Package java.util

Class HashMap<K,​V>

java.lang.Object
java.util.AbstractMap<K,​V>
java.util.HashMap<K,​V>
Type Parameters:
K - the type of keys maintained by this map
V - the type of mapped values
All Implemented Interfaces:
Serializable, Cloneable, Map<K,​V>
Direct Known Subclasses:
LinkedHashMap
public class HashMap<K,​V>
extends AbstractMap<K,​V>
implements Cloneable, Serializable
HashMap is an implementation of Map. All optional operations are supported.

All elements are permitted as keys or values, including null.

Note that the iteration order for HashMap is non-deterministic. If you want deterministic iteration, use LinkedHashMap.

Note: the implementation of HashMap is not synchronized. If one thread of several threads accessing an instance modifies the map structurally, access to the map needs to be synchronized. A structural modification is an operation that adds or removes an entry. Changes in the value of an entry are not structural changes.

The Iterator created by calling the iterator method may throw a ConcurrentModificationException if the map is structurally changed while an iterator is used to iterate over the elements. Only the remove method that is provided by the iterator allows for removal of elements during iteration. It is not possible to guarantee that this mechanism works in all cases of unsynchronized concurrent modification. It should only be used for debugging purposes.

See Also:
Serialized Form

  • Nested Class Summary

    Nested classes/interfaces inherited from class java.util.AbstractMap

    AbstractMap.SimpleEntry<K,​V>, AbstractMap.SimpleImmutableEntry<K,​V>

    Nested classes/interfaces inherited from interface java.util.Map

    Map.Entry<K,​V>

  • Constructor Summary

    Constructors
    Constructor Description
    HashMap()
    Constructs a new empty HashMap instance.
    HashMap​(int capacity)
    Constructs a new HashMap instance with the specified capacity.
    HashMap​(int capacity, float loadFactor)
    Constructs a new HashMap instance with the specified capacity and load factor.
    HashMap​(Map<? extends K,​? extends V> map)
    Constructs a new HashMap instance containing the mappings from the specified map.

  • Method Summary

    Modifier and Type Method Description
    void clear()
    Removes all mappings from this hash map, leaving it empty.
    Object clone()
    Returns a shallow copy of this map.
    boolean containsKey​(Object key)
    Returns whether this map contains the specified key.
    boolean containsValue​(Object value)
    Returns whether this map contains the specified value.
    Set<Map.Entry<K,​V>> entrySet()
    Returns a set containing all of the mappings in this map.
    V get​(Object key)
    Returns the value of the mapping with the specified key.
    boolean isEmpty()
    Returns whether this map is empty.
    Set<K> keySet()
    Returns a set of the keys contained in this map.
    V put​(K key, V value)
    Maps the specified key to the specified value.
    void putAll​(Map<? extends K,​? extends V> map)
    Copies all the mappings in the specified map to this map.
    V remove​(Object key)
    Removes the mapping with the specified key from this map.
    int size()
    Returns the number of elements in this map.
    Collection<V> values()
    Returns a collection of the values contained in this map.

    Methods inherited from class java.util.AbstractMap

    equals, hashCode, toString

    Methods inherited from class java.lang.Object

    finalize, getClass, notify, notifyAll, wait, wait, wait

  • Constructor Details

    • HashMap

      public HashMap()
      Constructs a new empty HashMap instance.

    • HashMap

      public HashMap​(int capacity)
      Constructs a new HashMap instance with the specified capacity.
      Parameters:
      capacity - the initial capacity of this hash map.
      Throws:
      IllegalArgumentException - when the capacity is less than zero.

    • HashMap

      public HashMap​(int capacity, float loadFactor)
      Constructs a new HashMap instance with the specified capacity and load factor.
      Parameters:
      capacity - the initial capacity of this hash map.
      loadFactor - the initial load factor.
      Throws:
      IllegalArgumentException - when the capacity is less than zero or the load factor is less or equal to zero or NaN.

    • HashMap

      public HashMap​(Map<? extends K,​? extends V> map)
      Constructs a new HashMap instance containing the mappings from the specified map.
      Parameters:
      map - the mappings to add.

  • Method Details

    • clone

      public Object clone()
      Returns a shallow copy of this map.
      Overrides:
      clone in class AbstractMap<K,​V>
      Returns:
      a shallow copy of this map.

