Class AbstractMap<K,V>

java.lang.Object
java.util.AbstractMap<K,V>
Type Parameters:
K - the type of keys maintained by this map
V - the type of mapped values
All Implemented Interfaces:
Map<K,V>
Direct Known Subclasses:
ConcurrentHashMap,ConcurrentSkipListMap,EnumMap,HashMap,IdentityHashMap,TreeMap,WeakHashMap
public abstract classAbstractMap<K,V>extendsObjectimplementsMap<K,V>
This class provides a skeletal implementation of theMap 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 theentrySet method, which returns a set-view of the map's mappings. Typically, the returned set will, in turn, be implemented atopAbstractSet. This set should not support theadd orremove methods, and its iterator should not support theremove method.

To implement a modifiable map, the programmer must additionally override this class'sput method (which otherwise throws anUnsupportedOperationException), and the iterator returned byentrySet().iterator() must additionally implement itsremove method.

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

Since:
1.2
See Also:

  • Constructor Details

    • AbstractMap

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

  • Method Details

    • size

      public int size()
      Returns the number of key-value mappings in this map. If the map contains more thanInteger.MAX_VALUE elements, returnsInteger.MAX_VALUE.
      Specified by:
      size in interface Map<K,V>
      Implementation Requirements:
      This implementation returnsentrySet().size().
      Returns:
      the number of key-value mappings in this map

    • isEmpty

      public boolean isEmpty()
      Returnstrue if this map contains no key-value mappings.
      Specified by:
      isEmpty in interface Map<K,V>
      Implementation Requirements:
      This implementation returnssize() == 0.
      Returns:
      true if this map contains no key-value mappings

    • containsValue

      public boolean containsValue(Object value)
      Returnstrue if this map maps one or more keys to the specified value. More formally, returnstrue if and only if this map contains at least one mapping to a valuev such thatObjects.equals(value, v). This operation will probably require time linear in the map size for most implementations of theMap interface.
      Specified by:
      containsValue in interface Map<K,V>
      Implementation Requirements:
      This implementation iterates overentrySet() 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.
      Parameters:
      value - value whose presence in this map is to be tested
      Returns:
      true if this map maps one or more keys to the specified value
      Throws:
      ClassCastException - if the value is of an inappropriate type for this map (optional)
      NullPointerException - if the specified value is null and this map does not permit null values (optional)

    • containsKey

      public boolean containsKey(Object key)
      Returnstrue if this map contains a mapping for the specified key. More formally, returnstrue if and only if this map contains a mapping for a keyk such thatObjects.equals(key, k). (There can be at most one such mapping.)
      Specified by:
      containsKey in interface Map<K,V>
      Implementation Requirements:
      This implementation iterates overentrySet() 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.
      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:
      ClassCastException - if the key is of an inappropriate type for this map (optional)
      NullPointerException - if the specified key is null and this map does not permit null keys (optional)

    • get

      public V get(Object key)
      Returns the value to which the specified key is mapped, ornull if this map contains no mapping for the key.

      More formally, if this map contains a mapping from a keyk to a valuev such thatObjects.equals(key, k), then this method returnsv; otherwise it returnsnull. (There can be at most one such mapping.)

      If this map permits null values, then a return value ofnull does notnecessarily indicate that the map contains no mapping for the key; it's also possible that the map explicitly maps the key tonull. ThecontainsKey operation may be used to distinguish these two cases.

      Specified by:
      get in interface Map<K,V>
      Implementation Requirements:
      This implementation iterates overentrySet() 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.
      Parameters:
      key - the key whose associated value is to be returned
      Returns:
      the value to which the specified key is mapped, ornull if this map contains no mapping for the key
      Throws:
      ClassCastException - if the key is of an inappropriate type for this map (optional)
      NullPointerException - if the specified key is null and this map does not permit null keys (optional)

    • 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 the key, the old value is replaced by the specified value. (A mapm is said to contain a mapping for a keyk if and only ifm.containsKey(k) would returntrue.)
      Specified by:
      put in interface Map<K,V>
      Implementation Requirements:
      This implementation always throws anUnsupportedOperationException.
      Parameters:
      key - key with which the specified value is to be associated
      value - value to be associated with the specified key
      Returns:
      the previous value associated withkey, ornull if there was no mapping forkey. (Anull return can also indicate that the map previously associatednull withkey, if the implementation supportsnull values.)
      Throws:
      UnsupportedOperationException - if theput 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
      NullPointerException - if the specified key or value is null and this map does not permit null keys or values
      IllegalArgumentException - if some property of the specified key or value prevents it from being stored in this map

    • remove

      public V remove(Object key)
      Removes the mapping for a key from this map if it is present (optional operation). More formally, if this map contains a mapping from keyk to valuev such thatObjects.equals(key, k), that mapping is removed. (The map can contain at most one such mapping.)

      Returns the value to which this map previously associated the key, ornull if the map contained no mapping for the key.

      If this map permits null values, then a return value ofnull does notnecessarily indicate that the map contained no mapping for the key; it's also possible that the map explicitly mapped the key tonull.

