// A map that associates an object of 'Massive' with a Vector3. The number of key-value pairs
// is not limited.
//
// TODO: define further classes and methods for the binary search tree and the implementation
//  of MassiveSet, if needed.
//
public class MassiveForceTreeMap {

    // TODO: define missing parts of this class.

    // 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) {

        // TODO: implement method.
        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) {

        // TODO: implement method.
        return null;
    }

    // Returns 'true' if this map contains a mapping for the specified key.
    //Precondition: key != null
    public boolean containsKey(Massive key) {

        // TODO: implement method.
        return false;
    }

    // Returns a readable representation of this map, in which key-value pairs are ordered
    // descending according to 'key.getMass()'.
    public String toString() {

        // TODO: implement method.
        return "";
    }

    // 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() {

        // TODO: implement method.
        return null;
    }
}

//TODO: Define additional class(es) implementing the binary search tree and the implementation
// of MassiveSet (either here or in a separate file).