This abstract class is a partial implementation of the Map interface that makes it easy to define simple custom Map implementations. To define an unmodifiable map, subclass AbstractMap and override the entrySet( ) method so that it returns a set of Map.Entry objects. (Note that you must also implement Map.Entry, of course.) The returned set should not support add( ) or remove( ), and its iterator should not support remove( ). In order to define a modifiable Map, you must additionally override the put( ) method and provide support for the remove( ) method of the iterator returned by enTRySet( ).iterator( ). In addition, it is conventional that all Map implementations define two constructors: one that accepts no arguments and another that accepts a Map of initial mappings.

AbstractMap defines all Map methods in terms of its entrySet( ) and put( ) methods and the remove( ) method of the entry set iterator. Note, however, that the implementation is based on a linear search of the Set returned by enTRySet( ) and is not efficient when the Map contains more than a handful of entries. Some subclasses may want to override additional AbstractMap methods to improve performance. HashMap and treeMap use different algorithms are are substantially more efficient.

Figure 16-3. java.util.AbstractMap<K,V>

public abstract class AbstractMap<K,V> implements Map<K,V> {
// Protected Constructors
     protected AbstractMap( );  
// Methods Implementing Map
     public void clear( );  
     public boolean containsKey(Object key);  
     public boolean containsValue(Object value);  
     public abstract Set<Map.Entry<K,V>> entrySet( );  
     public boolean equals(Object o);  
     public V get(Object key);  
     public int hashCode( );  
     public boolean isEmpty( );  
     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 Object
     public String toString( );  
// Protected Methods Overriding Object
1.4  protected Object clone( ) throws CloneNotSupportedException;  
}

Subclasses

EnumMap, HashMap, IdentityHashMap, TReeMap, WeakHashMap, java.util.concurrent.ConcurrentHashMap