HashMap

HashMap


  public
  
  
  
  class
  HashMap


  
    extends AbstractMap<K, V>


  
  
      implements
      
        Cloneable, 
      
        Map<K, V>, 
      
        Serializable

Known direct subclasses

LinkedHashMap<K, V>

Hash table and linked list implementation of the Map interface, with well-defined encounter order.

Hash table based implementation of the Map interface. This implementation provides all of the optional map operations, and permits null values and the null key. (The HashMap class is roughly equivalent to Hashtable, 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 and put), assuming the hash function disperses the elements properly among the buckets. Iteration over collection views requires time proportional to the "capacity" of the HashMap 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 of HashMap has two parameters that affect its performance: initial capacity and load factor. The capacity 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. The load 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 is rehashed (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 the HashMap class, including get and put). 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 a HashMap 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 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.

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, it must 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 the Collections.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" are fail-fast: if the map is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove method, the iterator will throw a ConcurrentModificationException. 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 throw ConcurrentModificationException 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.

See also:

Summary

Public constructors
`HashMap()` Constructs an empty `HashMap` with the default initial capacity (16) and the default load factor (0.75).
`HashMap(int initialCapacity)` Constructs an empty `HashMap` with the specified initial capacity and the default load factor (0.75).
`HashMap(int initialCapacity, float loadFactor)` Constructs an empty `HashMap` with the specified initial capacity and load factor.
`HashMap(Map<? extends K, ? extends V> m)` Constructs a new `HashMap` with the same mappings as the specified `Map`.

Public methods
`void`	`clear()` Removes all of the mappings from this map.
`Object`	`clone()` Returns a shallow copy of this `HashMap` instance: the keys and values themselves are not cloned.
`V`	`compute(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction)` Attempts to compute a mapping for the specified key and its current mapped value, or `null` if there is no current mapping (optional operation). This method will, on a best-effort basis, throw a `ConcurrentModificationException` if it is detected that the remapping function modifies this map during computation.
`V`	`computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction)` If the specified key is not already associated with a value (or is mapped to `null`), attempts to compute its value using the given mapping function and enters it into this map unless `null` (optional operation). This method will, on a best-effort basis, throw a `ConcurrentModificationException` if it is detected that the mapping function modifies this map during computation.
`V`	`computeIfPresent(K key, BiFunction<? super K, ? super V, ? extends V> 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). This method will, on a best-effort basis, throw a `ConcurrentModificationException` if it is detected that the remapping function modifies this map during computation.
`boolean`	`containsKey(Object key)` Returns `true` if this map contains a mapping for the specified key.
`boolean`	`containsValue(Object value)` Returns `true` if this map maps one or more keys to the specified value.
`Set<Entry<K, V>>`	`entrySet()` Returns a `Set` view of the mappings contained in this map.
`void`	`forEach(BiConsumer<? super K, ? super V> action)` Performs the given action for each entry in this map until all entries have been processed or the action throws an exception.
`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.
`V`	`getOrDefault(Object key, V defaultValue)` Returns the value to which the specified key is mapped, or `defaultValue` if this map contains no mapping for the key.
`boolean`	`isEmpty()` Returns `true` if this map contains no key-value mappings.
`Set<K>`	`keySet()` Returns a `Set` view of the keys contained in this map.
`V`	`merge(K key, V value, BiFunction<? super V, ? super V, ? extends V> 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). This method will, on a best-effort basis, throw a `ConcurrentModificationException` if it is detected that the remapping function modifies this map during computation.
`static <K, V> HashMap<K, V>`	`newHashMap(int numMappings)` Creates a new, empty HashMap suitable for the expected number of mappings.
`V`	`put(K key, V value)` Associates the specified value with the specified key in this map.
`void`	`putAll(Map<? extends K, ? extends V> m)` Copies all of the mappings from the specified map to this map.
`V`	`putIfAbsent(K key, V value)` If the specified key is not already associated with a value (or is mapped to `null`) associates it with the given value and returns `null`, else returns the current value (optional operation).
`boolean`	`remove(Object key, Object value)` Removes the entry for the specified key only if it is currently mapped to the specified value (optional operation).
`V`	`remove(Object key)` Removes the mapping for the specified key from this map if present.
`boolean`	`replace(K key, V oldValue, V newValue)` Replaces the entry for the specified key only if currently mapped to the specified value (optional operation).
`V`	`replace(K key, V value)` Replaces the entry for the specified key only if it is currently mapped to some value (optional operation).
`void`	`replaceAll(BiFunction<? super K, ? super V, ? extends V> function)` Replaces each entry's value with the result of invoking the given function on that entry until all entries have been processed or the function throws an exception (optional operation).
`int`	`size()` Returns the number of key-value mappings in this map.
`Collection<V>`	`values()` Returns a `Collection` view of the values contained in this map.

Inherited methods

From class


        
          java.util.AbstractMap

`void`	`clear()` Removes all of the mappings from this map (optional operation).
`Object`	`clone()` Returns a shallow copy of this `AbstractMap` instance: the keys and values themselves are not cloned.
`boolean`	`containsKey(Object key)` Returns `true` if this map contains a mapping for the specified key.
`boolean`	`containsValue(Object value)` Returns `true` if this map maps one or more keys to the specified value.
`abstract Set<Entry<K, V>>`	`entrySet()` Returns a `Set` view of the mappings contained in this map.
`boolean`	`equals(Object o)` Compares the specified object with this map for equality.
`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.
`int`	`hashCode()` Returns the hash code value for this map.
`boolean`	`isEmpty()` Returns `true` if this map contains no key-value mappings.
`Set<K>`	`keySet()` Returns a `Set` view of the keys contained in this map.
`V`	`put(K key, V value)` Associates the specified value with the specified key in this map (optional operation).
`void`	`putAll(Map<? extends K, ? extends V> m)` Copies all of the mappings from the specified map to this map (optional operation).
`V`	`remove(Object key)` Removes the mapping for a key from this map if it is present (optional operation).
`int`	`size()` Returns the number of key-value mappings in this map.
`String`	`toString()` Returns a string representation of this map.
`Collection<V>`	`values()` Returns a `Collection` view of the values contained in this map.

From class


        
          java.lang.Object

`Object`	`clone()` Creates and returns a copy of this object.
`boolean`	`equals(Object obj)` Indicates whether some other object is "equal to" this one.
`void`	`finalize()` Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.
`final Class<?>`	`getClass()` Returns the runtime class of this `Object`.
`int`	`hashCode()` Returns a hash code value for the object.
`final void`	`notify()` Wakes up a single thread that is waiting on this object's monitor.
`final void`	`notifyAll()` Wakes up all threads that are waiting on this object's monitor.
`String`	`toString()` Returns a string representation of the object.
`final void`	`wait(long timeoutMillis, int nanos)` Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed.
`final void`	`wait(long timeoutMillis)` Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed.
`final void`	`wait()` Causes the current thread to wait until it is awakened, typically by being notified or interrupted.

From interface


        
          java.util.Map

`abstract void`	`clear()` Removes all of the mappings from this map (optional operation).
`default V`	`compute(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction)` Attempts to compute a mapping for the specified key and its current mapped value, or `null` if there is no current mapping (optional operation).
`default V`	`computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction)` If the specified key is not already associated with a value (or is mapped to `null`), attempts to compute its value using the given mapping function and enters it into this map unless `null` (optional operation).
`default V`	`computeIfPresent(K key, BiFunction<? super K, ? super V, ? extends V> 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).
`abstract boolean`	`containsKey(Object key)` Returns `true` if this map contains a mapping for the specified key.
`abstract boolean`	`containsValue(Object value)` Returns `true` if this map maps one or more keys to the specified value.
`static <K, V> Map<K, V>`	`copyOf(Map<? extends K, ? extends V> map)` Returns an unmodifiable Map containing the entries of the given Map.
`static <K, V> Entry<K, V>`	`entry(K k, V v)` Returns an unmodifiable `Entry` containing the given key and value.
`abstract Set<Entry<K, V>>`	`entrySet()` Returns a `Set` view of the mappings contained in this map.
`abstract boolean`	`equals(Object o)` Compares the specified object with this map for equality.
`default void`	`forEach(BiConsumer<? super K, ? super V> action)` Performs the given action for each entry in this map until all entries have been processed or the action throws an exception.
`abstract 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.
`default V`	`getOrDefault(Object key, V defaultValue)` Returns the value to which the specified key is mapped, or `defaultValue` if this map contains no mapping for the key.
`abstract int`	`hashCode()` Returns the hash code value for this map.
`abstract boolean`	`isEmpty()` Returns `true` if this map contains no key-value mappings.
`abstract Set<K>`	`keySet()` Returns a `Set` view of the keys contained in this map.
`default V`	`merge(K key, V value, BiFunction<? super V, ? super V, ? extends V> 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> Map<K, V>`	`of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5)` Returns an unmodifiable map containing five mappings.
`static <K, V> Map<K, V>`	`of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4)` Returns an unmodifiable map containing four mappings.
`static <K, V> Map<K, V>`	`of(K k1, V v1)` Returns an unmodifiable map containing a single mapping.
`static <K, V> Map<K, V>`	`of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6, K k7, V v7, K k8, V v8)` Returns an unmodifiable map containing eight mappings.
`static <K, V> Map<K, V>`	`of(K k1, V v1, K k2, V v2)` Returns an unmodifiable map containing two mappings.
`static <K, V> Map<K, V>`	`of(K k1, V v1, K k2, V v2, K k3, V v3)` Returns an unmodifiable map containing three mappings.
`static <K, V> Map<K, V>`	`of()` Returns an unmodifiable map containing zero mappings.
`static <K, V> Map<K, V>`	`of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6, K k7, V v7, K k8, V v8, K k9, V v9)` Returns an unmodifiable map containing nine mappings.
`static <K, V> Map<K, V>`	`of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6)` Returns an unmodifiable map containing six mappings.
`static <K, V> Map<K, V>`	`of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6, K k7, V v7)` Returns an unmodifiable map containing seven mappings.
`static <K, V> Map<K, V>`	`of(K k1, V v1, K k2, V v2, K k3, V v3, K k4, V v4, K k5, V v5, K k6, V v6, K k7, V v7, K k8, V v8, K k9, V v9, K k10, V v10)` Returns an unmodifiable map containing ten mappings.
`static <K, V> Map<K, V>`	`ofEntries(Entry...<? extends K, ? extends V> entries)` Returns an unmodifiable map containing keys and values extracted from the given entries.
`abstract V`	`put(K key, V value)` Associates the specified value with the specified key in this map (optional operation).
`abstract void`	`putAll(Map<? extends K, ? extends V> m)` Copies all of the mappings from the specified map to this map (optional operation).
`default V`	`putIfAbsent(K key, V value)` If the specified key is not already associated with a value (or is mapped to `null`) associates it with the given value and returns `null`, else returns the current value (optional operation).
`default boolean`	`remove(Object key, Object value)` Removes the entry for the specified key only if it is currently mapped to the specified value (optional operation).
`abstract V`	`remove(Object key)` Removes the mapping for a key from this map if it is present (optional operation).
`default boolean`	`replace(K key, V oldValue, V newValue)` Replaces the entry for the specified key only if currently mapped to the specified value (optional operation).
`default V`	`replace(K key, V value)` Replaces the entry for the specified key only if it is currently mapped to some value (optional operation).
`default void`	`replaceAll(BiFunction<? super K, ? super V, ? extends V> function)` Replaces each entry's value with the result of invoking the given function on that entry until all entries have been processed or the function throws an exception (optional operation).
`abstract int`	`size()` Returns the number of key-value mappings in this map.
`abstract Collection<V>`	`values()` Returns a `Collection` view of the values contained in this map.

