### Question:
BidirectionalMap extends HashMap<K, V> implements Serializable { @Override public V put(K key, V value) { inverseMap.put(value, key); return super.put(key, value); }  @Override V remove(Object key); @Override V put(K key, V value); K getKey(V value); K removeValue(V value); @Override boolean equals(Object obj); @Override int hashCode();  }### Answer:
@Test public void bidirectionalMapPutTest(){ BidirectionalMap map = new BidirectionalMap(); Object previousValue = map.put("one", 1); assertNull(previousValue); }