Package java.util.concurrent

Class ConcurrentHashMap<K,​V>

  • Type Parameters:
    K - the type of keys maintained by this map
    V - the type of mapped values
    All Implemented Interfaces:
    Serializable, ConcurrentMap<K,​V>, Map<K,​V>
    public class ConcurrentHashMap<K,​V>
extends AbstractMap<K,​V>
implements ConcurrentMap<K,​V>, Serializable
    A hash table supporting full concurrency of retrievals and high expected concurrency for updates. This class obeys the same functional specification as Hashtable, and includes versions of methods corresponding to each method of Hashtable. However, even though all operations are thread-safe, retrieval operations do not entail locking, and there is not any support for locking the entire table in a way that prevents all access. This class is fully interoperable with Hashtable in programs that rely on its thread safety but not on its synchronization details.

    Retrieval operations (including get) generally do not block, so may overlap with update operations (including put and remove). Retrievals reflect the results of the most recently completed update operations holding upon their onset. (More formally, an update operation for a given key bears a happens-before relation with any (non-null) retrieval for that key reporting the updated value.) For aggregate operations such as putAll and clear, concurrent retrievals may reflect insertion or removal of only some entries. Similarly, Iterators and Enumerations return elements reflecting the state of the hash table at some point at or since the creation of the iterator/enumeration. They do not throw ConcurrentModificationException. However, iterators are designed to be used by only one thread at a time. Bear in mind that the results of aggregate status methods including size, isEmpty, and containsValue are typically useful only when a map is not undergoing concurrent updates in other threads. Otherwise the results of these methods reflect transient states that may be adequate for monitoring or estimation purposes, but not for program control.

    The table is dynamically expanded when there are too many collisions (i.e., keys that have distinct hash codes but fall into the same slot modulo the table size), with the expected average effect of maintaining roughly two bins per mapping (corresponding to a 0.75 load factor threshold for resizing). There may be much variance around this average as mappings are added and removed, but overall, this maintains a commonly accepted time/space tradeoff for hash tables. However, resizing this or any other kind of hash table may be a relatively slow operation. When possible, it is a good idea to provide a size estimate as an optional initialCapacity constructor argument. An additional optional loadFactor constructor argument provides a further means of customizing initial table capacity by specifying the table density to be used in calculating the amount of space to allocate for the given number of elements. Also, for compatibility with previous versions of this class, constructors may optionally specify an expected concurrencyLevel as an additional hint for internal sizing. Note that using many keys with exactly the same hashCode() is a sure way to slow down performance of any hash table. To ameliorate impact, when keys are Comparable, this class may use comparison order among keys to help break ties.

    This class and its views and iterators implement all of the optional methods of the Map and Iterator interfaces.

    Like Hashtable but unlike HashMap, this class does not allow null to be used as a key or value.

    Since:
    1.5
    See Also:
    Serialized Form

    • Constructor Summary

      Constructors 
      Constructor Description
      ConcurrentHashMap()
      Creates a new, empty map with the default initial table size (16).
      ConcurrentHashMap​(int initialCapacity)
      Creates a new, empty map with an initial table size accommodating the specified number of elements without the need to dynamically resize.
      ConcurrentHashMap​(int initialCapacity, float loadFactor)
      Creates a new, empty map with an initial table size based on the given number of elements (initialCapacity) and initial table density (loadFactor).
      ConcurrentHashMap​(int initialCapacity, float loadFactor, int concurrencyLevel)
      Creates a new, empty map with an initial table size based on the given number of elements (initialCapacity), table density (loadFactor), and number of concurrently updating threads (concurrencyLevel).
      ConcurrentHashMap​(Map<? extends K,​? extends V> m)
      Creates a new map with the same mappings as the given map.

    • Constructor Detail

      • ConcurrentHashMap

        public ConcurrentHashMap()
        Creates a new, empty map with the default initial table size (16).

      • ConcurrentHashMap

        public ConcurrentHashMap​(int initialCapacity)
        Creates a new, empty map with an initial table size accommodating the specified number of elements without the need to dynamically resize.
        Parameters:
        initialCapacity - The implementation performs internal sizing to accommodate this many elements.
        Throws:
        IllegalArgumentException - if the initial capacity of elements is negative

      • ConcurrentHashMap

        public ConcurrentHashMap​(Map<? extends K,​? extends V> m)
        Creates a new map with the same mappings as the given map.
        Parameters:
        m - the map

      • ConcurrentHashMap

        public ConcurrentHashMap​(int initialCapacity,
                         float loadFactor)
        Creates a new, empty map with an initial table size based on the given number of elements (initialCapacity) and initial table density (loadFactor).
        Parameters:
        initialCapacity - the initial capacity. The implementation performs internal sizing to accommodate this many elements, given the specified load factor.
        loadFactor - the load factor (table density) for establishing the initial table size
        Throws:
        IllegalArgumentException - if the initial capacity of elements is negative or the load factor is nonpositive
        Since:
        1.6