Public constructors

Added in API level 1

public HashMap ()

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

HashMap

Added in API level 1

public HashMap (int initialCapacity)

Constructs an empty HashMap with the specified initial capacity and the default load factor (0.75).

API Note:

To create a HashMap with an initial capacity that accommodates an expected number of mappings, use newHashMap.

Parameters
`initialCapacity`	`int`: the initial capacity.

Throws
`IllegalArgumentException`	if the initial capacity is negative.

HashMap

Added in API level 1

public HashMap (int initialCapacity, 
                float loadFactor)

Constructs an empty HashMap with the specified initial capacity and load factor.

API Note:

To create a HashMap with an initial capacity that accommodates an expected number of mappings, use newHashMap.

Parameters
`initialCapacity`	`int`: the initial capacity
`loadFactor`	`float`: the load factor

Throws
`IllegalArgumentException`	if the initial capacity is negative or the load factor is nonpositive

HashMap

Added in API level 1

public HashMap (Map<? extends K, ? extends V> m)

Constructs a new HashMap with the same mappings as the specified Map. The HashMap is created with default load factor (0.75) and an initial capacity sufficient to hold the mappings in the specified Map.

Parameters
`m`	`Map`: the map whose mappings are to be placed in this map

Throws
`NullPointerException`	if the specified map is null

Public methods

clear

Added in API level 1

public void clear ()

Removes all of the mappings from this map. The map will be empty after this call returns.

clone

Added in API level 1

public Object clone ()

Returns a shallow copy of this HashMap instance: the keys and values themselves are not cloned.

Returns
`Object`	a shallow copy of this map

compute

Added in API level 24

public V compute (K key, 
                BiFunction<? super K, ? super V, ? extends V> remappingFunction)

Attempts to compute a mapping for the specified key and its current mapped value, or null if there is no current mapping (optional operation). For example, to either create or append a String msg to a value mapping:

 map.compute(key, (k, v) -> (v == null) ? msg : v.concat(msg))

(Method merge() is often simpler to use for such purposes.)

If the remapping function returns null, 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 a ConcurrentModificationException if it is detected that the remapping function modifies this map during computation.

Parameters
`key`	`K`: key with which the specified value is to be associated
`remappingFunction`	`BiFunction`: the remapping function to compute a value

Returns
`V`	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

computeIfAbsent

Added in API level 24

public V computeIfAbsent (K key, 
                Function<? super K, ? extends V> mappingFunction)

If the specified key is not already associated with a value (or is mapped to null), attempts to compute its value using the given mapping function and enters it into this map unless null (optional operation).

If the mapping function returns null, 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 a ConcurrentModificationException if it is detected that the mapping function modifies this map during computation.

Parameters
`key`	`K`: key with which the specified value is to be associated
`mappingFunction`	`Function`: the mapping function to compute a value

Returns
`V`	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

Added in API level 24

public V computeIfPresent (K key, 
                BiFunction<? super K, ? super V, ? extends V> 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 returns null, 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 a ConcurrentModificationException if it is detected that the remapping function modifies this map during computation.

Parameters
`key`	`K`: key with which the specified value is to be associated
`remappingFunction`	`BiFunction`: the remapping function to compute a value

Returns
`V`	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

containsKey

Added in API level 1

public boolean containsKey (Object key)

Returns true if this map contains a mapping for the specified key.

Parameters
`key`	`Object`: The key whose presence in this map is to be tested

Returns
`boolean`	`true` if this map contains a mapping for the specified key.

containsValue

Added in API level 1

public boolean containsValue (Object value)

Returns true if this map maps one or more keys to the specified value.

