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,PrinterStateReasons
public classHashMap<K,V>extendsAbstractMap<K,V>implementsMap<K,V>,Cloneable,Serializable
Hash table based implementation of theMap interface. This implementation provides all of the optional map operations, and permitsnull values and thenull key. (TheHashMap class is roughly equivalent toHashtable, except that it is unsynchronized and permits nulls.) This class makes no guarantees as to the order of the map; in particular, it does not guarantee that the order will remain constant over time.

This implementation provides constant-time performance for the basic operations (get andput), assuming the hash function disperses the elements properly among the buckets. Iteration over collection views requires time proportional to the "capacity" of theHashMap instance (the number of buckets) plus its size (the number of key-value mappings). Thus, it's very important not to set the initial capacity too high (or the load factor too low) if iteration performance is important.

An instance ofHashMap has two parameters that affect its performance:initial capacity andload factor. Thecapacity is the number of buckets in the hash table, and the initial capacity is simply the capacity at the time the hash table is created. Theload factor is a measure of how full the hash table is allowed to get before its capacity is automatically increased. When the number of entries in the hash table exceeds the product of the load factor and the current capacity, the hash table isrehashed (that is, internal data structures are rebuilt) so that the hash table has approximately twice the number of buckets.

As a general rule, the default load factor (.75) offers a good tradeoff between time and space costs. Higher values decrease the space overhead but increase the lookup cost (reflected in most of the operations of theHashMap class, includingget andput). The expected number of entries in the map and its load factor should be taken into account when setting its initial capacity, so as to minimize the number of rehash operations. If the initial capacity is greater than the maximum number of entries divided by the load factor, no rehash operations will ever occur.

If many mappings are to be stored in aHashMap instance, creating it with a sufficiently large capacity will allow the mappings to be stored more efficiently than letting it perform automatic rehashing as needed to grow the table. Note that using many keys with the samehashCode() is a sure way to slow down performance of any hash table. To ameliorate impact, when keys areComparable, this class may use comparison order among keys to help break ties.

Note that this implementation is not synchronized. If multiple threads access a hash map concurrently, and at least one of the threads modifies the map structurally, itmust be synchronized externally. (A structural modification is any operation that adds or deletes one or more mappings; merely changing the value associated with a key that an instance already contains is not a structural modification.) This is typically accomplished by synchronizing on some object that naturally encapsulates the map. If no such object exists, the map should be "wrapped" using theCollections.synchronizedMap method. This is best done at creation time, to prevent accidental unsynchronized access to the map:

   Map m = Collections.synchronizedMap(new HashMap(...));

The iterators returned by all of this class's "collection view methods" arefail-fast: if the map is structurally modified at any time after the iterator is created, in any way except through the iterator's ownremove method, the iterator will throw aConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throwConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness:the fail-fast behavior of iterators should be used only to detect bugs.

This class is a member of the Java Collections Framework.

Since:
1.2
See Also:

  • Nested Class Summary

    Nested classes/interfaces declared in class java.util.AbstractMap

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

    Nested classes/interfaces declared in interface java.util.Map

    Map.Entry<K,V>

  • Constructor Summary

    Constructors
    Constructor
    Description
    Constructs an emptyHashMap with the default initial capacity (16) and the default load factor (0.75).
    HashMap(int initialCapacity)
    Constructs an emptyHashMap with the specified initial capacity and the default load factor (0.75).
    HashMap(int initialCapacity, float loadFactor)
    Constructs an emptyHashMap with the specified initial capacity and load factor.
    HashMap(Map<? extendsK, ? extendsV> m)
    Constructs a newHashMap with the same mappings as the specifiedMap.

  • Method Summary

    Modifier and Type
    Method
    Description
    void
    Removes all of the mappings from this map.
    Returns a shallow copy of thisHashMap instance: the keys and values themselves are not cloned.
    compute(K key,BiFunction<? superK, ? superV, ? extendsV> remappingFunction)
    Attempts to compute a mapping for the specified key and its current mapped value, ornull if there is no current mapping (optional operation).
    computeIfAbsent(K key,Function<? superK, ? extendsV> mappingFunction)
    If the specified key is not already associated with a value (or is mapped tonull), attempts to compute its value using the given mapping function and enters it into this map unlessnull (optional operation).
    computeIfPresent(K key,BiFunction<? superK, ? superV, ? extendsV> remappingFunction)
    If the value for the specified key is present and non-null, attempts to compute a new mapping given the key and its current mapped value (optional operation).
    boolean
    Returnstrue if this map contains a mapping for the specified key.
    boolean
    Returnstrue if this map maps one or more keys to the specified value.
    Returns aSet view of the mappings contained in this map.
    get(Object key)
    Returns the value to which the specified key is mapped, ornull if this map contains no mapping for the key.
    boolean
    Returnstrue if this map contains no key-value mappings.
    Returns aSet view of the keys contained in this map.
    merge(K key,V value,BiFunction<? superV, ? superV, ? extendsV> remappingFunction)
    If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value (optional operation).
    static <K,V> HashMap<K,V>
    newHashMap(int numMappings)
    Creates a new, empty HashMap suitable for the expected number of mappings.
    put(K key,V value)
    Associates the specified value with the specified key in this map.
    void
    putAll(Map<? extendsK, ? extendsV> m)
    Copies all of the mappings from the specified map to this map.
    Removes the mapping for the specified key from this map if present.
    int
    Returns the number of key-value mappings in this map.
    Returns aCollection view of the values contained in this map.