      The map will not contain a mapping for the specified key once the call returns.

      Specified by:
      remove in interface Map<K,V>
      Implementation Requirements:
      This implementation iterates overentrySet() searching for an entry with the specified key. If such an entry is found, its value is obtained with itsgetValue operation, the entry is removed from the collection (and the backing map) with the iterator'sremove 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 anUnsupportedOperationException if theentrySet iterator does not support theremove method and this map contains a mapping for the specified key.

      Parameters:
      key - key whose mapping is to be removed from the map
      Returns:
      the previous value associated withkey, ornull if there was no mapping forkey.
      Throws:
      UnsupportedOperationException - if theremove operation is not supported by this map
      ClassCastException - if the key is of an inappropriate type for this map (optional)
      NullPointerException - if the specified key is null and this map does not permit null keys (optional)

    • putAll

      public void putAll(Map<? extendsK, ? extendsV> m)
      Copies all of the mappings from the specified map to this map (optional operation). The effect of this call is equivalent to that of callingput(k, v) on this map once for each mapping from keyk to valuev in the specified map. The behavior of this operation is undefined if the specified map is modified while the operation is in progress. If the specified map has a definedencounter order, processing of its mappings generally occurs in that order.
      Specified by:
      putAll in interface Map<K,V>
      Implementation Requirements:
      This implementation iterates over the specified map'sentrySet() collection, and calls this map'sput operation once for each entry returned by the iteration.

      Note that this implementation throws anUnsupportedOperationException if this map does not support theput operation and the specified map is nonempty.

      Parameters:
      m - mappings to be stored in this map
      Throws:
      UnsupportedOperationException - if theputAll 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
      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
      IllegalArgumentException - if some property of a key or value in the specified map prevents it from being stored in this map

    • clear

      public void clear()
      Removes all of the mappings from this map (optional operation). The map will be empty after this call returns.
      Specified by:
      clear in interface Map<K,V>
      Implementation Requirements:
      This implementation callsentrySet().clear().

      Note that this implementation throws anUnsupportedOperationException if theentrySet does not support theclear operation.

      Throws:
      UnsupportedOperationException - if theclear operation is not supported by this map

    • keySet

      public Set<K> keySet()
      Returns aSet 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 (except through the iterator's ownremove operation), the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via theIterator.remove,Set.remove,removeAll,retainAll, andclear operations. It does not support theadd oraddAll operations.
      Specified by:
      keySet in interface Map<K,V>
      Implementation Requirements:
      This implementation returns a set that subclassesAbstractSet. The subclass's iterator method returns a "wrapper object" over this map'sentrySet() iterator. Thesize method delegates to this map'ssize method and thecontains method delegates to this map'scontainsKey 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.

      Returns:
      a set view of the keys contained in this map

    • values

      public Collection<V> values()
      Returns aCollection 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 (except through the iterator's ownremove operation), the results of the iteration are undefined. The collection supports element removal, which removes the corresponding mapping from the map, via theIterator.remove,Collection.remove,removeAll,retainAll andclear operations. It does not support theadd oraddAll operations.
      Specified by:
      values in interface Map<K,V>
      Implementation Requirements:
      This implementation returns a collection that subclassesAbstractCollection. The subclass's iterator method returns a "wrapper object" over this map'sentrySet() iterator. Thesize method delegates to this map'ssize method and thecontains method delegates to this map'scontainsValue 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.

      Returns:
      a collection view of the values contained in this map

    • equals

      public boolean equals(Object o)
      Compares the specified object with this map for equality. Returnstrue if the given object is also a map and the two maps represent the same mappings. More formally, two mapsm1 andm2 represent the same mappings ifm1.entrySet().equals(m2.entrySet()). This ensures that theequals method works properly across different implementations of theMap interface.
      Specified by:
      equals in interface Map<K,V>
      Overrides:
      equals in class Object
      Implementation Requirements:
      This implementation first checks if the specified object is this map; if so it returnstrue. Then, it checks if the specified object is a map whose size is identical to the size of this map; if not, it returnsfalse. If so, it iterates over this map'sentrySet 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.
      Parameters:
      o - object to be compared for equality with this map
      Returns:
      true if the specified object is equal to this map
      See Also:

    • 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'sentrySet() view. This ensures thatm1.equals(m2) implies thatm1.hashCode()==m2.hashCode() for any two mapsm1 andm2, as required by the general contract ofObject.hashCode().
      Specified by:
      hashCode in interface Map<K,V>
      Overrides:
      hashCode in class Object
      Implementation Requirements:
      This implementation iterates overentrySet(), callinghashCode() on each element (entry) in the set, and adding up the results.
      Returns:
      the hash code value for this map
      See Also:

    • 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'sentrySet 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 byString.valueOf(Object).
      Overrides:
      toString in class Object
      Returns:
      a string representation of this map

    • clone

      protected Object clone() throwsCloneNotSupportedException
      Returns a shallow copy of thisAbstractMap 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 theCloneable interface. Subclasses that override theclone method can also throw this exception to indicate that an instance cannot be cloned.
      See Also: