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## java.util
Class AbstractMap<K,V>

    java.lang.Object
      java.util.AbstractMap<K,V>

All Implemented Interfaces:
  Map<K,V>

Direct Known Subclasses:
  ConcurrentHashMap, EnumMap, HashMap, IdentityHashMap, TreeMap, WeakHashMap

    public abstract class AbstractMap<K,V>

extends Object
implements Map<K,V>

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 methods 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.

Since:
  1.2
See Also:
  `Map`, `Collection`

Nested Class Summary

Nested classes/interfaces inherited from interface java.util.Map
`Map.Entry<K,V>`

Constructor Summary
`protected` | `AbstractMap()` Sole constructor.

Method Summary
`void` | `clear()` Removes all mappings from this map (optional operation).
`protected Object` | `clone()` Returns a shallow copy of this `AbstractMap` instance: the keys and values themselves are not cloned.
`boolean` | `containsKey(Object key)` Returns `true` if this map contains a mapping for the specified key.
`boolean` | `containsValue(Object value)` Returns `true` if this map maps one or more keys to this value.
`abstract Set<Map.Entry<K,V>>` | `entrySet()` Returns a set view of the mappings contained in this map.
`boolean` | `equals(Object o)` Compares the specified object with this map for equality.
`V` | `get(Object key)` Returns the value to which this map maps the specified key.
`int` | `hashCode()` Returns the hash code value for this map.
`boolean` | `isEmpty()` Returns `true` if this map contains no key-value mappings.
`Set<K>` | `keySet()` Returns a Set view of the keys contained in this map.
`V` | `put(K key, V value)` Associates the specified value with the specified key in this map (optional operation).
`void` | `putAll(Map<? extends K,? extends V> t)` Copies all of the mappings from the specified map to this map (optional operation).
`V` | `remove(Object key)` Removes the mapping for this key from this map if present (optional operation).
`int` | `size()` Returns the number of key-value mappings in this map.
`String` | `toString()` Returns a string representation of this map.
`Collection<V>` | `values()` Returns a collection view of the values contained in this map.

Methods inherited from class java.lang.Object
`finalize, getClass, notify, notifyAll, wait, wait, wait`

Constructor Detail

### AbstractMap

    protected AbstractMap()

  Sole constructor. (For invocation by subclass constructors, typically implicit.)

Method Detail

### size

    public int size()

  Returns the number of key-value mappings in this map. If the map contains more than `Integer.MAX_VALUE` elements, returns `Integer.MAX_VALUE`.

  This implementation returns `entrySet().size()`.

  Specified by:
    `size` in interface `Map<K,V>`
  Returns:
    the number of key-value mappings in this map.

### isEmpty

    public boolean isEmpty()

  Returns `true` if this map contains no key-value mappings.

  This implementation returns `size() == 0`.

  Specified by:
    `isEmpty` in interface `Map<K,V>`
  Returns:
    `true` if this map contains no key-value mappings.

### containsValue

    public boolean containsValue(Object value)

  Returns `true` if this map maps one or more keys to this value. More formally, returns `true` if and only if this map contains at least one mapping to a value `v` such that `(value==null ? v==null : value.equals(v))`. This operation will probably require time linear in the map size for most implementations of map.

  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.

  Specified by:
    `containsValue` in interface `Map<K,V>`
  Parameters:
    `value` - value whose presence in this map is to be tested.
  Returns:
    `true` if this map maps one or more keys to this value.

### containsKey

    public boolean containsKey(Object key)

  Returns `true` if this map contains a mapping for the specified key.

  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.

  Specified by:
    `containsKey` in interface `Map<K,V>`
  Parameters:
    `key` - key whose presence in this map is to be tested.
  Returns:
    `true` if this map contains a mapping for the specified key.
  Throws:
    `NullPointerException` - if the key is `null` and this map does not permit `null` keys.

### get

    public V get(Object key)

  Returns the value to which this map maps the specified key. Returns `null` if the map contains no mapping for this key. A return value of `null` does not necessarily indicate that the map contains no mapping for the key; it's also possible that the map explicitly maps the key to `null`. The containsKey operation may be used to distinguish these two cases.

  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.

  Specified by:
    `get` in interface `Map<K,V>`
  Parameters:
    `key` - key whose associated value is to be returned.
  Returns:
    the value to which this map maps the specified key.
  Throws:
    `NullPointerException` - if the key is `null` and this map does not permit `null` keys.
  See Also:
    `containsKey(Object)`

### put

    public V put(K key,
                 V value)

  Associates the specified value with the specified key in this map (optional operation). If the map previously contained a mapping for this key, the old value is replaced.

  This implementation always throws an `UnsupportedOperationException`.

  Specified by:
    `put` in interface `Map<K,V>`
  Parameters:
    `key` - key with which the specified value is to be associated.
    `value` - value to be associated with the specified key.
  Returns:
    previous value associated with specified key, or `null` if there was no mapping for key. (A `null` return can also indicate that the map previously associated `null` with the specified key, if the implementation supports `null` values.)
  Throws:
    `UnsupportedOperationException` - if the `put` operation is not supported by this map.
    `ClassCastException` - if the class of the specified key or value prevents it from being stored in this map.
    `IllegalArgumentException` - if some aspect of this key or value * prevents it from being stored in this map.
    `NullPointerException` - if this map does not permit `null` keys or values, and the specified key or value is `null`.

### remove

    public V remove(Object key)

  Removes the mapping for this key from this map if present (optional operation).

  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.

  Specified by:
    `remove` in interface `Map<K,V>`
  Parameters:
    `key` - key whose mapping is to be removed from the map.
  Returns:
    previous value associated with specified key, or `null` if there was no entry for key. (A `null` return can also indicate that the map previously associated `null` with the specified key, if the implementation supports `null` values.)
  Throws:
    `UnsupportedOperationException` - if the `remove` operation is not supported by this map.

### putAll

    public void putAll(Map<? extends K,? extends V> t)

  Copies all of the mappings from the specified map to this map (optional operation). These mappings will replace any mappings that this map had for any of the keys currently in the specified map.

  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.

  Specified by:
    `putAll` in interface `Map<K,V>`
  Parameters:
    `t` - mappings to be stored in this map.
  Throws:
    `UnsupportedOperationException` - if the `putAll` operation is not supported by this map.
    `ClassCastException` - if the class of a key or value in the specified map prevents it from being stored in this map.
    `IllegalArgumentException` - if some aspect of a key or value in the specified map prevents it from being stored in this map.
    `NullPointerException` - if the specified map is `null`, or if this map does not permit `null` keys or values, and the specified map contains `null` keys or values.

### clear

    public void clear()

  Removes all mappings from this map (optional operation).

  This implementation calls `entrySet().clear()`. Note that this implementation throws an `UnsupportedOperationException` if the `entrySet` does not support the `clear` operation.

  Specified by:
    `clear` in interface `Map<K,V>`
  Throws:
    `UnsupportedOperationException` - clear is not supported by this map.

### keySet

    public Set<K> keySet()

  Returns a Set view of the keys contained in this map. The Set is backed by the map, so changes to the map are reflected in the Set, and vice-versa. (If the map is modified while an iteration over the Set is in progress, the results of the iteration are undefined.) The Set supports element removal, which removes the corresponding entry from the map, via the Iterator.remove, Set.remove, removeAll retainAll, and clear operations. It does not support the add or addAll operations.

  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.

  Specified by:
    `keySet` in interface `Map<K,V>`
  Returns:
    a Set view of the keys contained in this map.

### values

    public Collection<V> values()

  Returns a collection view of the values contained in this map. The collection is backed by the map, so changes to the map are reflected in the collection, and vice-versa. (If the map is modified while an iteration over the collection is in progress, the results of the iteration are undefined.) The collection supports element removal, which removes the corresponding entry from the map, via the `Iterator.remove`, `Collection.remove`, `removeAll`, `retainAll` and `clear` operations. It does not support the `add` or `addAll` operations.

  This implementation returns a collection that subclasses abstract collection. 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.

  Specified by:
    `values` in interface `Map<K,V>`
  Returns:
    a collection view of the values contained in this map.

### entrySet

    public abstract Set<Map.Entry<K,V>> entrySet()

  Returns a set view of the mappings contained in this map. Each element in this set is a Map.Entry. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. (If the map is modified while an iteration over the set is in progress, the results of the iteration are undefined.) The set supports element removal, which removes the corresponding entry from the map, via the `Iterator.remove`, `Set.remove`, `removeAll`, `retainAll` and `clear` operations. It does not support the `add` or `addAll` operations.

  Specified by:
    `entrySet` in interface `Map<K,V>`
  Returns:
    a set view of the mappings contained in this map.

### equals

    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 `t1` and `t2` represent the same mappings if `t1.keySet().equals(t2.keySet())` and for every key `k` in `t1.keySet()`, `(t1.get(k)==null ? t2.get(k)==null : t1.get(k).equals(t2.get(k)))` . 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 set; 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.

  Specified by:
    `equals` in interface `Map<K,V>`
  Overrides:
    `equals` in class `Object`
  Parameters:
    `o` - object to be compared for equality with this map.
  Returns:
    `true` if the specified object is equal to this map.
  See Also:
    `Object.hashCode()`, `Hashtable`

### hashCode

    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 `t1.equals(t2)` implies that `t1.hashCode()==t2.hashCode()` for any two maps `t1` and `t2`, as required by the general contract of Object.hashCode.

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

  Specified by:
    `hashCode` in interface `Map<K,V>`
  Overrides:
    `hashCode` in class `Object`
  Returns:
    the hash code value for this map.
  See Also:
    `Map.Entry.hashCode()`, `Object.hashCode()`, `Object.equals(Object)`, `Set.equals(Object)`

### toString

    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)`.

  This implementation creates an empty string buffer, appends a left brace, and iterates over the map's `entrySet` view, appending the string representation of each `map.entry` in turn. After appending each entry except the last, the string `", "` is appended. Finally a right brace is appended. A string is obtained from the stringbuffer, and returned.

  Overrides:
    `toString` in class `Object`
  Returns:
    a String representation of this map.

### clone

    protected Object clone()
                    throws CloneNotSupportedException

  Returns a shallow copy of this `AbstractMap` instance: the keys and values themselves are not cloned.

  Overrides:
    `clone` in class `Object`
  Returns:
    a shallow copy of this map.
  Throws:
    `CloneNotSupportedException` - if the object's class does not support the `Cloneable` interface. Subclasses that override the `clone` method can also throw this exception to indicate that an instance cannot be cloned.
  See Also:
    `Cloneable`

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For further API reference and developer documentation, see Java 2 SDK SE Developer Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.

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