public ConcurrentHashMap() 
{
        this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);
}

Creates a new, empty map with a default initial capacity (16), load factor (0.75) and concurrencyLevel (16).

 public ConcurrentHashMap(int initialCapacity) 
{
        this(initialCapacity, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);
}

Creates a new, empty map with the specified initial capacity, and with default load factor (0.75) and concurrencyLevel (16).

Parameters:

initialCapacity - the initial capacity. The implementation performs internal sizing to accommodate this many elements.

Throws:

IllegalArgumentException - if the initial capacity of elements is negative.

 public ConcurrentHashMap(Map<? extends K, ? extends V> m) 
{
        this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,
                      DEFAULT_INITIAL_CAPACITY),
             DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);
        putAll(m);
}

Creates a new map with the same mappings as the given map. The map is created with a capacity of 1.5 times the number of mappings in the given map or 16 (whichever is greater), and a default load factor (0.75) and concurrencyLevel (16).

Parameters:

m - the map

 public ConcurrentHashMap(int initialCapacity,
    float loadFactor) 
{
        this(initialCapacity, loadFactor, DEFAULT_CONCURRENCY_LEVEL);
}

Creates a new, empty map with the specified initial capacity and load factor and with the default concurrencyLevel (16).

Parameters:

initialCapacity - The implementation performs internal sizing to accommodate this many elements.

loadFactor - the load factor threshold, used to control resizing. Resizing may be performed when the average number of elements per bin exceeds this threshold.

Throws:

IllegalArgumentException - if the initial capacity of elements is negative or the load factor is nonpositive

since: 1.6 -

 public ConcurrentHashMap(int initialCapacity,
    float loadFactor,
    int concurrencyLevel) 
{
        if (!(loadFactor  > 0) || initialCapacity <  0 || concurrencyLevel < = 0)
            throw new IllegalArgumentException();
        if (concurrencyLevel  > MAX_SEGMENTS)
            concurrencyLevel = MAX_SEGMENTS;
        // Find power-of-two sizes best matching arguments
        int sshift = 0;
        int ssize = 1;
        while (ssize <  concurrencyLevel) {
            ++sshift;
            ssize < < = 1;
        }
        this.segmentShift = 32 - sshift;
        this.segmentMask = ssize - 1;
        if (initialCapacity  > MAXIMUM_CAPACITY)
            initialCapacity = MAXIMUM_CAPACITY;
        int c = initialCapacity / ssize;
        if (c * ssize <  initialCapacity)
            ++c;
        int cap = MIN_SEGMENT_TABLE_CAPACITY;
        while (cap <  c)
            cap < < = 1;
        // create segments and segments[0]
        Segment< K,V > s0 =
            new Segment< K,V >(loadFactor, (int)(cap * loadFactor),
                             (HashEntry< K,V >[])new HashEntry[cap]);
        Segment< K,V >[] ss = (Segment< K,V >[])new Segment[ssize];
        UNSAFE.putOrderedObject(ss, SBASE, s0); // ordered write of segments[0]
        this.segments = ss;
}

Creates a new, empty map with the specified initial capacity, load factor and concurrency level.

Parameters:

initialCapacity - the initial capacity. The implementation performs internal sizing to accommodate this many elements.

loadFactor - the load factor threshold, used to control resizing. Resizing may be performed when the average number of elements per bin exceeds this threshold.

concurrencyLevel - the estimated number of concurrently updating threads. The implementation performs internal sizing to try to accommodate this many threads.

Throws:

IllegalArgumentException - if the initial capacity is negative or the load factor or concurrencyLevel are nonpositive.

 public  void clear() 
{
        final Segment< K,V >[] segments = this.segments;
        for (int j = 0; j <  segments.length; ++j) {
            Segment< K,V > s = segmentAt(segments, j);
            if (s != null)
                s.clear();
        }
}

Removes all of the mappings from this map.

 public boolean contains(Object value) 
{
        return containsValue(value);
}

Legacy method testing if some key maps into the specified value in this table. This method is identical in functionality to #containsValue , and exists solely to ensure full compatibility with class java.util.Hashtable , which supported this method prior to introduction of the Java Collections framework.

 public boolean containsKey(Object key) 
{
        Segment< K,V > s; // same as get() except no need for volatile value read
        HashEntry< K,V >[] tab;
        int h = hash(key.hashCode());
        long u = (((h  > > > segmentShift) & segmentMask) < <  SSHIFT) + SBASE;
        if ((s = (Segment< K,V >)UNSAFE.getObjectVolatile(segments, u)) != null &&
            (tab = s.table) != null) {
            for (HashEntry< K,V > e = (HashEntry< K,V >) UNSAFE.getObjectVolatile
                     (tab, ((long)(((tab.length - 1) & h)) < <  TSHIFT) + TBASE);
                 e != null; e = e.next) {
                K k;
                if ((k = e.key) == key || (e.hash == h && key.equals(k)))
                    return true;
            }
        }
        return false;
}

Tests if the specified object is a key in this table.

 public boolean containsValue(Object value) 
{
        // Same idea as size()
        if (value == null)
            throw new NullPointerException();
        final Segment< K,V >[] segments = this.segments;
        boolean found = false;
        long last = 0;
        int retries = -1;
        try {
            outer: for (;;) {
                if (retries++ == RETRIES_BEFORE_LOCK) {
                    for (int j = 0; j <  segments.length; ++j)
                        ensureSegment(j).lock(); // force creation
                }
                long hashSum = 0L;
                int sum = 0;
                for (int j = 0; j <  segments.length; ++j) {
                    HashEntry< K,V >[] tab;
                    Segment< K,V > seg = segmentAt(segments, j);
                    if (seg != null && (tab = seg.table) != null) {
                        for (int i = 0 ; i <  tab.length; i++) {
                            HashEntry< K,V > e;
                            for (e = entryAt(tab, i); e != null; e = e.next) {
                                V v = e.value;
                                if (v != null && value.equals(v)) {
                                    found = true;
                                    break outer;
                                }
                            }
                        }
                        sum += seg.modCount;
                    }
                }
                if (retries  > 0 && sum == last)
                    break;
                last = sum;
            }
        } finally {
            if (retries  > RETRIES_BEFORE_LOCK) {
                for (int j = 0; j <  segments.length; ++j)
                    segmentAt(segments, j).unlock();
            }
        }
        return found;
}

Returns true if this map maps one or more keys to the specified value. Note: This method requires a full internal traversal of the hash table, and so is much slower than method containsKey.

 public Enumeration<V> elements() 
{
        return new ValueIterator();
}

Returns an enumeration of the values in this table.

 static final HashEntry<K, V> entryAt(HashEntry<K, V>[] tab,
    int i) 
{
        return (tab == null) ? null :
            (HashEntry< K,V >) UNSAFE.getObjectVolatile
            (tab, ((long)i < <  TSHIFT) + TBASE);
}

Gets the ith element of given table (if nonnull) with volatile read semantics. Note: This is manually integrated into a few performance-sensitive methods to reduce call overhead.

 static final HashEntry<K, V> entryForHash(Segment<K, V> seg,
    int h) 
{
        HashEntry< K,V >[] tab;
        return (seg == null || (tab = seg.table) == null) ? null :
            (HashEntry< K,V >) UNSAFE.getObjectVolatile
            (tab, ((long)(((tab.length - 1) & h)) < <  TSHIFT) + TBASE);
}

Gets the table entry for the given segment and hash

 public Set<K, V> entrySet() 
{
        Set< Map.Entry< K,V > > es = entrySet;
        return (es != null) ? es : (entrySet = new 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. 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.

 public V get(Object key) 
{
        Segment< K,V > s; // manually integrate access methods to reduce overhead
        HashEntry< K,V >[] tab;
        int h = hash(key.hashCode());
        long u = (((h  > > > segmentShift) & segmentMask) < <  SSHIFT) + SBASE;
        if ((s = (Segment< K,V >)UNSAFE.getObjectVolatile(segments, u)) != null &&
            (tab = s.table) != null) {
            for (HashEntry< K,V > e = (HashEntry< K,V >) UNSAFE.getObjectVolatile
                     (tab, ((long)(((tab.length - 1) & h)) < <  TSHIFT) + TBASE);
                 e != null; e = e.next) {
                K k;
                if ((k = e.key) == key || (e.hash == h && key.equals(k)))
                    return e.value;
            }
        }
        return null;
}

Returns the value to which the specified key is mapped, or {@code null} if this map contains no mapping for the key.

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

 public boolean isEmpty() 
{
        /*
         * Sum per-segment modCounts to avoid mis-reporting when
         * elements are concurrently added and removed in one segment
         * while checking another, in which case the table was never
         * actually empty at any point. (The sum ensures accuracy up
         * through at least 1< < 31 per-segment modifications before
         * recheck.)  Methods size() and containsValue() use similar
         * constructions for stability checks.
         */
        long sum = 0L;
        final Segment< K,V >[] segments = this.segments;
        for (int j = 0; j <  segments.length; ++j) {
            Segment< K,V > seg = segmentAt(segments, j);
            if (seg != null) {
                if (seg.count != 0)
                    return false;
                sum += seg.modCount;
            }
        }
        if (sum != 0L) { // recheck unless no modifications
            for (int j = 0; j <  segments.length; ++j) {
                Segment< K,V > seg = segmentAt(segments, j);
                if (seg != null) {
                    if (seg.count != 0)
                        return false;
                    sum -= seg.modCount;
                }
            }
            if (sum != 0L)
                return false;
        }
        return true;
}

Returns true if this map contains no key-value mappings.

 public Set<K> keySet() 
{
        Set< K > ks = keySet;
        return (ks != null) ? ks : (keySet = new 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.

 public Enumeration<K> keys() 
{
        return new KeyIterator();
}

Returns an enumeration of the keys in this table.

 public V put(K key,
    V value) 
{
        Segment< K,V > s;
        if (value == null)
            throw new NullPointerException();
        int hash = hash(key.hashCode());
        int j = (hash  > > > segmentShift) & segmentMask;
        if ((s = (Segment< K,V >)UNSAFE.getObject          // nonvolatile; recheck
             (segments, (j < <  SSHIFT) + SBASE)) == null) //  in ensureSegment
            s = ensureSegment(j);
        return s.put(key, hash, value, false);
}

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.

 public  void putAll(Map<? extends K, ? extends V> m) 
{
        for (Map.Entry< ? extends K, ? extends V > e : m.entrySet())
            put(e.getKey(), e.getValue());
}

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.

 public V putIfAbsent(K key,
    V value) 
{
        Segment< K,V > s;
        if (value == null)
            throw new NullPointerException();
        int hash = hash(key.hashCode());
        int j = (hash  > > > segmentShift) & segmentMask;
        if ((s = (Segment< K,V >)UNSAFE.getObject
             (segments, (j < <  SSHIFT) + SBASE)) == null)
            s = ensureSegment(j);
        return s.put(key, hash, value, true);
}

{@inheritDoc}

 public V remove(Object key) 
{
        int hash = hash(key.hashCode());
        Segment< K,V > s = segmentForHash(hash);
        return s == null ? null : s.remove(key, hash, null);
}

Removes the key (and its corresponding value) from this map. This method does nothing if the key is not in the map.

 public boolean remove(Object key,
    Object value) 
{
        int hash = hash(key.hashCode());
        Segment< K,V > s;
        return value != null && (s = segmentForHash(hash)) != null &&
            s.remove(key, hash, value) != null;
}

{@inheritDoc}

 public V replace(K key,
    V value) 
{
        int hash = hash(key.hashCode());
        if (value == null)
            throw new NullPointerException();
        Segment< K,V > s = segmentForHash(hash);
        return s == null ? null : s.replace(key, hash, value);
}

{@inheritDoc}

 public boolean replace(K key,
    V oldValue,
    V newValue) 
{
        int hash = hash(key.hashCode());
        if (oldValue == null || newValue == null)
            throw new NullPointerException();
        Segment< K,V > s = segmentForHash(hash);
        return s != null && s.replace(key, hash, oldValue, newValue);
}

{@inheritDoc}

 static final Segment<K, V> segmentAt(Segment<K, V>[] ss,
    int j) 
{
        long u = (j < <  SSHIFT) + SBASE;
        return ss == null ? null :
            (Segment< K,V >) UNSAFE.getObjectVolatile(ss, u);
}

Gets the jth element of given segment array (if nonnull) with volatile element access semantics via Unsafe. (The null check can trigger harmlessly only during deserialization.) Note: because each element of segments array is set only once (using fully ordered writes), some performance-sensitive methods rely on this method only as a recheck upon null reads.

 static final  void setEntryAt(HashEntry<K, V>[] tab,
    int i,
    HashEntry<K, V> e) 
{
        UNSAFE.putOrderedObject(tab, ((long)i < <  TSHIFT) + TBASE, e);
}

Sets the ith element of given table, with volatile write semantics. (See above about use of putOrderedObject.)

 public int size() 
{
        // Try a few times to get accurate count. On failure due to
        // continuous async changes in table, resort to locking.
        final Segment< K,V >[] segments = this.segments;
        int size;
        boolean overflow; // true if size overflows 32 bits
        long sum;         // sum of modCounts
        long last = 0L;   // previous sum
        int retries = -1; // first iteration isn't retry
        try {
            for (;;) {
                if (retries++ == RETRIES_BEFORE_LOCK) {
                    for (int j = 0; j <  segments.length; ++j)
                        ensureSegment(j).lock(); // force creation
                }
                sum = 0L;
                size = 0;
                overflow = false;
                for (int j = 0; j <  segments.length; ++j) {
                    Segment< K,V > seg = segmentAt(segments, j);
                    if (seg != null) {
                        sum += seg.modCount;
                        int c = seg.count;
                        if (c <  0 || (size += c) <  0)
                            overflow = true;
                    }
                }
                if (sum == last)
                    break;
                last = sum;
            }
        } finally {
            if (retries  > RETRIES_BEFORE_LOCK) {
                for (int j = 0; j <  segments.length; ++j)
                    segmentAt(segments, j).unlock();
            }
        }
        return overflow ? Integer.MAX_VALUE : 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.

 public Collection<V> values() 
{
        Collection< V > vs = values;
        return (vs != null) ? vs : (values = new 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.