      • ConcurrentHashMap

        public ConcurrentHashMap​(int initialCapacity,
                         float loadFactor,
                         int concurrencyLevel)
        Creates a new, empty map with an initial table size based on the given number of elements (initialCapacity), table density (loadFactor), and number of concurrently updating threads (concurrencyLevel).
        Parameters:
        initialCapacity - the initial capacity. The implementation performs internal sizing to accommodate this many elements, given the specified load factor.
        loadFactor - the load factor (table density) for establishing the initial table size
        concurrencyLevel - the estimated number of concurrently updating threads. The implementation may use this value as a sizing hint.
        Throws:
        IllegalArgumentException - if the initial capacity is negative or the load factor or concurrencyLevel are nonpositive

    • Method Detail

      • size

        public int size()
        Returns the number of mappings in this Map.

        This implementation returns its entry set's size.

        Specified by:
        size in interface Map<K,​V>
        Overrides:
        size in class AbstractMap<K,​V>
        Returns:
        the number of mappings in this Map.

      • isEmpty

        public boolean isEmpty()
        Returns whether this map is empty.

        This implementation compares size() to 0.

        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:
        Map.size()

      • 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.equals(k), then this method returns v; otherwise it returns null. (There can be at most one such mapping.)

        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.
        Throws:
        NullPointerException - if the specified key is null

      • containsKey

        public boolean containsKey​(Object key)
        Tests if the specified object is a key in this table.
        Specified by:
        containsKey in interface Map<K,​V>
        Overrides:
        containsKey in class AbstractMap<K,​V>
        Parameters:
        key - possible key
        Returns:
        true if and only if the specified object is a key in this table, as determined by the equals method; false otherwise
        Throws:
        NullPointerException - if the specified key is null

      • containsValue

        public boolean containsValue​(Object value)
        Returns true if this map maps one or more keys to the specified value. Note: This method may require a full traversal of the map, and is much slower than method containsKey.
        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
        Throws:
        NullPointerException - if the specified value is null

      • put

        public V put​(K key,
             V value)
        Maps the specified key to the specified value in this table. Neither the key nor the value can be null.

        The value can be retrieved by calling the get method with a key that is equal to the original key.

        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 with key, or null if there was no mapping for key
        Throws:
        NullPointerException - if the specified key or value is null

      • putAll

        public void putAll​(Map<? extends K,​? extends V> m)
        Copies all of the mappings from the specified map to this one. These mappings 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

      • remove

        public V remove​(Object key)
        Removes the key (and its corresponding value) from this map. This method does nothing if the key is not in the map.
        Specified by:
        remove in interface Map<K,​V>
        Overrides:
        remove in class AbstractMap<K,​V>
        Parameters:
        key - the key that needs to be removed
        Returns:
        the previous value associated with key, or null if there was no mapping for key
        Throws:
        NullPointerException - if the specified key is null

      • 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. The set supports element removal, which removes the corresponding mapping from this map, via the Iterator.remove, Set.remove, removeAll, retainAll, and clear operations. It does not support the add or addAll operations.

        The view's iterator is a "weakly consistent" iterator that will never throw ConcurrentModificationException, and guarantees to traverse elements as they existed upon construction of the iterator, and may (but is not guaranteed to) reflect any modifications subsequent to construction.

        Specified by:
        keySet in interface Map<K,​V>
        Overrides:
        keySet in class AbstractMap<K,​V>
        Returns:
        the set view

      • 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. The collection supports element removal, which removes the corresponding mapping from this map, via the Iterator.remove, Collection.remove, removeAll, retainAll, and clear operations. It does not support the add or addAll operations.

        The view's iterator is a "weakly consistent" iterator that will never throw ConcurrentModificationException, and guarantees to traverse elements as they existed upon construction of the iterator, and may (but is not guaranteed to) reflect any modifications subsequent to construction.

        Specified by:
        values in interface Map<K,​V>
        Overrides:
        values in class AbstractMap<K,​V>
        Returns:
        the collection view

      • entrySet

        public Set<Map.Entry<K,​V>> entrySet()
        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. The set supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Set.remove, removeAll, retainAll, and clear operations.

        The view's iterator is a "weakly consistent" iterator that will never throw ConcurrentModificationException, and guarantees to traverse elements as they existed upon construction of the iterator, and may (but is not guaranteed to) reflect any modifications subsequent to construction.

