diff --git a/src/BodyForceMap.java b/src/BodyForceMap.java
index 35711b9..ab12eeb 100644
--- a/src/BodyForceMap.java
+++ b/src/BodyForceMap.java
@@ -6,6 +6,7 @@ public class BodyForceMap {
     //TODO: declare variables.
 
     // Initializes this map with an initial capacity.
+    // Precondition: initialCapacity > 0.
     public BodyForceMap(int initialCapacity) {
 
         //TODO: define constructor.
@@ -13,6 +14,7 @@ public class BodyForceMap {
 
     // 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(Body key, Vector3 force) {
 
         //TODO: implement method.
@@ -20,7 +22,8 @@ public class BodyForceMap {
     }
 
     // Returns the value associated with the specified key, i.e. the returns the force vector
-    // associated with the specified body.
+    // associated with the specified body. Returns 'null' if the key is not contained in this map.
+    // Precondition: key != null.
     public Vector3 get(Body key) {
 
         //TODO: implement method.
diff --git a/src/BodyQueue.java b/src/BodyQueue.java
index ef38f4d..f40d41e 100644
--- a/src/BodyQueue.java
+++ b/src/BodyQueue.java
@@ -8,6 +8,7 @@ public class BodyQueue {
     //TODO: declare variables.
 
     // Initializes this queue with an initial capacity.
+    // Precondition: initialCapacity > 0.
     public BodyQueue(int initialCapacity) {
 
         //TODO: define constructor.
@@ -16,6 +17,7 @@ public class BodyQueue {
     // Initializes this queue as an independent copy of the specified queue.
     // Calling methods of this queue will not affect the specified queue
     // and vice versa.
+    // Precondition: q != null.
     public BodyQueue(BodyQueue q) {
 
         //TODO: define constructor.