    Methods declared in class java.util.AbstractMap

    equals,hashCode,toString

    Methods declared in class java.lang.Object

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

  • Constructor Details

    • HashMap

      public HashMap(int initialCapacity, float loadFactor)
      Constructs an emptyHashMap with the specified initial capacity and load factor.
      API Note:
      To create aHashMap with an initial capacity that accommodates an expected number of mappings, usenewHashMap.
      Parameters:
      initialCapacity - the initial capacity
      loadFactor - the load factor
      Throws:
      IllegalArgumentException - if the initial capacity is negative or the load factor is nonpositive

    • HashMap

      public HashMap(int initialCapacity)
      Constructs an emptyHashMap with the specified initial capacity and the default load factor (0.75).
      API Note:
      To create aHashMap with an initial capacity that accommodates an expected number of mappings, usenewHashMap.
      Parameters:
      initialCapacity - the initial capacity.
      Throws:
      IllegalArgumentException - if the initial capacity is negative.

    • HashMap

      public HashMap()
      Constructs an emptyHashMap with the default initial capacity (16) and the default load factor (0.75).

    • HashMap

      public HashMap(Map<? extendsK, ? extendsV> m)
      Constructs a newHashMap with the same mappings as the specifiedMap. TheHashMap is created with default load factor (0.75) and an initial capacity sufficient to hold the mappings in the specifiedMap.
      Parameters:
      m - the map whose mappings are to be placed in this map
      Throws:
      NullPointerException - if the specified map is null

  • Method Details

    • size

      public int size()
      Returns the number of key-value mappings in this map.
      Specified by:
      size in interface Map<K,V>
      Overrides:
      size in class AbstractMap<K,V>
      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>
      Overrides:
      isEmpty in class AbstractMap<K,V>
      Returns:
      true if this map contains no key-value mappings

    • 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 that(key==null ? k==null : key.equals(k)), then this method returnsv; otherwise it returnsnull. (There can be at most one such mapping.)

      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>
      Overrides:
      get in class AbstractMap<K,V>
      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
      See Also:

    • containsKey

      public boolean containsKey(Object key)
      Returnstrue if this map contains a mapping for the specified key.
      Specified by:
      containsKey in interface Map<K,V>
      Overrides:
      containsKey in class AbstractMap<K,V>
      Parameters:
      key - The key whose presence in this map is to be tested
      Returns:
      true if this map contains a mapping for the specified key.

    • put

      public V put(K key,V value)
      Associates the specified value with the specified key in this map. If the map previously contained a mapping for the key, the old value is replaced.
      Specified by:
      put in interface Map<K,V>
      Overrides:
      put in class AbstractMap<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 withkey, ornull if there was no mapping forkey. (Anull return can also indicate that the map previously associatednull withkey.)

    • putAll

      public void putAll(Map<? extendsK, ? extendsV> m)
      Copies all of the mappings from the specified map to this map. These mappings will replace any mappings that this map had for any of the keys currently in the specified map.
      Specified by:
      putAll in interface Map<K,V>
      Overrides:
      putAll in class AbstractMap<K,V>
      Parameters:
      m - mappings to be stored in this map
      Throws:
      NullPointerException - if the specified map is null

    • remove

      public V remove(Object key)
      Removes the mapping for the specified key from this map if present.
      Specified by:
      remove in interface Map<K,V>
      Overrides:
      remove in class AbstractMap<K,V>
      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. (Anull return can also indicate that the map previously associatednull withkey.)

    • clear

      public void clear()
      Removes all of the mappings from this map. The map will be empty after this call returns.
      Specified by:
      clear in interface Map<K,V>
      Overrides:
      clear in class AbstractMap<K,V>

    • containsValue

      public boolean containsValue(Object value)
      Returnstrue if this map maps one or more keys to the specified value.
      Specified by:
      containsValue in interface Map<K,V>
      Overrides:
      containsValue in class AbstractMap<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

    • 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>
      Overrides:
      keySet in class AbstractMap<K,V>
      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>
      Overrides:
      values in class AbstractMap<K,V>
      Returns:
      a view of the values contained in this map

    • entrySet

      public Set<Map.Entry<K,V>> entrySet()
      Returns aSet 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 ownremove operation, or through thesetValue 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 theIterator.remove,Set.remove,removeAll,retainAll andclear operations. It does not support theadd oraddAll operations.
      Specified by:
      entrySet in interface Map<K,V>
      Returns:
      a set view of the mappings contained in this map

    • computeIfAbsent

      public V computeIfAbsent(K key,Function<? superK, ? extendsV> mappingFunction)
      If the specified key is not already associated with a value (or is mapped tonull), attempts to compute its value using the given mapping function and enters it into this map unlessnull (optional operation).

