klasse `MassiveForceHashMap`, die eine Hash-Tabelle
+ mit Schlüssel vom Typ `Massive` und Wert vom Typ `Vector3` implementiert. Die Klasse ist ähnlich
+ zur Klasse `BodyForceTreeMap`. Die Unterschiede sind:
+ - Der Typ des Schlüssels ist der gemeinsame Obertyp von `Body` und `NamedBody` (`Massive`).
+   Dadurch lassen sich Objekte beider Klassen gemeinsam in der Hash-Tabelle speichern.
+ - Die Schlüssel-Werte-Paare sind nicht nach Masse sortiert. Stattdessen wird der Hash-Wert zur
+   Suche benutzt.
+ - Es gibt eine zusätzliche Methode `keyList()`. Die Methoden `equals` und `hashCode` werden
+   redefiniert.
+
+**5. Implementierung von `Simulation5`:**
+
+ Implementieren Sie die Simulationsschleife unter Verwendung eines Objekts vom Typ
+ `MassiveForceHashMap`. Die Methode `keyList()` hilft beim Iterieren der Hash-Tabelle.
+ Kollisionen von Himmelskörpern müssen in dieser Simulation nicht berücksichtigt werden.
+
+### Hinweise:
+
+- Verwenden Sie bei der Implementierung von `MassiveForceHashMap` eine geeignete Kollisionsbehandlung
+  für gleiche Hash-Werte. Als Vorlage können Sie den Beispielcode aus dem Skriptum nutzen.
+
+### Denkanstöße (ohne Bewertung)
+
+1. Wie könnte man vorgehen, wenn man - wie in früheren Simulationen - Himmelskörper im Fall von
+   Kollisionen verschmelzen will?
+2. Was ändert sich am Verhalten von `MassiveForceHashMap`, wenn man in `Body` die Methoden
+   `equals` und `hashCode` überschreiben würde?
+
+#### _Punkteaufteilung_
+
+- Implementierung von `Massive` in `NamedBody`: 1 Punkt
+- Implementierung von `Massive` in `Body`: 0.5 Punkt
+- Implementierung von `MassiveForceHashMap`: 2 Punkte
+- Implementierung von `MassiveLinkedList`: 0.5 Punkte
+- Implementierung von `Simulation5`: 1 Punkte
+ 
+- Gesamt: 5 Punkte

angabe/Aufgabenblatt6.md

@@ -0,0 +1,75 @@
+# Aufgabenblatt 6
+
+## Allgemeine Anmerkungen
+
+Ihre Lösung für dieses Aufgabenblatt ist bis Montag, 23.5. 11h durch `git commit` und `git push`
+abzugeben. Mit der Angabe werden die Dateien `MassiveIterable.java`, `MassiveIterator.java`, 
+`MassiveSet.java`, `MassiveForceTreeMap.java`, `Simulation6.java` und `Aufgabe6Test.java` 
+mitgeliefert.
+
+Wenn Sie zusätzlich zu den gefragten Klassen weitere Klassen definieren, achten Sie darauf, dass
+die Klassennamen mit `My` beginnen, um Konflikte mit späteren Aufgabenblättern zu vermeiden.
+
+## Ziel
+
+Ziel der Aufgabe ist die Anwendung der Konzepte: Iterator, Kopie vs. Sichtweise, Sortieren
+(siehe Skriptum Seite 91-109).
+
+## Beschreibung der gegebenen Dateien
+
+- [MassiveIterable](../src/MassiveIterable.java) ist ein Interface, das iterierbare Objekte mit
+Elementen vom Typ `Massive` spezifiziert. Verändern Sie diese Datei bitte nicht.
+- [MassiveIterator](../src/MassiveIterator.java) ist ein Interface, das einen Iterator über
+Elemente vom Typ `Massive` spezifiziert. Verändern Sie diese Datei bitte nicht.
+- [MassiveSet](../src/MassiveSet.java) ist ein Interface, das iterierbare Mengen mit 
+`Massive`-Elementen spezifiziert. Verändern Sie diese Datei bitte nicht.
+- [MassiveForceTreeMap](../src/MassiveForceTreeMap.java) ist das Gerüst für eine Implementierung
+einer assoziativen Datenstruktur, die ein `Massive`-Objekt mit der auf das Objekt wirkenden Kraft
+assoziiert.
+- [Simulation6](../src/Simulation6.java) ist ein Gerüst für eine ausführbare Klasse. Hier soll
+die Simulation analog zur Klasse `Simulation` implementiert werden (damit Sie Ihre [ursprüngliche
+ Datei](../src/Simulation.java) nicht überschreiben müssen).
+- [Aufgabe6Test](../src/Aufgabe6Test.java) ist eine vorgegebene Klasse, die Sie zum Testen Ihrer
+Implementierung verwenden sollten. Bei einer fehlerfreien Implementierung sollten bei der
+Ausführung dieser Klasse keine Exceptions geworfen werden und alle Tests als erfolgreich ("successful")
+ausgegeben werden. Entfernen Sie die Kommentarzeichen, um diese Klasse verwenden zu können. Sie
+müssen diese Klasse nicht weiter verändern, können aber eigene Testfälle hinzufügen.
+
+## Aufgaben
+
+Ihre Aufgaben sind folgende:
+
+**1. Implementieren Sie die Klasse `MassiveForceTreeMap`.**
+
+ Implementieren Sie die Klasse `MassiveForceTreeMap`. `MassiveForceTreeMap` ist wie 
+ `BodyForceTreeMap` aufgebaut, mit dem Unterschied, dass der Typ des Schlüssels statt `Body` nun 
+ der Typ `Massive` ist. Weiters soll die Klasse Methode `getKeys()` zur Verfügung stellen, 
+ die eine `MassiveSet`-Sichtweise auf die Menge der Schlüssel liefert. Änderungen an dem 
+ zurückgelieferten `MassiveSet`-Objekt wirken sich auf das zugrunde 
+ liegende `MassiveForceTreeMap`-Objekt aus. Die Methode `toList()` liefert dagegen eine 
+ unabhängige Liste (Kopie) mit allen Schlüsseln der Map. Für die Implementierung von 
+ `MassiveSet` können Sie einen eigenen Klassennamen beginnend mit `My` wählen. Die Definition kann 
+ in einer eigenen Datei oder in der Datei `MassiveForceTreeMap.java` erfolgen.
+ 
+**2. Adaptieren Sie die Klasse `HierarchicalSystem`:**
+
+ Die Klasse `HierarchicalSystem` soll so geändert werden, dass sie das gegebene 
+ Interface `MassiveIterable` implementiert. Die Reihenfolge der vom Iterator gelieferten 
+ Elemente ist nicht festgelegt. Sie dürfen für die Implementierung bei Bedarf Ihren Klassen 
+ `NamedBodyForcePair` und `HierarchicalSystem` neue, nicht angegebene Methoden hinzufügen.
+ Die Verwendung von Klassen des Java-Collection-Frameworks (z.B. java.util.Stack) ist erlaubt
+ (aber nicht notwendig).
+ 
+**3. Implementierung von `Simulation6`:**
+
+ Implementieren Sie die Simulationsschleife unter Verwendung eines Objekts vom Typ
+ `MassiveForceTreeMap`. Die Methode `getKeys()` hilft beim Iterieren der gespeicherten Schlüssel.
+ Kollisionen von Himmelskörpern müssen in dieser Simulation nicht berücksichtigt werden.
+
+#### _Punkteaufteilung_
+
+- Implementierung von `MassiveForceTreeMap`: 3 Punkte
+- Implementierung von `MassiveIterable` in `HierarchicalSystem`: 1.5 Punkte
+- Implementierung von `Simulation6`: 0.5 Punkte
+ 
+- Gesamt: 5 Punkte

src/Aufgabe5Test.java

@@ -0,0 +1,96 @@
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+public class Aufgabe5Test {
+
+    @Test
+    public void testEP2() {
+        //test classes NamedBody and MassiveForceHashMap
+
+        // create 12 named bodies
+        NamedBody sun1, mercury1, venus1, earth1, moon1, mars1, deimos1, phobos1, vesta1, pallas1, hygiea1, ceres1;
+
+        // create a nameless body
+        Body earth2 = new Body(SolSystem4.EARTH);
+
+        // create the same 12 named body-force pairs
+        sun1 = new NamedBody(SolSystem4.SUN_NAMED);
+        earth1 = new NamedBody(SolSystem4.EARTH_NAMED);
+        moon1 = new NamedBody(SolSystem4.MOON_NAMED);
+        mars1 = new NamedBody(SolSystem4.MARS_NAMED);
+        deimos1 = new NamedBody(SolSystem4.DEIMOS_NAMED);
+        phobos1 = new NamedBody(SolSystem4.PHOBOS_NAMED);
+        mercury1 = new NamedBody(SolSystem4.MERCURY_NAMED);
+        venus1 = new NamedBody(SolSystem4.VENUS_NAMED);
+        vesta1 = new NamedBody(SolSystem4.VESTA_NAMED);
+        pallas1 = new NamedBody(SolSystem4.PALLAS_NAMED);
+        hygiea1 = new NamedBody(SolSystem4.HYGIEA_NAMED);
+        ceres1 = new NamedBody(SolSystem4.CERES_NAMED);
+
+        NamedBody sun2 = new NamedBody("Sun", 1.9895E30, new Vector3(0.1, 0.0, 0.0), new Vector3(0.0, 0.0, 0.0));
+        NamedBody earth3 = new NamedBody("Earth", 1, new Vector3(0, 0, 0), new Vector3(0, 0, 0));
+        assertEquals(sun1, sun2);
+        assertEquals(sun2.hashCode(), sun1.hashCode());
+        assertEquals(earth1, earth3);
+        assertEquals(earth3.hashCode(), earth1.hashCode());
+
+        // check basic functions of 'MassiveForceHashMap'
+        MassiveForceHashMap map = new MassiveForceHashMap();
+        map.put(sun1, new Vector3(0, 0, 0));
+        map.put(mercury1, new Vector3(0, 0, 0));
+        map.put(venus1, new Vector3(0, 0, 0));
+        map.put(earth1, new Vector3(0, 0, 0));
+        map.put(moon1, new Vector3(0, 0, 0));
+        map.put(mars1, new Vector3(0, 0, 0));
+        map.put(deimos1, new Vector3(0, 0, 0));
+        map.put(phobos1, new Vector3(0, 0, 0));
+        map.put(vesta1, new Vector3(0, 0, 0));
+        map.put(pallas1, new Vector3(0, 0, 0));
+        map.put(hygiea1, new Vector3(0, 0, 0));
+        map.put(ceres1, new Vector3(0, 0, 0));
+        map.put(mars1, new Vector3(0, 0, 0)); // inserted twice
+        assertEquals(12, map.keyList().size());
+
+        assertTrue(map.toString().contains("Mars"));
+        assertTrue(map.toString().contains("Deimos"));
+        assertTrue(map.toString().contains("Moon"));
+        assertTrue(map.toString().contains("Earth"));
+
+        MassiveLinkedList bl = map.keyList();
+        boolean allThere = true;
+        while (bl.size() > 0) {
+            allThere &= map.containsKey(bl.pollFirst());
+        }
+        assertTrue(allThere);
+        assertFalse(map.containsKey(new Body(0, new Vector3(0, 0, 0), new Vector3(0, 0, 0))));
+        assertFalse(map.containsKey(new NamedBody("Omuamua", 0, new Vector3(0, 0, 0), new Vector3(0, 0, 0))));
+
+        int hashCode1 = map.hashCode();
+
+        Vector3 f = new Vector3(5, 5, 5);
+        map.put(earth3, f);
+        assertEquals(f, map.get(earth1));
+        assertNull(map.get(earth2));
+
+        int hashCode2 = map.hashCode();
+        assertEquals(map, map);
+        assertEquals(hashCode2, map.hashCode());
+        assertNotEquals(hashCode1, hashCode2);
+    }
+
+    @Test
+    public void testDelKey() {
+        MassiveForceHashMap map = new MassiveForceHashMap();
+        NamedBody sun1 = new NamedBody(SolSystem4.SUN_NAMED);
+        NamedBody earth1 = new NamedBody(SolSystem4.EARTH_NAMED);
+        NamedBody moon1 = new NamedBody(SolSystem4.MOON_NAMED);
+        map.put(sun1, new Vector3());
+        map.put(earth1, new Vector3());
+        map.put(moon1, new Vector3());
+        assertNotNull(map.get(sun1));
+        assertNotNull(map.delete(sun1));
+        assertNull(map.get(sun1));
+        assertNull(map.delete(sun1));
+    }
+}

src/Aufgabe6Test.java

@@ -0,0 +1,116 @@
+import org.junit.jupiter.api.Test;
+
+import java.util.HashSet;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+public class Aufgabe6Test {
+
+    @Test
+    public void testEP2() {
+        NamedBody sun1, mercury1, venus1, earth1, moon1, mars1, deimos1, phobos1, vesta1, pallas1, hygiea1, ceres1;
+
+        // create the same 12 named body-force pairs
+        sun1 = new NamedBody(SolSystem4.SUN_NAMED);
+        earth1 = new NamedBody(SolSystem4.EARTH_NAMED);
+        moon1 = new NamedBody(SolSystem4.MOON_NAMED);
+        mars1 = new NamedBody(SolSystem4.MARS_NAMED);
+        deimos1 = new NamedBody(SolSystem4.DEIMOS_NAMED);
+        phobos1 = new NamedBody(SolSystem4.PHOBOS_NAMED);
+        mercury1 = new NamedBody(SolSystem4.MERCURY_NAMED);
+        venus1 = new NamedBody(SolSystem4.VENUS_NAMED);
+        vesta1 = new NamedBody(SolSystem4.VESTA_NAMED);
+        pallas1 = new NamedBody(SolSystem4.PALLAS_NAMED);
+        hygiea1 = new NamedBody(SolSystem4.HYGIEA_NAMED);
+        ceres1 = new NamedBody(SolSystem4.CERES_NAMED);
+
+        // check basic functions of 'MassiveForceHashMap'
+        MassiveForceTreeMap map = new MassiveForceTreeMap();
+        map.put(sun1, new Vector3(0, 0, 0));
+        map.put(mercury1, new Vector3(0, 0, 0));
+        map.put(venus1, new Vector3(0, 0, 0));
+        map.put(earth1, new Vector3(0, 0, 0));
+        map.put(moon1, new Vector3(0, 0, 0));
+        map.put(mars1, new Vector3(0, 0, 0));
+        map.put(deimos1, new Vector3(0, 0, 0));
+        map.put(phobos1, new Vector3(0, 0, 0));
+        map.put(vesta1, new Vector3(0, 0, 0));
+        map.put(pallas1, new Vector3(0, 0, 0));
+        map.put(hygiea1, new Vector3(0, 0, 0));
+        map.put(ceres1, new Vector3(0, 0, 0));
+        map.put(mars1, new Vector3(0, 0, 0)); // inserted twice
+
+        HashSet<Massive> set1 = new HashSet<>();
+        set1.add(sun1);
+        set1.add(mercury1);
+        set1.add(venus1);
+        set1.add(earth1);
+        set1.add(moon1);
+        set1.add(mars1);
+        set1.add(deimos1);
+        set1.add(phobos1);
+        set1.add(vesta1);
+        set1.add(pallas1);
+        set1.add(hygiea1);
+        set1.add(ceres1);
+
+        assertTrue(map.toString().contains("Mars"));
+        assertTrue(map.toString().contains("Deimos"));
+        assertTrue(map.toString().contains("Moon"));
+        assertTrue(map.toString().contains("Earth"));
+
+        assertEquals(12, map.getKeys().size());
+
+        assertTrue(map.getKeys().contains(mars1));
+        assertTrue(map.getKeys().contains(new NamedBody("Mars", 6.41712E23, new Vector3(0, 0, 0), new Vector3(0, 0, 0))));
+        assertFalse(map.getKeys().contains(new Body(6.41712E23, new Vector3(0, 0, 0), new Vector3(0, 0, 0))));
+
+        HashSet<Massive> set2 = new HashSet<>();
+        for (Massive m : map.getKeys()) {
+            set2.add(m);
+        }
+
+        assertEquals(set1, set2);
+
+        MassiveLinkedList list = map.getKeys().toList();
+        while (list.size() > 0) {
+            set1.remove(list.pollLast());
+        }
+        assertTrue(set1.isEmpty());
+
+        map.getKeys().remove(mars1);
+        assertFalse(map.containsKey(mars1));
+        assertEquals(11, map.getKeys().size());
+        map.getKeys().clear();
+        assertEquals(0, map.getKeys().size());
+
+        NamedBodyForcePair sun2, mercury2, venus2, earth2, moon2, mars2, deimos2, phobos2, vesta2, pallas2, hygiea2, ceres2;
+
+        //test classes NamedBody and MassiveForceHashMap
+
+        // create 12 named bodies
+        // create the same 12 named body-force pairs
+        sun2 = new NamedBodyForcePair(SolSystem4.SUN_NAMED);
+        earth2 = new NamedBodyForcePair(SolSystem4.EARTH_NAMED);
+        moon2 = new NamedBodyForcePair(SolSystem4.MOON_NAMED);
+        mars2 = new NamedBodyForcePair(SolSystem4.MARS_NAMED);
+        deimos2 = new NamedBodyForcePair(SolSystem4.DEIMOS_NAMED);
+        phobos2 = new NamedBodyForcePair(SolSystem4.PHOBOS_NAMED);
+        mercury2 = new NamedBodyForcePair(SolSystem4.MERCURY_NAMED);
+        venus2 = new NamedBodyForcePair(SolSystem4.VENUS_NAMED);
+        vesta2 = new NamedBodyForcePair(SolSystem4.VESTA_NAMED);
+        pallas2 = new NamedBodyForcePair(SolSystem4.PALLAS_NAMED);
+        hygiea2 = new NamedBodyForcePair(SolSystem4.HYGIEA_NAMED);
+        ceres2 = new NamedBodyForcePair(SolSystem4.CERES_NAMED);
+
+        CosmicSystem earthSystem = new HierarchicalSystem(earth2, moon2);
+        CosmicSystem marsSystem = new HierarchicalSystem(mars2, deimos2, phobos2);
+        HierarchicalSystem solarSystem = new HierarchicalSystem(sun2, mercury2, venus2, earthSystem, marsSystem, vesta2, pallas2, hygiea2, ceres2);
+
+        int count = 0;
+        for (Massive b : solarSystem) {
+            count++;
+        }
+        assertEquals(12, count);
+    }
+}

