IdentityHashMapK,V

IdentityHashMap<K,V>

java.util

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Java 1.4

cloneable serializable collection

This Map implementation has a API that is very similar to HashMap , and uses an internal hashtable, like HashMap does. However, it behaves differently from HashMap in one very important way. When testing two keys to see if they are equal, HashMap , LinkedHashMap and treeMap use the equals( ) method to determine whether the two objects are indistinguishable in terms of their content or state. IdentityHashMap is different: it uses the == operator to determine whether the two key objects are identicalwhether they are exactly the same object. This one difference in how key equality is tested has profound ramifications for the behavior of the Map . In most cases, the equality testing of a HashMap , LinkedHashMap or TReeMap is the appropriate behavior, and you should use one of those classes. For certain purposes, however, the identity testing of IdentityHashMap is what is required.

Figure 16-27. java.util.IdentityHashMap<K,V>

 public class  IdentityHashMap<K,V>  extends AbstractMap<K,V> implements Map<K,V>,          Serializable, Cloneable {  // Public Constructors  public  IdentityHashMap  ( );        public  IdentityHashMap  (int  expectedMaxSize  );        public  IdentityHashMap  (Map<? extends K,? extends V>  m  );  // Methods Implementing Map  public void  clear  ( );        public boolean  containsKey  (Object  key  );        public boolean  containsValue  (Object  value  );        public Set<Map.Entry<K,V>>  entrySet  ( );        public boolean  equals  (Object  o  );        public V  get  (Object  key  );        public int  hashCode  ( );        public boolean  isEmpty  ( );  default:true  public Set<K>  keySet  ( );        public V  put  (K  key  , V  value  );        public void  putAll  (Map<? extends K,? extends V>  t  );        public V  remove  (Object  key  );        public int  size  ( );        public Collection<V>  values  ( );  // Public Methods Overriding AbstractMap  public Object  clone  ( );   }