    • isEmpty

      public boolean isEmpty()
      Returns whether this map is empty.
      Specified by:
      isEmpty in interface Map<K,​V>
      Overrides:
      isEmpty in class AbstractMap<K,​V>
      Returns:
      true if this map has no elements, false otherwise.
      See Also:
      size()

    • size

      public int size()
      Returns the number of elements in this map.
      Specified by:
      size in interface Map<K,​V>
      Overrides:
      size in class AbstractMap<K,​V>
      Returns:
      the number of elements in this map.

    • get

      public V get​(Object key)
      Returns the value of the mapping with the specified key.
      Specified by:
      get in interface Map<K,​V>
      Overrides:
      get in class AbstractMap<K,​V>
      Parameters:
      key - the key.
      Returns:
      the value of the mapping with the specified key, or null if no mapping for the specified key is found.

    • containsKey

      public boolean containsKey​(Object key)
      Returns whether this map contains the specified key.
      Specified by:
      containsKey in interface Map<K,​V>
      Overrides:
      containsKey in class AbstractMap<K,​V>
      Parameters:
      key - the key to search for.
      Returns:
      true if this map contains the specified key, false otherwise.

    • containsValue

      public boolean containsValue​(Object value)
      Returns whether this map contains the specified value.
      Specified by:
      containsValue in interface Map<K,​V>
      Overrides:
      containsValue in class AbstractMap<K,​V>
      Parameters:
      value - the value to search for.
      Returns:
      true if this map contains the specified value, false otherwise.

    • put

      public V put​(K key, V value)
      Maps the specified key to the specified value.
      Specified by:
      put in interface Map<K,​V>
      Overrides:
      put in class AbstractMap<K,​V>
      Parameters:
      key - the key.
      value - the value.
      Returns:
      the value of any previous mapping with the specified key or null if there was no such mapping.

    • putAll

      public void putAll​(Map<? extends K,​? extends V> map)
      Copies all the mappings in the specified map to this map. These mappings will replace all mappings that this map had for any of the keys currently in the given map.
      Specified by:
      putAll in interface Map<K,​V>
      Overrides:
      putAll in class AbstractMap<K,​V>
      Parameters:
      map - the map to copy mappings from.

    • remove

      public V remove​(Object key)
      Removes the mapping with the specified key from this map.
      Specified by:
      remove in interface Map<K,​V>
      Overrides:
      remove in class AbstractMap<K,​V>
      Parameters:
      key - the key of the mapping to remove.
      Returns:
      the value of the removed mapping or null if no mapping for the specified key was found.

    • clear

      public void clear()
      Removes all mappings from this hash map, leaving it empty.
      Specified by:
      clear in interface Map<K,​V>
      Overrides:
      clear in class AbstractMap<K,​V>
      See Also:
      isEmpty(), size

    • keySet

      public Set<K> keySet()
      Returns a set of the keys contained in this map. The set is backed by this map so changes to one are reflected by the other. The set does not support adding.
      Specified by:
      keySet in interface Map<K,​V>
      Overrides:
      keySet in class AbstractMap<K,​V>
      Returns:
      a set of the keys.

    • values

      public Collection<V> values()
      Returns a collection of the values contained in this map. The collection is backed by this map so changes to one are reflected by the other. The collection supports remove, removeAll, retainAll and clear operations, and it does not support add or addAll operations.

      This method returns a collection which is the subclass of AbstractCollection. The iterator method of this subclass returns a "wrapper object" over the iterator of map's entrySet(). The size method wraps the map's size method and the contains method wraps the map's containsValue method.

      The collection is created when this method is called for the first time and returned in response to all subsequent calls. This method may return different collections when multiple concurrent calls occur, since no synchronization is performed.

      Specified by:
      values in interface Map<K,​V>
      Overrides:
      values in class AbstractMap<K,​V>
      Returns:
      a collection of the values contained in this map.

    • entrySet

      public Set<Map.Entry<K,​V>> entrySet()
      Returns a set containing all of the mappings in this map. Each mapping is an instance of Map.Entry. As the set is backed by this map, changes in one will be reflected in the other.
      Specified by:
      entrySet in interface Map<K,​V>
      Specified by:
      entrySet in class AbstractMap<K,​V>
      Returns:
      a set of the mappings.