src/Body.java

@@ -3,7 +3,7 @@ import codedraw.CodeDraw;
 /**
  * This class represents celestial bodies like stars, planets, asteroids, etc...
  */
-public class Body {
+public class Body implements Massive {
     private final double mass;
     private Vector3 massCenter; // position of the mass center.
     private Vector3 currentMovement;

src/BodyForceTreeMap.java

@@ -4,7 +4,7 @@
  */
 public class BodyForceTreeMap {
     private int size = 0;
-    private BodyForceTreeMapItem root = null;
+    private Item root = null;
 
     /**
      * Adds a new key-value association to this map. If the key already exists in this map,
@@ -13,30 +13,30 @@ public class BodyForceTreeMap {
      */
     public Vector3 put(Body key, Vector3 value) {
         if (root == null) {
-            root = new BodyForceTreeMapItem(key, value);
+            root = new Item(key, value);
             size++;
             return null;
         }
 
-        BodyForceTreeMapItem item = root;
+        Item item = root;
         while (item != null) {
-            if (item.key() == key) {
-                Vector3 old = item.value();
-                item.setValue(value);
+            if (item.key == key) {
+                Vector3 old = item.value;
+                item.value = value;
                 return old;
-            } else if (item.key().mass() > key.mass()) {
-                if (item.left() != null) {
-                    item = item.left();
+            } else if (item.key.mass() > key.mass()) {
+                if (item.left != null) {
+                    item = item.left;
                 } else {
-                    item.setLeft(new BodyForceTreeMapItem(key, value));
+                    item.setLeft(new Item(key, value));
                     size++;
                     break;
                 }
             } else {
-                if (item.right() != null) {
-                    item = item.right();
+                if (item.right != null) {
+                    item = item.right;
                 } else{
-                    item.setRight(new BodyForceTreeMapItem(key, value));
+                    item.setRight(new Item(key, value));
                     size++;
                     break;
                 }
@@ -52,14 +52,14 @@ public class BodyForceTreeMap {
      * Precondition: key != null.
      */
     public Vector3 get(Body key) {
-        BodyForceTreeMapItem item = root;
+        Item item = root;
         while (item != null) {
-            if (item.key() == key) {
-                return item.value();
-            } else if (item.key().mass() > key.mass()) {
-                item = item.left();
+            if (item.key == key) {
+                return item.value;
+            } else if (item.key.mass() > key.mass()) {
+                item = item.left;
             } else {
-                item = item.right();
+                item = item.right;
             }
         }
         return null;
@@ -69,14 +69,14 @@ public class BodyForceTreeMap {
      * Returns 'true' if this map contains a mapping for the specified key.
      */
     public boolean containsKey(Body key) {
-        BodyForceTreeMapItem item = root;
+        Item item = root;
         while (item != null) {
-            if (item.key() == key) {
+            if (item.key == key) {
                 return true;
-            } else if (item.key().mass() > key.mass()) {
-                item = item.left();
+            } else if (item.key.mass() > key.mass()) {
+                item = item.left;
             } else {
-                item = item.right();
+                item = item.right;
             }
         }
         return false;
@@ -86,14 +86,14 @@ public class BodyForceTreeMap {
         return this.size;
     }
 
-    private String toString(BodyForceTreeMapItem item) {
+    private String toString(Item item) {
         String s = "";
         if (item == null) {
             return s;
         }
-        s += this.toString(item.right());
-        s += String.format("{%s: %s}\n", item.key(), item.value());
-        s += this.toString(item.left());
+        s += this.toString(item.right);
+        s += String.format("{%s: %s}\n", item.key, item.value);
+        s += this.toString(item.left);
         return s;
     }
 
@@ -103,53 +103,29 @@ public class BodyForceTreeMap {
      */
     @Override
     public String toString() {
-        return toString(root);
-    }
+        return (root != null) ? toString(root) : "";
     }
 
-class BodyForceTreeMapItem {
+    private static class Item {
         private final Body key;
         private Vector3 value;
-    private BodyForceTreeMapItem parent;
-    private BodyForceTreeMapItem left;
-    private BodyForceTreeMapItem right;
+        private Item parent;
+        private Item left;
+        private Item right;
 
-    public BodyForceTreeMapItem(Body key, Vector3 value) {
+        public Item(Body key, Vector3 value) {
             this.key = key;
             this.value = value;
         }
 
-    public Body key() {
-        return this.key;
-    }
-
-    public void setValue(Vector3 value) {
-        this.value = value;
-    }
-
-    public Vector3 value() {
-        return this.value;
-    }
-
-    public BodyForceTreeMapItem left() {
-        return this.left;
-    }
-
-    public BodyForceTreeMapItem right() {
-        return this.right;
-    }
-
-    public BodyForceTreeMapItem parent() {
-        return this.parent;
-    }
-
-    public void setLeft(BodyForceTreeMapItem left) {
+        public void setLeft(Item left) {
             this.left = left;
             if (left != null) left.parent = this;
         }
 
-    public void setRight(BodyForceTreeMapItem right) {
+        public void setRight(Item right) {
             this.right = right;
             if (right != null) right.parent = this;
         }
     }
+}

src/BodyLinkedList.java

@@ -6,8 +6,8 @@ import java.util.Iterator;
  */
 public class BodyLinkedList implements Iterable<Body> {
     private int size = 0;
-    private BodyLinkedListItem first;
-    private BodyLinkedListItem last;
+    private Item first;
+    private Item last;
 
     /**
      * Initializes 'this' as an empty list.
@@ -35,11 +35,11 @@ public class BodyLinkedList implements Iterable<Body> {
      */
     public void addFirst(Body body) {
         if (first == null) {
-            first = new BodyLinkedListItem(body);
+            first = new Item(body);
             last = first;
         } else {
-            first.setPrev(new BodyLinkedListItem(body));
-            first = first.prev();
+            first.setPrev(new Item(body));
+            first = first.prev;
         }
         size++;
     }
@@ -49,11 +49,11 @@ public class BodyLinkedList implements Iterable<Body> {
      */
     public void addLast(Body body) {
         if (last == null) {
-            last = new BodyLinkedListItem(body);
+            last = new Item(body);
             first = last;
         } else {
-            last.setNext(new BodyLinkedListItem(body));
-            last = last.next();
+            last.setNext(new Item(body));
+            last = last.next;
         }
         size++;
     }
@@ -63,7 +63,7 @@ public class BodyLinkedList implements Iterable<Body> {
      * Returns 'null' if the list is empty.
      */
     public Body getLast() {
-        return (last != null) ? last.body() : null;
+        return (last != null) ? last.body : null;
     }
 
     /**
@@ -71,7 +71,7 @@ public class BodyLinkedList implements Iterable<Body> {
      * Returns 'null' if the list is empty.
      */
     public Body getFirst() {
-        return (first != null) ? first.body() : null;
+        return (first != null) ? first.body : null;
     }
 
     /**
@@ -82,8 +82,8 @@ public class BodyLinkedList implements Iterable<Body> {
         if (first == null) {
             return null;
         }
-        Body b = first.body();
-        first = first.next();
+        Body b = first.body;
+        first = first.next;
         if (first != null) first.setPrev(null);
         size--;
         return b;
@@ -97,8 +97,8 @@ public class BodyLinkedList implements Iterable<Body> {
         if (last == null) {
             return null;
         }
-        Body b = last.body();
-        last = last.prev();
+        Body b = last.body;
+        last = last.prev;
         if (last != null) last.setNext(null);
         size--;
         return b;
@@ -117,28 +117,28 @@ public class BodyLinkedList implements Iterable<Body> {
             return;
         }
 