        Specified by:
        entrySet in interface Map<K,​V>
        Specified by:
        entrySet in class AbstractMap<K,​V>
        Returns:
        the set view

      • hashCode

        public int hashCode()
        Returns the hash code value for this Map, i.e., the sum of, for each key-value pair in the map, key.hashCode() ^ value.hashCode().
        Specified by:
        hashCode in interface Map<K,​V>
        Overrides:
        hashCode in class AbstractMap<K,​V>
        Returns:
        the hash code value for this map
        See Also:
        Object.equals(java.lang.Object)

      • toString

        public String toString()
        Returns a string representation of this map. The string representation consists of a list of key-value mappings (in no particular order) 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.
        Overrides:
        toString in class AbstractMap<K,​V>
        Returns:
        a string representation of this map

      • equals

        public boolean equals​(Object o)
        Compares the specified object with this map for equality. Returns true if the given object is a map with the same mappings as this map. This operation may return misleading results if either map is concurrently modified during execution of this method.
        Specified by:
        equals in interface Map<K,​V>
        Overrides:
        equals in class AbstractMap<K,​V>
        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()

      • putIfAbsent

        public V putIfAbsent​(K key,
                     V value)
        If the specified key is not already associated with a value, associate it with the given value. This is equivalent to 
         
 if (!map.containsKey(key))
   return map.put(key, value);
 else
   return map.get(key);
        except that the action is performed atomically.
        Specified by:
        putIfAbsent in interface ConcurrentMap<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 the specified key, or null if there was no mapping for the key
        Throws:
        NullPointerException - if the specified key or value is null

      • remove

        public boolean remove​(Object key,
                      Object value)
        Removes the entry for a key only if currently mapped to a given value. This is equivalent to 
         
 if (map.containsKey(key) && map.get(key).equals(value)) {
   map.remove(key);
   return true;
 } else
   return false;
        except that the action is performed atomically.
        Specified by:
        remove in interface ConcurrentMap<K,​V>
        Parameters:
        key - key with which the specified value is associated
        value - value expected to be associated with the specified key
        Returns:
        true if the value was removed
        Throws:
        NullPointerException - if the specified key is null

      • replace

        public boolean replace​(K key,
                       V oldValue,
                       V newValue)
        Replaces the entry for a key only if currently mapped to a given value. This is equivalent to 
         
 if (map.containsKey(key) && map.get(key).equals(oldValue)) {
   map.put(key, newValue);
   return true;
 } else
   return false;
        except that the action is performed atomically.
        Specified by:
        replace in interface ConcurrentMap<K,​V>
        Parameters:
        key - key with which the specified value is associated
        oldValue - value expected to be associated with the specified key
        newValue - value to be associated with the specified key
        Returns:
        true if the value was replaced
        Throws:
        NullPointerException - if any of the arguments are null

      • replace

        public V replace​(K key,
                 V value)
        Replaces the entry for a key only if currently mapped to some value. This is equivalent to 
         
 if (map.containsKey(key)) {
   return map.put(key, value);
 } else
   return null;
        except that the action is performed atomically.
        Specified by:
        replace in interface ConcurrentMap<K,​V>
        Parameters:
        key - key with which the specified value is associated
        value - value to be associated with the specified key
        Returns:
        the previous value associated with the specified key, or null if there was no mapping for the key
        Throws:
        NullPointerException - if the specified key or value is null

      • contains

        public boolean contains​(Object value)
        Legacy method testing if some key maps into the specified value in this table. This method is identical in functionality to containsValue(Object), and exists solely to ensure full compatibility with class Hashtable, which supported this method prior to introduction of the Java Collections framework.
        Parameters:
        value - a value to search for
        Returns:
        true if and only if some key maps to the value argument in this table as determined by the equals method; false otherwise
        Throws:
        NullPointerException - if the specified value is null

      • keys

        public Enumeration<K> keys()
        Returns an enumeration of the keys in this table.
        Returns:
        an enumeration of the keys in this table
        See Also:
        keySet()

      • elements

        public Enumeration<V> elements()
        Returns an enumeration of the values in this table.
        Returns:
        an enumeration of the values in this table
        See Also:
        values()

      • mappingCount

        public long mappingCount()
        Returns the number of mappings. This method should be used instead of size() because a ConcurrentHashMap may contain more mappings than can be represented as an int. The value returned is an estimate; the actual count may differ if there are concurrent insertions or removals.
        Returns:
        the number of mappings
        Since:
        1.8

      • newKeySet

        public static <K> ConcurrentHashMap.KeySetView<K,​Boolean> newKeySet()
        Creates a new Set backed by a ConcurrentHashMap from the given type to Boolean.TRUE.
        Type Parameters:
        K - the element type of the returned set
        Returns:
        the new set
        Since:
        1.8

      • newKeySet

        public static <K> ConcurrentHashMap.KeySetView<K,​Boolean> newKeySet​(int initialCapacity)
        Creates a new Set backed by a ConcurrentHashMap from the given type to Boolean.TRUE.
        Type Parameters:
        K - the element type of the returned set
        Parameters:
        initialCapacity - The implementation performs internal sizing to accommodate this many elements.
        Returns:
        the new set
        Throws:
        IllegalArgumentException - if the initial capacity of elements is negative
        Since:
        1.8