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java.util
abstract public class: AbstractMap [javadoc | source]
java.lang.Object
   java.util.AbstractMap

All Implemented Interfaces:
    Map

This class provides a skeletal implementation of the Map interface, to minimize the effort required to implement this interface.

To implement an unmodifiable map, the programmer needs only to extend this class and provide an implementation for the entrySet method, which returns a set-view of the map's mappings. Typically, the returned set will, in turn, be implemented atop AbstractSet. This set should not support the add or remove methods, and its iterator should not support the remove method.

To implement a modifiable map, the programmer must additionally override this class's put method (which otherwise throws an UnsupportedOperationException), and the iterator returned by entrySet().iterator() must additionally implement its remove method.

The programmer should generally provide a void (no argument) and map constructor, as per the recommendation in the Map interface specification.

The documentation for each non-abstract method in this class describes its implementation in detail. Each of these methods may be overridden if the map being implemented admits a more efficient implementation.

This class is a member of the Java Collections Framework.

Nested Class Summary:
public static class  AbstractMap.SimpleEntry  An Entry maintaining a key and a value. The value may be changed using the setValue method. This class facilitates the process of building custom map implementations. For example, it may be convenient to return arrays of SimpleEntry instances in method Map.entrySet().toArray
public static class  AbstractMap.SimpleImmutableEntry  An Entry maintaining an immutable key and value. This class does not support method setValue. This class may be convenient in methods that return thread-safe snapshots of key-value mappings. 
Field Summary
transient volatile  Set<K> keySet    Each of these fields are initialized to contain an instance of the appropriate view the first time this view is requested. The views are stateless, so there's no reason to create more than one of each. 
transient volatile  Collection<V> values     
Constructor:
 protected AbstractMap() 
Method from java.util.AbstractMap Summary:
clear,   clone,   containsKey,   containsValue,   entrySet,   equals,   get,   hashCode,   isEmpty,   keySet,   put,   putAll,   remove,   size,   toString,   values
Methods from java.lang.Object:
clone,   equals,   finalize,   getClass,   hashCode,   notify,   notifyAll,   toString,   wait,   wait,   wait
Method from java.util.AbstractMap Detail:
 public  void clear() 
    {@inheritDoc}

    This implementation calls entrySet().clear().

    Note that this implementation throws an UnsupportedOperationException if the entrySet does not support the clear operation.

 protected Object clone() throws CloneNotSupportedException 
    Returns a shallow copy of this AbstractMap instance: the keys and values themselves are not cloned.
 public boolean containsKey(Object key) 
    {@inheritDoc}

    This implementation iterates over entrySet() searching for an entry with the specified key. If such an entry is found, true is returned. If the iteration terminates without finding such an entry, false is returned. Note that this implementation requires linear time in the size of the map; many implementations will override this method.

 public boolean containsValue(Object value) 
    {@inheritDoc}

    This implementation iterates over entrySet() searching for an entry with the specified value. If such an entry is found, true is returned. If the iteration terminates without finding such an entry, false is returned. Note that this implementation requires linear time in the size of the map.

 abstract public Set<K, V> entrySet()
 public boolean equals(Object o) 
    Compares the specified object with this map for equality. Returns true if the given object is also a map and the two maps represent the same mappings. More formally, two maps m1 and m2 represent the same mappings if m1.entrySet().equals(m2.entrySet()). This ensures that the equals method works properly across different implementations of the Map interface.

    This implementation first checks if the specified object is this map; if so it returns true. Then, it checks if the specified object is a map whose size is identical to the size of this map; if not, it returns false. If so, it iterates over this map's entrySet collection, and checks that the specified map contains each mapping that this map contains. If the specified map fails to contain such a mapping, false is returned. If the iteration completes, true is returned.

 public V get(Object key) 
    {@inheritDoc}

    This implementation iterates over entrySet() searching for an entry with the specified key. If such an entry is found, the entry's value is returned. If the iteration terminates without finding such an entry, null is returned. Note that this implementation requires linear time in the size of the map; many implementations will override this method.

 public int hashCode() 
    Returns the hash code value for this map. The hash code of a map is defined to be the sum of the hash codes of each entry in the map's entrySet() view. This ensures that m1.equals(m2) implies that m1.hashCode()==m2.hashCode() for any two maps m1 and m2, as required by the general contract of Object#hashCode .

    This implementation iterates over entrySet(), calling hashCode() on each element (entry) in the set, and adding up the results.

 public boolean isEmpty() 
    {@inheritDoc}

    This implementation returns size() == 0.

 public Set<K> keySet() 
    {@inheritDoc}

    This implementation returns a set that subclasses AbstractSet . The subclass's iterator method returns a "wrapper object" over this map's entrySet() iterator. The size method delegates to this map's size method and the contains method delegates to this map's containsKey method.

    The set is created the first time this method is called, and returned in response to all subsequent calls. No synchronization is performed, so there is a slight chance that multiple calls to this method will not all return the same set.

 public V put(K key,
    V value) 
    {@inheritDoc}

    This implementation always throws an UnsupportedOperationException.

 public  void putAll(Map<? extends K, ? extends V> m) 
    {@inheritDoc}

    This implementation iterates over the specified map's entrySet() collection, and calls this map's put operation once for each entry returned by the iteration.

    Note that this implementation throws an UnsupportedOperationException if this map does not support the put operation and the specified map is nonempty.

 public V remove(Object key) 
    {@inheritDoc}

    This implementation iterates over entrySet() searching for an entry with the specified key. If such an entry is found, its value is obtained with its getValue operation, the entry is removed from the collection (and the backing map) with the iterator's remove operation, and the saved value is returned. If the iteration terminates without finding such an entry, null is returned. Note that this implementation requires linear time in the size of the map; many implementations will override this method.

    Note that this implementation throws an UnsupportedOperationException if the entrySet iterator does not support the remove method and this map contains a mapping for the specified key.

 public int size() 
    {@inheritDoc}

    This implementation returns entrySet().size().

 public String toString() 
    Returns a string representation of this map. The string representation consists of a list of key-value mappings in the order returned by the map's entrySet view's iterator, enclosed in braces ("{}"). Adjacent mappings are separated by the characters ", " (comma and space). Each key-value mapping is rendered as the key followed by an equals sign ("=") followed by the associated value. Keys and values are converted to strings as by String#valueOf(Object) .
 public Collection<V> values() 
    {@inheritDoc}

    This implementation returns a collection that subclasses AbstractCollection . The subclass's iterator method returns a "wrapper object" over this map's entrySet() iterator. The size method delegates to this map's size method and the contains method delegates to this map's containsValue method.

    The collection is created the first time this method is called, and returned in response to all subsequent calls. No synchronization is performed, so there is a slight chance that multiple calls to this method will not all return the same collection.