-        BodyLinkedListItem item = first;
+        Item item = first;
         for (int j = 0; j < i; j++) {
-            item = item.next();
+            item = item.next;
         }
 
-        item.prev().setNext(new BodyLinkedListItem(body));
-        item.setPrev(item.prev().next());
+        item.prev.setNext(new Item(body));
+        item.setPrev(item.prev.next);
         size++;
     }
 
-    private Body removeItem(BodyLinkedListItem item) {
+    private Body removeItem(Item item) {
         if (item == first) {
-            first = item.next();
+            first = item.next;
             if (first != null) first.setPrev(null);
         } else if (item == last) {
-            last = item.prev();
+            last = item.prev;
             if (last != null) last.setNext(null);
         } else {
-            item.next().setPrev(item.prev());
+            item.next.setPrev(item.prev);
         }
         size--;
-        return item.body();
+        return item.body;
     }
 
     /**
@@ -146,19 +146,19 @@ public class BodyLinkedList implements Iterable<Body> {
      * Precondition: i >= 0 && i < size().
      */
     public Body get(int i) {
-        BodyLinkedListItem item;
+        Item item;
         if (i < size / 2) {
             item = first;
             for (int j = 0; j < i; j++) {
-                item = item.next();
+                item = item.next;
             }
         } else {
             item = last;
             for (int j = size - 1; j > i; j--) {
-                item = item.prev();
+                item = item.prev;
             }
         }
-        return item.body();
+        return item.body;
     }
 
     /**
@@ -170,12 +170,9 @@ public class BodyLinkedList implements Iterable<Body> {
             return -1;
         }
 
-        BodyLinkedListItem item = first;
-        for (int i = 0; i < size; i++) {
-            if (item.body() == body) {
-                return i;
-            }
-            item = item.next();
+        Item item = first;
+        for (int i = 0; i < size; i++, item = item.next) {
+            if (item.body == body) return i;
         }
 
         return -1;
@@ -187,8 +184,8 @@ public class BodyLinkedList implements Iterable<Body> {
      */
     public BodyLinkedList removeCollidingWith(Body body) {
         BodyLinkedList removed = new BodyLinkedList();
-        for (BodyLinkedListItem item = first; item != null; item = item.next()) {
-            if (body != item.body() && body.collidesWith(item.body())) {
+        for (Item item = first; item != null; item = item.next) {
+            if (body != item.body && body.collidesWith(item.body)) {
                 removed.addLast(this.removeItem(item));
             }
         }
@@ -205,57 +202,45 @@ public class BodyLinkedList implements Iterable<Body> {
     @Override
     public Iterator<Body> iterator() {
         return new Iterator<>() {
-            BodyLinkedListItem ptr = first;
+            Item ptr = first;
             boolean yieldedFirst = false;
 
             @Override
             public boolean hasNext() {
-                return ptr != null && (!yieldedFirst || ptr.next() != null);
+                return ptr != null && (!yieldedFirst || ptr.next != null);
             }
 
             @Override
             public Body next() {
                 if (!yieldedFirst) {
                     yieldedFirst = true;
-                    return ptr.body();
+                } else {
+                    ptr = ptr.next;
                 }
-                ptr = ptr.next();
-                return ptr.body();
+                return ptr.body;
             }
         };
     }
-}
 
-class BodyLinkedListItem {
+    private static class Item {
         private final Body body;
-    private BodyLinkedListItem prev;
-    private BodyLinkedListItem next;
+        private Item prev;
+        private Item next;
 
-    public BodyLinkedListItem(Body body) {
+        public Item(Body body) {
             this.body = body;
             this.prev = null;
             this.next = null;
         }
 
-    public Body body() {
-        return body;
-    }
-
-    public BodyLinkedListItem prev() {
-        return prev;
-    }
-
-    public void setPrev(BodyLinkedListItem prev) {
+        public void setPrev(Item prev) {
             this.prev = prev;
             if (prev != null) prev.next = this;
         }
 
-    public BodyLinkedListItem next() {
-        return next;
-    }
-
-    public void setNext(BodyLinkedListItem next) {
+        public void setNext(Item next) {
             this.next = next;
             if (next != null) next.prev = this;
         }
     }
+}

src/HierarchicalSystem.java

@@ -5,11 +5,11 @@ import codedraw.CodeDraw;
  * and an arbitrary number of subsystems (of type 'CosmicSystem') in its orbit.
  * This class implements 'CosmicSystem'.
  */
-public class HierarchicalSystem implements CosmicSystem {
+public class HierarchicalSystem implements CosmicSystem, MassiveIterable {
 
     private final NamedBodyForcePair central;
-    private final CosmicSystem[] orbit;
-    private final CosmicSystem[] all;
+    private CosmicSystem[] orbit;
+    private CosmicSystem[] all;
 
     /**
      * Initializes this system with a name and a central body.
@@ -110,4 +110,70 @@ public class HierarchicalSystem implements CosmicSystem {
 
         return sb.toString();
     }
+
+    /**
+     * Puts the system 'cs' at the first place in the orbit of this system.
+     * Precondition: cs != null
+     */
+    public boolean putFirst(CosmicSystem cs) {
+        CosmicSystem[] old = orbit;
+        orbit = new CosmicSystem[old.length + 1];
+        all = new CosmicSystem[old.length + 2];
+
+        orbit[0] = cs;
+        System.arraycopy(old, 0, orbit, 1, old.length);
+        all[0] = central;
+        System.arraycopy(orbit, 0, all, 1, orbit.length);
+
+        return true;
+    }
+
+    @Override
+    public MassiveIterator iterator() {
+        return new MassiveIterator() {
+            private int i = 0;
+            private MassiveIterator cur = null;
+
+            @Override
+            public Massive next() {
+                if (cur != null && cur.hasNext()) return cur.next();
+
+                for (; i < all.length; i++) {
+                    CosmicSystem sys = all[i];
+                    if (sys instanceof NamedBodyForcePair m) {
+                        i++;
+                        return m.getBody();
+                    } else if (sys instanceof HierarchicalSystem hs) {
+                        cur = hs.iterator();
+                        if (cur.hasNext()) {
+                            i++;
+                            return cur.next();
+                        }
+                    }
+                }
+
+                return null;
+            }
+
+            @Override
+            public boolean hasNext() {
+                if (cur != null && cur.hasNext()) return true;
+
+                for (; i < all.length; i++) {
+                    CosmicSystem sys = all[i];
+                    if (sys instanceof NamedBodyForcePair) {
+                        return true;
+                    } else if (sys instanceof HierarchicalSystem hs) {
+                        cur = hs.iterator();
+                        if (cur.hasNext()) {
+                            i++;
+                            return true;
+                        }
+                    }
+                }
+
+                return false;
+            }
+        };
+    }
 }

src/Massive.java

@@ -0,0 +1,73 @@
+/**
+ * Represents a coherent mass with a mass center in 3D space. Has two naming schemes for its
+ * methods. Please, do not change this interface definition!
+ */
+public interface Massive extends Drawable {
+
+    /**
+     * Returns the mass.
+     */
+    default double mass() {
+        return getMass();
+    }
+
+    /**
+     * Returns the mass center.
+     */
+    default Vector3 massCenter() {
+        return getMassCenter();
+    }
+
+    /**
+     * Returns the mass.
+     */
+    default double getMass() {
+        return mass();
+    }
+
+    /**
+     * Returns the mass center.
+     */
+    default Vector3 getMassCenter() {
+        return massCenter();
+    }
+
+    /**
+     * Returns the approximate radius of 'this', assuming it is a coherent round mass.
+     * (It is assumed that the radius r is related to the mass m by r = m ^ 0.5,
+     * where m and r measured in solar units.)
+     */
+    default double getRadius() {
+        return radius();
+    }
+
+    /**
+     * Returns the approximate radius of 'this', assuming it is a coherent round mass.
+     * (It is assumed that the radius r is related to the mass m by r = m ^ 0.5,
+     * where m and r measured in solar units.)
+     */
+    default double radius() {
+        return SpaceDraw.massToRadius(mass());
+    }
+
+    /**
+     * Returns a vector representing the gravitational force exerted by 'b' on this mass.
+     * The gravitational Force F is calculated by F = G*(m1*m2)/(r*r), with m1 and m2 being the
+     * masses of the objects interacting, r being the distance between the centers of the masses
+     * and G being the gravitational constant.
+     */
+    default Vector3 gravitationalForce(Massive b) {
+        Vector3 direction = b.massCenter().minus(this.massCenter());
+        double distance = direction.length();
+        direction.normalize();
+        double force = Simulation.G * this.mass() * b.mass() / (distance * distance);
+        return direction.times(force);
+    }
+
+    /**
+     * Centers this mass at a new position, according to the specified force vector 'force' exerted
+     * on it, and updates the current velocity vector accordingly.
+     * (Velocity depends on the mass of 'this', its current velocity and the exerted force.)
+     */
+    void move(Vector3 force);
+}

