Implement AB6, Aufgabe 1+2

src/BodyForceTreeMap.java

@@ -103,10 +103,10 @@ public class BodyForceTreeMap {
      */
     @Override
     public String toString() {
-        return toString(root);
+        return (root != null) ? toString(root) : "";
     }
 
-    private class Item {
+    private static class Item {
         private final Body key;
         private Vector3 value;
         private Item parent;