Class HashMap<K,V>

java.lang.Object
java.util.AbstractMap<K,V>
java.util.HashMap<K,V>
All Implemented Interfaces:
Map<K,V>
Direct Known Subclasses:
LinkedHashMap, Properties
public class HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>
HashMap is an implementation of Map. All optional operations (adding and removing) are supported. Keys and values can be any objects.

  • 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: @throws IllegalArgumentException when the capacity is less than zero or the load factor is less or equal to zero.

    • 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

    • clear

      public void clear()

      Removes all mappings from this hash map, leaving it empty.

      See also

      • #isEmpty

      • #size

      Specified by:
      clear in interface Map<K,V>
      Overrides:
      clear in class AbstractMap<K,V>

    • containsKey

      public boolean containsKey(Object key)

      Returns whether this map contains the specified key.

      Parameters
      • key: the key to search for.
      Returns
      Specified by:
      containsKey in interface Map<K,V>
      Overrides:
      containsKey in class AbstractMap<K,V>
      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.

      Parameters
      • value: the value to search for.
      Returns
      Specified by:
      containsValue in interface Map<K,V>
      Overrides:
      containsValue in class AbstractMap<K,V>
      Returns:
      true if this map contains the specified value, false otherwise.

    • 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.

      Returns

      a set of the mappings.

      Specified by:
      entrySet in interface Map<K,V>
      Specified by:
      entrySet in class AbstractMap<K,V>

    • get

      public V get(Object key)

      Returns the value of the mapping with the specified key.

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

    • isEmpty

      public boolean isEmpty()

      Returns whether this map is empty.

      Returns
      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()

    • 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.

      Returns

      a set of the keys.

      Specified by:
      keySet in interface Map<K,V>
      Overrides:
      keySet in class AbstractMap<K,V>

    • put

      public V put(K key, V value)

      Maps the specified key to the specified value.

      Parameters

      • key: the key.

      • value: the value.

      Returns
      Specified by:
      put in interface Map<K,V>
      Overrides:
      put in class AbstractMap<K,V>
      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.

      Parameters
      • map: the map to copy mappings from.
      Throws
      • NullPointerException: if map is null.
      Specified by:
      putAll in interface Map<K,V>
      Overrides:
      putAll in class AbstractMap<K,V>

    • remove

      public V remove(Object key)

      Removes the mapping with the specified key from this map.

      Parameters
      • key: the key of the mapping to remove.
      Returns
      Specified by:
      remove in interface Map<K,V>
      Overrides:
      remove in class AbstractMap<K,V>
      Returns:
      the value of the removed mapping or null if no mapping for the specified key was found.

    • size

      public int size()

      Returns the number of elements in this map.

      Returns

      the number of elements in this map.

      Specified by:
      size in interface Map<K,V>
      Overrides:
      size in class AbstractMap<K,V>

    • 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.

      Returns

      a collection of the values contained in this map.

      Specified by:
      values in interface Map<K,V>
      Overrides:
      values in class AbstractMap<K,V>