src/MassiveForceHashMap.java

@@ -0,0 +1,181 @@
+/**
+ * A hash map that associates a 'Massive'-object with a Vector3 (typically this is the force
+ * exerted on the object). The number of key-value pairs is not limited.
+ */
+public class MassiveForceHashMap {
+    private int size;
+    private Massive[] keys;
+    private Vector3[] values;
+
+    /**
+     * Initializes 'this' as an empty map.
+     */
+    public MassiveForceHashMap() {
+        this(16);
+    }
+
+    public MassiveForceHashMap(int capacity) {
+        this.size = 0;
+        this.keys = new Massive[capacity];
+        this.values = new Vector3[capacity];
+    }
+
+    /**
+     * Adds a new key-value association to this map. If the key already exists in this map,
+     * the value is replaced and the old value is returned. Otherwise 'null' is returned.
+     * Precondition: key != null.
+     */
+    public Vector3 put(Massive key, Vector3 value) {
+        if (size > keys.length / 2) doubleCapacity();
+
+        int idx = ((key.hashCode() % keys.length) + keys.length) % keys.length;
+        for (int i = 0; i < keys.length; i++) {
+            int pos = (idx + i) % keys.length;
+            if (values[pos] == null) {
+                keys[pos] = key;
+                values[pos] = value;
+                size++;
+                return null;
+            } else if (keys[pos].equals(key)) {
+                Vector3 old = values[pos];
+                values[pos] = value;
+                return old;
+            }
+        }
+        throw new RuntimeException();
+    }
+
+    private void doubleCapacity() {
+        Massive[] oldKeys = keys;
+        Vector3[] oldValues = values;
+        keys = new Massive[keys.length * 2];
+        values = new Vector3[values.length * 2];
+        size = 0;
+
+        for (int i = 0; i < oldKeys.length; i++) {
+            Massive k = oldKeys[i];
+            Vector3 v = oldValues[i];
+            if (v != null) put(k, v);
+        }
+    }
+
+    /**
+     * Returns the value associated with the specified key, i.e. the method returns the force vector
+     * associated with the specified key. Returns 'null' if the key is not contained in this map.
+     * Precondition: key != null.
+     */
+    public Vector3 get(Massive key) {
+        int pos = find(key);
+        return (pos == -1) ? null : values[pos];
+    }
+
+    private int find(Massive key) {
+        int idx = ((key.hashCode() % keys.length) + keys.length) % keys.length;
+        for (int i = 0; i < keys.length; i++) {
+            int pos = (idx + i) % keys.length;
+            if (keys[pos] == null) {
+                break;
+            } else if (keys[pos].equals(key)) {
+                return pos;
+            }
+        }
+        return -1;
+    }
+
+    /**
+     * Deletes the mapping for the specified key from this map if present.
+     * Returns the previous value associated with key, or null if there was
+     * no mapping for key.
+     * Precondition: key != null
+     */
+    public Vector3 delete(Massive key) {
+        int pos = find(key);
+        if (pos == -1) return null;
+
+        Vector3 val = values[pos];
+        values[pos] = null;
+        return val;
+    }
+
+    /**
+     * Returns 'true' if this map contains a mapping for the specified key.
+     */
+    public boolean containsKey(Massive key) {
+        return this.get(key) != null;
+    }
+
+    /**
+     * Returns a readable representation of this map, with all key-value pairs. Their order is not
+     * defined.
+     */
+    @Override
+    public String toString() {
+        if (size == 0) return "{}";
+
+        StringBuilder sb = new StringBuilder("{");
+        for (int i = 0; i < keys.length; i++) {
+            Massive k = keys[i];
+            Vector3 v = values[i];
+            if (k != null && v != null) {
+                sb.append("\n  ");
+                sb.append(k);
+                sb.append(": ");
+                sb.append(v);
+                sb.append(",");
+            }
+        }
+        sb.deleteCharAt(sb.length() - 1);
+        sb.append("\n}");
+        return sb.toString();
+    }
+
+    /**
+     * Compares `this` with the specified object for equality. Returns `true` if the specified
+     * `o` is not `null` and is of type `MassiveForceHashMap` and both `this` and `o` have equal
+     * key-value pairs, i.e. the number of key-value pairs is the same in both maps and every
+     * key-value pair in `this` equals one key-value pair in `o`. Two key-value pairs are
+     * equal if the two keys are equal and the two values are equal. Otherwise, `false` is returned.
+     */
+    @Override
+    public boolean equals(Object o) {
+        if (!(o instanceof MassiveForceHashMap map) || map.size != size)
+            return false;
+
+        for (int i = 0; i < keys.length; i++) {
+            Massive k1 = keys[i];
+            Vector3 v1 = values[i];
+            if (k1 == null || v1 == null) continue;
+            Vector3 v2 = map.get(k1);
+            if (!v1.equals(v2)) return false;
+        }
+        return true;
+    }
+
+    /**
+     * Returns the hashCode of `this`.
+     */
+    @Override
+    public int hashCode() {
+        int hash = 0;
+        for (int i = 0; i < keys.length; i++) {
+            Massive k = keys[i];
+            Vector3 v = values[i];
+            if (k == null || v == null) continue;
+            hash ^= k.hashCode();
+            hash ^= v.hashCode();
+        }
+        return hash;
+    }
+
+    /**
+     * Returns a list of all the keys in no specified order.
+     */
+    public MassiveLinkedList keyList() {
+        MassiveLinkedList list = new MassiveLinkedList();
+        for (int i = 0; i < keys.length; i++) {
+            if (keys[i] != null && values[i] != null)
+                list.addLast(keys[i]);
+        }
+        return list;
+    }
+}