      If the mapping function returnsnull, no mapping is recorded. If the mapping function itself throws an (unchecked) exception, the exception is rethrown, and no mapping is recorded. The most common usage is to construct a new object serving as an initial mapped value or memoized result, as in:

       map.computeIfAbsent(key, k -> new Value(f(k)));

      Or to implement a multi-value map,Map<K,Collection<V>>, supporting multiple values per key:

       map.computeIfAbsent(key, k -> new HashSet<V>()).add(v);

      The mapping function should not modify this map during computation.

      This method will, on a best-effort basis, throw aConcurrentModificationException if it is detected that the mapping function modifies this map during computation.

      Specified by:
      computeIfAbsent in interface Map<K,V>
      Parameters:
      key - key with which the specified value is to be associated
      mappingFunction - the mapping function to compute a value
      Returns:
      the current (existing or computed) value associated with the specified key, or null if the computed value is null
      Throws:
      ConcurrentModificationException - if it is detected that the mapping function modified this map

    • computeIfPresent

      public V computeIfPresent(K key,BiFunction<? superK, ? superV, ? extendsV> remappingFunction)
      If the value for the specified key is present and non-null, attempts to compute a new mapping given the key and its current mapped value (optional operation).

      If the remapping function returnsnull, the mapping is removed. If the remapping function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.

      The remapping function should not modify this map during computation.

      This method will, on a best-effort basis, throw aConcurrentModificationException if it is detected that the remapping function modifies this map during computation.

      Specified by:
      computeIfPresent in interface Map<K,V>
      Parameters:
      key - key with which the specified value is to be associated
      remappingFunction - the remapping function to compute a value
      Returns:
      the new value associated with the specified key, or null if none
      Throws:
      ConcurrentModificationException - if it is detected that the remapping function modified this map

    • compute

      public V compute(K key,BiFunction<? superK, ? superV, ? extendsV> remappingFunction)
      Attempts to compute a mapping for the specified key and its current mapped value, ornull if there is no current mapping (optional operation). For example, to either create or append aString msg to a value mapping:
       map.compute(key, (k, v) -> (v == null) ? msg : v.concat(msg))
      (Methodmerge() is often simpler to use for such purposes.)

      If the remapping function returnsnull, the mapping is removed (or remains absent if initially absent). If the remapping function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.

      The remapping function should not modify this map during computation.

      This method will, on a best-effort basis, throw aConcurrentModificationException if it is detected that the remapping function modifies this map during computation.

      Specified by:
      compute in interface Map<K,V>
      Parameters:
      key - key with which the specified value is to be associated
      remappingFunction - the remapping function to compute a value
      Returns:
      the new value associated with the specified key, or null if none
      Throws:
      ConcurrentModificationException - if it is detected that the remapping function modified this map

    • merge

      public V merge(K key,V value,BiFunction<? superV, ? superV, ? extendsV> remappingFunction)
      If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value (optional operation). Otherwise, replaces the associated value with the results of the given remapping function, or removes if the result isnull. This method may be of use when combining multiple mapped values for a key. For example, to either create or append aString msg to a value mapping:
       map.merge(key, msg, String::concat)

      If the remapping function returnsnull, the mapping is removed. If the remapping function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.

      The remapping function should not modify this map during computation.

      This method will, on a best-effort basis, throw aConcurrentModificationException if it is detected that the remapping function modifies this map during computation.

      Specified by:
      merge in interface Map<K,V>
      Parameters:
      key - key with which the resulting value is to be associated
      value - the non-null value to be merged with the existing value associated with the key or, if no existing value or a null value is associated with the key, to be associated with the key
      remappingFunction - the remapping function to recompute a value if present
      Returns:
      the new value associated with the specified key, or null if no value is associated with the key
      Throws:
      ConcurrentModificationException - if it is detected that the remapping function modified this map

    • clone

      public Object clone()
      Returns a shallow copy of thisHashMap instance: the keys and values themselves are not cloned.
      Overrides:
      clone in class AbstractMap<K,V>
      Returns:
      a shallow copy of this map
      See Also:

    • newHashMap

      public static <K,V> HashMap<K,V> newHashMap(int numMappings)
      Creates a new, empty HashMap suitable for the expected number of mappings. The returned map uses the default load factor of 0.75, and its initial capacity is generally large enough so that the expected number of mappings can be added without resizing the map.
      Type Parameters:
      K - the type of keys maintained by the new map
      V - the type of mapped values
      Parameters:
      numMappings - the expected number of mappings
      Returns:
      the newly created map
      Throws:
      IllegalArgumentException - if numMappings is negative
      Since:
      19