java.util

Class 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 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 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:
    Map, Collection

    • Constructor Summary

      Constructors

      Modifier Constructor and Description
      protected AbstractMap()
      Sole constructor.

    • Method Summary

      Modifier and Type Method and Description
      void clear()
      Removes all of the 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 the specified 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 the specified key is mapped, or null if this map contains no mapping for the 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> m)
      Copies all of the mappings from the specified map to this map (optional operation).
      V remove(Object key)
      Removes the mapping for a key from this map if it is 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.

    • 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.
        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.
        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 the specified 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 the Map interface.
        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 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)
        Returns true if this map contains a mapping for the specified key. More formally, returns true if and only if this map contains a mapping for a key k such that (key==null ? k==null : key.equals(k)). (There can be at most one such mapping.)
        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:
        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, or null if this map contains no mapping for the key.

        More formally, if this map contains a mapping from a key k to a value v such that (key==null ? k==null : key.equals(k)), then this method returns v; otherwise it returns null. (There can be at most one such mapping.)

        If this map permits null values, then 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.

        Specified by:
        get in interface  Map<K,V>
        Parameters:
        key - the key whose associated value is to be returned
        Returns:
        the value to which the specified key is mapped, or null 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 map m is said to contain a mapping for a key k if and only if m.containsKey(k) would return true.)
        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:
        the previous value associated with key, or null if there was no mapping for key. (A null return can also indicate that the map previously associated null with 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
        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 key k to value v such that (key==null ? k==null : key.equals(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, or null if the map contained no mapping for the key.

        If this map permits null values, then a return value of null does not necessarily indicate that the map contained no mapping for the key; it's also possible that the map explicitly mapped the key to null.

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

        Specified by:
        remove in interface  Map<K,V>
        Parameters:
        key - key whose mapping is to be removed from the map
        Returns:
        the previous value associated with key, or null if there was no mapping for key.
        Throws:
        UnsupportedOperationException - if the remove 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<? extends K,? extends V> 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 calling put(k, v) on this map once for each mapping from key k to value v in the specified map. The behavior of this operation is undefined if the specified map is modified while the operation is in progress.
        Specified by:
        putAll in interface  Map<K,V>
        Parameters:
        m - 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
        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>
        Throws:
        UnsupportedOperationException - if the clear operation 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 (except through the iterator's own remove operation), the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping 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:
        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 (except through the iterator's own remove operation), the results of the iteration are undefined. The collection supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Collection.remove, removeAll, retainAll and clear operations. It does not support the add or addAll operations.
        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()
        Description copied from interface: Map
        Returns a Set view of the mappings 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 own remove operation, or through the setValue operation on a map entry returned by the iterator) the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping 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 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.
        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(), HashMap

      • 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 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().
        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.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).
        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