src/MassiveForceTreeMap.java

@@ -0,0 +1,234 @@
+import codedraw.CodeDraw;
+
+/**
+ * A map that associates an object of 'Massive' with a Vector3. The number of key-value pairs
+ * is not limited.
+ */
+public class MassiveForceTreeMap implements MassiveSet {
+    private int size = 0;
+    private Item root;
+
+    /**
+     * Adds a new key-value association to this map. If the key already exists in this map,
+     * the value is replaced and the old value is returned. Otherwise 'null' is returned.
+     * Precondition: key != null.
+     */
+    public Vector3 put(Massive key, Vector3 value) {
+        if (root == null) {
+            root = new Item(key, value);
+            size++;
+            return null;
+        }
+
+        Item item = root;
+        while (item != null) {
+            if (item.key.equals(key)) {
+                Vector3 old = item.value;
+                item.value = value;
+                return old;
+            } else if (item.key.mass() > key.mass()) {
+                if (item.left != null) {
+                    item = item.left;
+                } else {
+                    item.setLeft(new Item(key, value));
+                    size++;
+                    break;
+                }
+            } else {
+                if (item.right != null) {
+                    item = item.right;
+                } else{
+                    item.setRight(new Item(key, value));
+                    size++;
+                    break;
+                }
+            }
+        }
+
+        return null;
+    }
+
+    /**
+     * Returns the value associated with the specified key, i.e. the method returns the force vector
+     * associated with the specified key. Returns 'null' if the key is not contained in this map.
+     * Precondition: key != null.
+     */
+    public Vector3 get(Massive key) {
+        Item item = root;
+        while (item != null) {
+            if (item.key.equals(key)) {
+                return item.value;
+            } else if (item.key.mass() > key.mass()) {
+                item = item.left;
+            } else {
+                item = item.right;
+            }
+        }
+        return null;
+    }
+
+    /**
+     * Returns 'true' if this map contains a mapping for the specified key.
+     * Precondition: key != null
+     */
+    public boolean containsKey(Massive key) {
+        Item item = root;
+        while (item != null) {
+            if (item.key.equals(key)) {
+                return true;
+            } else if (item.key.mass() > key.mass()) {
+                item = item.left;
+            } else {
+                item = item.right;
+            }
+        }
+        return false;
+    }
+
+    private String toString(Item item) {
+        String s = "";
+        if (item == null) {
+            return s;
+        }
+        s += this.toString(item.right);
+        s += String.format("{%s: %s}\n", item.key, item.value);
+        s += this.toString(item.left);
+        return s;
+    }
+
+    /**
+     * Returns a readable representation of this map, in which key-value pairs are ordered
+     * descending according to 'key.getMass()'.
+     */
+    public String toString() {
+        return (root != null) ? toString(root) : "";
+    }
+
+    /**
+     * Returns a `MassiveSet` view of the keys contained in this tree map. Changing the
+     * elements of the returned `MassiveSet` object also affects the keys in this tree map.
+     */
+    public MassiveSet getKeys() {
+        return this;
+    }
+
+    @Override
+    public void draw(CodeDraw cd) {
+
+    }
+
+    @Override
+    public MassiveIterator iterator() {
+        return new MassiveIterator() {
+            private Item next = root.getLeftLeaf();
+
+            @Override
+            public Massive next() {
+                if (next == null) return null;
+                Massive m = next.key;
+                Item newNext = (next.right != null) ? next.right.getLeftLeaf() : next.parent;
+                while (newNext != null && newNext.right == next) {
+                    next = newNext;
+                    newNext = newNext.parent;
+                }
+                next = newNext;
+                return m;
+            }
+
+            @Override
+            public boolean hasNext() {
+                return next != null;
+            }
+        };
+    }
+
+    @Override
+    public boolean contains(Massive element) {
+        return containsKey(element);
+    }
+
+    @Override
+    public void remove(Massive element) {
+        Item item = root;
+        while (item != null) {
+            if (item.key.equals(element)) {
+                Item newP = null;
+                if (item.left != null) {
+                    newP = item.left.getRightLeaf();
+                } else if (item.right != null) {
+                    newP = item.right.getLeftLeaf();
+                }
+                if (item.parent.left == item) {
+                    item.parent.setLeft(newP);
+                } else {
+                    item.parent.setRight(newP);
+                }
+                size--;
+                return;
+            } else if (item.key.mass() > element.mass()) {
+                item = item.left;
+            } else {
+                item = item.right;
+            }
+        }
+    }
+
+    @Override
+    public void clear() {
+        size = 0;
+        root = null;
+    }
+
+    @Override
+    public int size() {
+        return size;
+    }
+
+    @Override
+    public MassiveLinkedList toList() {
+        MassiveLinkedList list = new MassiveLinkedList();
+        for (Massive m : this) {
+            list.addLast(m);
+        }
+        return list;
+    }
+
+    private static class Item {
+        private final Massive key;
+        private Vector3 value;
+        private Item parent;
+        private Item left;
+        private Item right;
+
+        public Item(Massive key, Vector3 value) {
+            this.key = key;
+            this.value = value;
+        }
+
+        public void setLeft(Item left) {
+            this.left = left;
+            if (left != null) left.parent = this;
+        }
+
+        public void setRight(Item right) {
+            this.right = right;
+            if (right != null) right.parent = this;
+        }
+
+        public Item getLeftLeaf() {
+            Item cur = this;
+            while (cur.left != null) {
+                cur = cur.left;
+            }
+            return cur;
+        }
+
+        public Item getRightLeaf() {
+            Item cur = this;
+            while (cur.right != null) {
+                cur = cur.right;
+            }
+            return cur;
+        }
+    }
+}

src/MassiveIterable.java

@@ -0,0 +1,11 @@
+/**
+ * Iterable objects with 'Massive' elements.
+ */
+public interface MassiveIterable extends Iterable<Massive> {
+
+    /**
+     * Returns an iterator over elements of 'Massive'.
+     */
+    @Override
+    MassiveIterator iterator();
+}

src/MassiveIterator.java

@@ -0,0 +1,20 @@
+import java.util.Iterator;
+
+/**
+ * An iterator over elements of 'Massive'.
+ */
+public interface MassiveIterator extends Iterator<Massive> {
+
+    /**
+     * Returns the next element in the iteration.
+     * (Returns 'null' if the iteration has no more elements.)
+     */
+    @Override
+    Massive next();
+
+    /**
+     * Returns 'true' if the iteration has more elements.
+     */
+    @Override
+    boolean hasNext();
+}

src/MassiveLinkedList.java

@@ -0,0 +1,218 @@
+import java.util.Iterator;
+
+/**
+ * A list of massive objects implemented as a linked list.
+ * The number of elements of the list is not limited.
+ */
+public class MassiveLinkedList implements Iterable<Massive> {
+    private int size = 0;
+    private Item first;
+    private Item last;
+
+    /**
+     * Initializes 'this' as an empty list.
+     */
+    public MassiveLinkedList() {
+        first = null;
+        last = null;
+    }
+
+    /**
+     * Initializes 'this' as an independent copy of the specified list 'list'.
+     * Calling methods of this list will not affect the specified list 'list'
+     * and vice versa.
+     * Precondition: list != null.
+     */
+    public MassiveLinkedList(BodyLinkedList list) {
+        this.size = 0;
+        for (Body b : list) {
+            this.addLast(b);
+        }
+    }
+
+    /**
+     * Inserts the specified element 'body' at the beginning of this list.
+     */
+    public void addFirst(Massive body) {
+        if (first == null) {
+            first = new Item(body);
+            last = first;
+        } else {
+            first.setPrev(new Item(body));
+            first = first.prev;
+        }
+        size++;
+    }
+
+    /**
+     * Appends the specified element 'body' to the end of this list.
+     */
+    public void addLast(Massive body) {
+        if (last == null) {
+            last = new Item(body);
+            first = last;
+        } else {
+            last.setNext(new Item(body));
+            last = last.next;
+        }
+        size++;
+    }
+
+    /**
+     * Returns the last element in this list.
+     * Returns 'null' if the list is empty.
+     */
+    public Massive getLast() {
+        return (last != null) ? last.body : null;
+    }
+
+    /**
+     * Returns the first element in this list.
+     * Returns 'null' if the list is empty.
+     */
+    public Massive getFirst() {
+        return (first != null) ? first.body : null;
+    }
+
+    /**
+     * Retrieves and removes the first element in this list.
+     * Returns 'null' if the list is empty.
+     */
+    public Massive pollFirst() {
+        if (first == null) {
+            return null;
+        }
+        Massive m = first.body;
+        first = first.next;
+        if (first != null) first.setPrev(null);
+        size--;
+        return m;
+    }
+
+    /**
+     * Retrieves and removes the last element in this list.
+     * Returns 'null' if the list is empty.
+     */
+    public Massive pollLast() {
+        if (last == null) {
+            return null;
+        }
+        Massive m = last.body;
+        last = last.prev;
+        if (last != null) last.setNext(null);
+        size--;
+        return m;
+    }
+
+    /**
+     * Inserts the specified element at the specified position in this list.
+     * Precondition: i >= 0 && i <= size().
+     */
+    public void add(int i, Massive m) {
+        if (first == null || i == 0) {
+            addFirst(m);
+            return;
+        } else if (i == size) {
+            addLast(m);
+            return;
+        }
+
+        Item item = first;
+        for (int j = 0; j < i; j++) {
+            item = item.next;
+        }
+
+        item.prev.setNext(new Item(m));
+        item.setPrev(item.prev.next);
+        size++;
+    }
+
+    /**
+     * Returns the element at the specified position in this list.
+     * Precondition: i >= 0 && i < size().
+     */
+    public Massive get(int i) {
+        Item item;
+        if (i < size / 2) {
+            item = first;
+            for (int j = 0; j < i; j++) {
+                item = item.next;
+            }
+        } else {
+            item = last;
+            for (int j = size - 1; j > i; j--) {
+                item = item.prev;
+            }
+        }
+        return item.body;
+    }
+
+    /**
+     * Returns the index of the first occurrence of the specified element in this list, or -1 if
+     * this list does not contain the element.
+     */
+    public int indexOf(Massive m) {
+        if (first == null) {
+            return -1;
+        }
+
+        Item item = first;
+        for (int i = 0; i < size; i++, item = item.next) {
+            if (item.body.equals(m)) return i;
+        }
+
+        return -1;
+    }
+
+    /**
+     * Returns the number of elements in this list.
+     */
+    public int size() {
+        return size;
+    }
+
+    @Override
+    public Iterator<Massive> iterator() {
+        return new Iterator<>() {
+            Item ptr = first;
+            boolean yieldedFirst = false;
+
+            @Override
+            public boolean hasNext() {
+                return ptr != null && (!yieldedFirst || ptr.next != null);
+            }
+
+            @Override
+            public Massive next() {
+                if (!yieldedFirst) {
+                    yieldedFirst = true;
+                } else {
+                    ptr = ptr.next;
+                }
+                return ptr.body;
+            }
+        };
+    }
+
+    private static class Item {
+        private final Massive body;
+        private Item prev;
+        private Item next;
+
+        public Item(Massive body) {
+            this.body = body;
+            this.prev = null;
+            this.next = null;
+        }
+
+        public void setPrev(Item prev) {
+            this.prev = prev;
+            if (prev != null) prev.next = this;
+        }
+
+        public void setNext(Item next) {
+            this.next = next;
+            if (next != null) next.prev = this;
+        }
+    }
+}

