@Override
    public ListMap<K, V> clone(){
        ListMap<K, V> clone = new ListMap<>(size());
        clone.putAll(this);
        return clone;
    }
Implementation of a Map that favors iteration speed rather than
get/put speed.

@author Kirill Vainer
jme3-core/src/main/java/com/jme3/util/ListMap.java