src/MassiveSet.java

@@ -0,0 +1,31 @@
+/**
+ * A collection of 'Massive' objects in which there are no duplicates.
+ */
+public interface MassiveSet extends MassiveIterable, Drawable {
+
+    /**
+     * Returns 'true' if the set has the specified element (i.e., has an element equal to the
+     * specified element).
+     */
+    boolean contains(Massive element);
+
+    /**
+     * Removes the specified element from the set.
+     */
+    void remove(Massive element);
+
+    /**
+     * Removes all elements from the set.
+     */
+    void clear();
+
+    /**
+     * Returns the number of elements in the set.
+     */
+    int size();
+
+    /**
+     * Returns an object of 'MassiveLinkedList' with all elements of 'this'.
+     */
+    MassiveLinkedList toList();
+}

src/NamedBody.java

@@ -0,0 +1,79 @@
+import codedraw.CodeDraw;
+
+public class NamedBody implements Massive {
+
+    private final String name;
+    private final Body body;
+
+    /**
+     * Initializes this with name, mass, current position and movement.
+     */
+    public NamedBody(String name, double mass, Vector3 massCenter, Vector3 currentMovement) {
+        this(name, new Body(mass, massCenter, currentMovement));
+    }
+
+    public NamedBody(String name, Body body) {
+        this.name = name;
+        this.body = body;
+    }
+
+    public NamedBody(NamedBody other) {
+        this(other.name, new Body(other.body));
+    }
+
+    /**
+     * Returns the name of the body.
+     */
+    public String getName() {
+        return name;
+    }
+
+    public Body getBody() {
+        return body;
+    }
+
+    public Vector3 getMassCenter() {
+        return body.getMassCenter();
+    }
+
+    public double getMass() {
+        return body.getMass();
+    }
+
+    /**
+     * Compares `this` with the specified object. Returns `true` if the specified `o` is not
+     * `null` and is of type `NamedBody` and both `this` and `o` have equal names.
+     * Otherwise, `false` is returned.
+     */
+    @Override
+    public boolean equals(Object o) {
+        if (!(o instanceof NamedBody b)) return false;
+        return this.name.equals(b.name);
+    }
+
+    /**
+     * Returns the hashCode of `this`.
+     */
+    @Override
+    public int hashCode() {
+        return this.name.hashCode();
+    }
+
+    /**
+     * Returns a readable representation including the name of this body.
+     */
+    @Override
+    public String toString() {
+        return this.getName();
+    }
+
+    @Override
+    public void move(Vector3 force) {
+        body.move(force);
+    }
+
+    @Override
+    public void draw(CodeDraw cd) {
+        body.draw(cd);
+    }
+}

src/NamedBodyForcePair.java

@@ -6,8 +6,7 @@ import codedraw.CodeDraw;
  */
 public class NamedBodyForcePair implements CosmicSystem {
 
-    private final String name;
-    private final Body body;
+    private final NamedBody body;
     private final Vector3 force = new Vector3();
 
     /**
@@ -15,27 +14,30 @@ public class NamedBodyForcePair implements CosmicSystem {
      * is initialized with a zero vector.
      */
     public NamedBodyForcePair(String name, double mass, Vector3 massCenter, Vector3 currentMovement) {
-        this(name, new Body(mass, massCenter, currentMovement));
+        this(new NamedBody(name, mass, massCenter, currentMovement));
     }
 
-    public NamedBodyForcePair(String name, Body b) {
-        this.body = b;
-        this.name = name;
+    public NamedBodyForcePair(String name, Body body) {
+        this(new NamedBody(name, body));
+    }
+
+    public NamedBodyForcePair(NamedBody body) {
+        this.body = body;
     }
 
     public NamedBodyForcePair(NamedBodyForcePair other) {
-        this(other.name, new Body(other.body));
+        this(new NamedBody(other.body));
     }
 
     public Body getBody() {
-        return body;
+        return body.getBody();
     }
 
     /**
      * Returns the name of the body.
      */
     public String getName() {
-        return name;
+        return body.getName();
     }
 
     @Override
@@ -70,13 +72,13 @@ public class NamedBodyForcePair implements CosmicSystem {
 
     @Override
     public void addForceTo(CosmicSystem cs) {
-        cs.addForceFrom(body);
+        cs.addForceFrom(body.getBody());
     }
 
     @Override
     public BodyLinkedList getBodies() {
         BodyLinkedList list = new BodyLinkedList();
-        list.addFirst(body);
+        list.addFirst(body.getBody());
         return list;
     }
 

src/Simulation5.java

@@ -0,0 +1,113 @@
+import codedraw.CodeDraw;
+
+import java.awt.*;
+import java.util.Random;
+
+/**
+ * Simulates the formation of a massive solar system.
+ */
+public class Simulation5 {
+
+    // gravitational constant
+    public static final double G = 6.6743e-11;
+
+    // one astronomical unit (AU) is the average distance of earth to the sun.
+    public static final double AU = 150e9; // meters
+
+    // one light year
+    public static final double LY = 9.461e15; // meters
+
+    // some further constants needed in the simulation
+    public static final double SUN_MASS = 1.989e30; // kilograms
+    public static final double SUN_RADIUS = 696340e3; // meters
+    public static final double EARTH_MASS = 5.972e24; // kilograms
+    public static final double EARTH_RADIUS = 6371e3; // meters
+
+    // set some system parameters
+    public static final double SECTION_SIZE = 10 * AU; // the size of the square region in space
+
+    public static final int NUMBER_OF_BODIES = 22;
+    public static final double OVERALL_SYSTEM_MASS = 20 * SUN_MASS; // kilograms
+
+    // all quantities are based on units of kilogram respectively second and meter.
+
+    /**
+     * The main simulation method using instances of other classes.
+     */
+    public static void main(String[] args) {
+        // simulation
+        CodeDraw cd = new CodeDraw();
+
+        // create solar system with 12 bodies
+        NamedBody sun = new NamedBody(SolSystem4.SUN_NAMED);
+        NamedBody earth = new NamedBody(SolSystem4.EARTH_NAMED);
+        NamedBody moon = new NamedBody(SolSystem4.MOON_NAMED);
+        NamedBody mars = new NamedBody(SolSystem4.MARS_NAMED);
+        NamedBody deimos = new NamedBody(SolSystem4.DEIMOS_NAMED);
+        NamedBody phobos = new NamedBody(SolSystem4.PHOBOS_NAMED);
+        NamedBody mercury = new NamedBody(SolSystem4.MERCURY_NAMED);
+        NamedBody venus = new NamedBody(SolSystem4.VENUS_NAMED);
+        NamedBody vesta = new NamedBody(SolSystem4.VESTA_NAMED);
+        NamedBody pallas = new NamedBody(SolSystem4.PALLAS_NAMED);
+        NamedBody hygiea = new NamedBody(SolSystem4.HYGIEA_NAMED);
+        NamedBody ceres = new NamedBody(SolSystem4.CERES_NAMED);
+
+        // create some additional bodies
+        Body[] bodies = new Body[NUMBER_OF_BODIES];
+
+        Random random = new Random(2022);
+
+        for (int i = 0; i < bodies.length; i++) {
+            bodies[i] = new Body(
+                    Math.abs(random.nextGaussian()) * OVERALL_SYSTEM_MASS / bodies.length,
+                    new Vector3(0.2 * random.nextGaussian() * AU, 0.2 * random.nextGaussian() * AU, 0.2 * random.nextGaussian() * AU),
+                    new Vector3(0 + random.nextGaussian() * 5e3, 0 + random.nextGaussian() * 5e3, 0 + random.nextGaussian() * 5e3)
+            );
+        }
+
+        MassiveForceHashMap forceOnBody = new MassiveForceHashMap();
+        forceOnBody.put(sun, new Vector3());
+        forceOnBody.put(earth, new Vector3());
+        forceOnBody.put(moon, new Vector3());
+        forceOnBody.put(mars, new Vector3());
+        forceOnBody.put(deimos, new Vector3());
+        forceOnBody.put(phobos, new Vector3());
+        forceOnBody.put(mercury, new Vector3());
+        forceOnBody.put(venus, new Vector3());
+        forceOnBody.put(vesta, new Vector3());
+        forceOnBody.put(pallas, new Vector3());
+        forceOnBody.put(hygiea, new Vector3());
+        forceOnBody.put(ceres, new Vector3());
+
+        for (Body b : bodies) {
+            forceOnBody.put(b, new Vector3());
+        }
+
+        long seconds = 0;
+        while (true) {
+            seconds++;
+
+            for (Massive b1 : forceOnBody.keyList()) {
+                Vector3 force = new Vector3();
+                for (Massive b2 : forceOnBody.keyList()) {
+                    if (b1 != b2) {
+                        force = force.plus(b1.gravitationalForce(b2));
+                    }
+                }
+                forceOnBody.put(b1, force);
+            }
+
+            for (Massive body : forceOnBody.keyList()) {
+                body.move(forceOnBody.get(body));
+            }
+
+            if ((seconds % 3600) == 0) {
+                cd.clear(Color.BLACK);
+                for (Massive body : forceOnBody.keyList()) {
+                    body.draw(cd);
+                }
+                cd.show();
+            }
+        }
+    }
+}

src/Simulation6.java

@@ -0,0 +1,113 @@
+import codedraw.CodeDraw;
+
+import java.awt.*;
+import java.util.Random;
+
+/**
+ * Simulates the formation of a massive solar system.
+ */
+public class Simulation6 {
+
+    // gravitational constant
+    public static final double G = 6.6743e-11;
+
+    // one astronomical unit (AU) is the average distance of earth to the sun.
+    public static final double AU = 150e9; // meters
+
+    // one light year
+    public static final double LY = 9.461e15; // meters
+
+    // some further constants needed in the simulation
+    public static final double SUN_MASS = 1.989e30; // kilograms
+    public static final double SUN_RADIUS = 696340e3; // meters
+    public static final double EARTH_MASS = 5.972e24; // kilograms
+    public static final double EARTH_RADIUS = 6371e3; // meters
+
+    // set some system parameters
+    public static final double SECTION_SIZE = 10 * AU; // the size of the square region in space
+
+    public static final int NUMBER_OF_BODIES = 22;
+    public static final double OVERALL_SYSTEM_MASS = 20 * SUN_MASS; // kilograms
+
+    // all quantities are based on units of kilogram respectively second and meter.
+
+    /**
+     * The main simulation method using instances of other classes.
+     */
+    public static void main(String[] args) {
+        // simulation
+        CodeDraw cd = new CodeDraw();
+
+        // create solar system with 12 bodies
+        NamedBody sun = new NamedBody(SolSystem4.SUN_NAMED);
+        NamedBody earth = new NamedBody(SolSystem4.EARTH_NAMED);
+        NamedBody moon = new NamedBody(SolSystem4.MOON_NAMED);
+        NamedBody mars = new NamedBody(SolSystem4.MARS_NAMED);
+        NamedBody deimos = new NamedBody(SolSystem4.DEIMOS_NAMED);
+        NamedBody phobos = new NamedBody(SolSystem4.PHOBOS_NAMED);
+        NamedBody mercury = new NamedBody(SolSystem4.MERCURY_NAMED);
+        NamedBody venus = new NamedBody(SolSystem4.VENUS_NAMED);
+        NamedBody vesta = new NamedBody(SolSystem4.VESTA_NAMED);
+        NamedBody pallas = new NamedBody(SolSystem4.PALLAS_NAMED);
+        NamedBody hygiea = new NamedBody(SolSystem4.HYGIEA_NAMED);
+        NamedBody ceres = new NamedBody(SolSystem4.CERES_NAMED);
+
+        // create some additional bodies
+        Body[] bodies = new Body[NUMBER_OF_BODIES];
+
+        Random random = new Random(2022);
+
+        for (int i = 0; i < bodies.length; i++) {
+            bodies[i] = new Body(
+                    Math.abs(random.nextGaussian()) * OVERALL_SYSTEM_MASS / bodies.length,
+                    new Vector3(0.2 * random.nextGaussian() * AU, 0.2 * random.nextGaussian() * AU, 0.2 * random.nextGaussian() * AU),
+                    new Vector3(0 + random.nextGaussian() * 5e3, 0 + random.nextGaussian() * 5e3, 0 + random.nextGaussian() * 5e3)
+            );
+        }
+
+        MassiveForceTreeMap forceOnBody = new MassiveForceTreeMap();
+        forceOnBody.put(sun, new Vector3());
+        forceOnBody.put(earth, new Vector3());
+        forceOnBody.put(moon, new Vector3());
+        forceOnBody.put(mars, new Vector3());
+        forceOnBody.put(deimos, new Vector3());
+        forceOnBody.put(phobos, new Vector3());
+        forceOnBody.put(mercury, new Vector3());
+        forceOnBody.put(venus, new Vector3());
+        forceOnBody.put(vesta, new Vector3());
+        forceOnBody.put(pallas, new Vector3());
+        forceOnBody.put(hygiea, new Vector3());
+        forceOnBody.put(ceres, new Vector3());
+
+        for (Body b : bodies) {
+            forceOnBody.put(b, new Vector3());
+        }
+
+        long seconds = 0;
+        while (true) {
+            seconds++;
+
+            for (Massive b1 : forceOnBody.getKeys()) {
+                Vector3 force = new Vector3();
+                for (Massive b2 : forceOnBody.getKeys()) {
+                    if (b1 != b2) {
+                        force = force.plus(b1.gravitationalForce(b2));
+                    }
+                }
+                forceOnBody.put(b1, force);
+            }
+
+            for (Massive body : forceOnBody.getKeys()) {
+                body.move(forceOnBody.get(body));
+            }
+
+            if ((seconds % 3600) == 0) {
+                cd.clear(Color.BLACK);
+                for (Massive body : forceOnBody.getKeys()) {
+                    body.draw(cd);
+                }
+                cd.show();
+            }
+        }
+    }
+}

src/SolSystem4.java

@@ -12,16 +12,16 @@ public class SolSystem4 {
     public static final Body HYGIEA = new Body(8.32E19, new Vector3(-3.983943433707043E11, 2.325833000024021E11, -2.233667695713672E10), new Vector3(-6931.864585548552, -15686.8108598699, -690.5791992347208));
     public static final Body CERES = new Body(9.394E20, new Vector3(3.781372641419032E11, 1.96718960466285E11, -6.366459168068592E10), new Vector3(-8555.324226752316, 14718.33755980907, 2040.230135060142));
 
-    public static final NamedBodyForcePair SUN_NAMED = new NamedBodyForcePair("Sun", SUN);
-    public static final NamedBodyForcePair EARTH_NAMED = new NamedBodyForcePair("Earth", EARTH);
-    public static final NamedBodyForcePair MOON_NAMED = new NamedBodyForcePair("Moon", MOON);
-    public static final NamedBodyForcePair MARS_NAMED = new NamedBodyForcePair("Mars", MARS);
-    public static final NamedBodyForcePair DEIMOS_NAMED = new NamedBodyForcePair("Deimos", DEIMOS);
-    public static final NamedBodyForcePair PHOBOS_NAMED = new NamedBodyForcePair("Phobos", PHOBOS);
-    public static final NamedBodyForcePair MERCURY_NAMED = new NamedBodyForcePair("Mercury", MERCURY);
-    public static final NamedBodyForcePair VENUS_NAMED = new NamedBodyForcePair("Venus", VENUS);
-    public static final NamedBodyForcePair VESTA_NAMED = new NamedBodyForcePair("Vesta", VESTA);
-    public static final NamedBodyForcePair PALLAS_NAMED = new NamedBodyForcePair("Pallas", PALLAS);
-    public static final NamedBodyForcePair HYGIEA_NAMED = new NamedBodyForcePair("Hygiea", HYGIEA);
-    public static final NamedBodyForcePair CERES_NAMED = new NamedBodyForcePair("Ceres", CERES);
+    public static final NamedBody SUN_NAMED = new NamedBody("Sun", SUN);
+    public static final NamedBody EARTH_NAMED = new NamedBody("Earth", EARTH);
+    public static final NamedBody MOON_NAMED = new NamedBody("Moon", MOON);
+    public static final NamedBody MARS_NAMED = new NamedBody("Mars", MARS);
+    public static final NamedBody DEIMOS_NAMED = new NamedBody("Deimos", DEIMOS);
+    public static final NamedBody PHOBOS_NAMED = new NamedBody("Phobos", PHOBOS);
+    public static final NamedBody MERCURY_NAMED = new NamedBody("Mercury", MERCURY);
+    public static final NamedBody VENUS_NAMED = new NamedBody("Venus", VENUS);
+    public static final NamedBody VESTA_NAMED = new NamedBody("Vesta", VESTA);
+    public static final NamedBody PALLAS_NAMED = new NamedBody("Pallas", PALLAS);
+    public static final NamedBody HYGIEA_NAMED = new NamedBody("Hygiea", HYGIEA);
+    public static final NamedBody CERES_NAMED = new NamedBody("Ceres", CERES);
 }