Parameters
`value`	`Object`: value whose presence in this map is to be tested

Returns
`boolean`	`true` if this map maps one or more keys to the specified value

entrySet

Added in API level 1

public Set<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. If the map is modified while an iteration over the set is in progress (except through the iterator's own remove operation, or through the setValue 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 the Iterator.remove, Set.remove, removeAll, retainAll and clear operations. It does not support the add or addAll operations.

Returns
`Set<Entry<K, V>>`	a set view of the mappings contained in this map

forEach

Added in API level 24

public void forEach (BiConsumer<? super K, ? super V> action)

Performs the given action for each entry in this map until all entries have been processed or the action throws an exception. Unless otherwise specified by the implementing class, actions are performed in the order of entry set iteration (if an iteration order is specified.) Exceptions thrown by the action are relayed to the caller.

Parameters
`action`	`BiConsumer`: The action to be performed for each entry

get

Added in API level 1

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

A return value of null does not necessarily indicate that the map contains no mapping for the key; it's also possible that the map explicitly maps the key to null. The containsKey operation may be used to distinguish these two cases.

Parameters
`key`	`Object`: the key whose associated value is to be returned

Returns
`V`	the value to which the specified key is mapped, or `null` if this map contains no mapping for the key

See also:

put(Object,Object)

getOrDefault

Added in API level 24

public V getOrDefault (Object key, 
                V defaultValue)

Returns the value to which the specified key is mapped, or defaultValue if this map contains no mapping for the key.

Parameters
`key`	`Object`: the key whose associated value is to be returned
`defaultValue`	`V`: the default mapping of the key

Returns
`V`	the value to which the specified key is mapped, or `defaultValue` if this map contains no mapping for the key

isEmpty

Added in API level 1

public boolean isEmpty ()

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

Returns
`boolean`	`true` if this map contains no key-value mappings

keySet

Added in API level 1

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. If the map is modified while an iteration over the set is in progress (except through the iterator's own remove operation), the results of the iteration are undefined. 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.

Returns
`Set<K>`	a set view of the keys contained in this map

merge

Added in API level 24

public V merge (K key, 
                V value, 
                BiFunction<? super V, ? super V, ? extends V> 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 is null. This method may be of use when combining multiple mapped values for a key. For example, to either create or append a String msg to a value mapping:

 map.merge(key, msg, String::concat)

The remapping function should not modify this map during computation.

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

Parameters
`key`	`K`: key with which the resulting value is to be associated
`value`	`V`: 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`	`BiFunction`: the remapping function to recompute a value if present

Returns
`V`	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

newHashMap

Added in API level 35

public static 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.

Parameters
`numMappings`	`int`: the expected number of mappings

Returns
`HashMap<K, V>`	the newly created map

Throws
`IllegalArgumentException`	if numMappings is negative

put

Added in API level 1

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.

Parameters
`key`	`K`: key with which the specified value is to be associated
`value`	`V`: value to be associated with the specified key

Returns
`V`	the previous value associated with `key`, or `null` if there was no mapping for `key`. (A `null` return can also indicate that the map previously associated `null` with `key`.)

putAll

Added in API level 1

public void putAll (Map<? extends K, ? extends V> 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.

Parameters
`m`	`Map`: mappings to be stored in this map

Throws
`NullPointerException`	if the specified map is null

putIfAbsent

Added in API level 24

public V putIfAbsent (K key, 
                V value)

If the specified key is not already associated with a value (or is mapped to null) associates it with the given value and returns null, else returns the current value (optional operation).

Parameters
`key`	`K`: key with which the specified value is to be associated
`value`	`V`: value to be associated with the specified key

Returns
`V`	the previous value associated with the specified key, or `null` if there was no mapping for the key. (A `null` return can also indicate that the map previously associated `null` with the key, if the implementation supports null values.)

remove

Added in API level 24

public boolean remove (Object key, 
                Object value)

Removes the entry for the specified key only if it is currently mapped to the specified value (optional operation).

Parameters
`key`	`Object`: key with which the specified value is associated
`value`	`Object`: value expected to be associated with the specified key

Returns
`boolean`	`true` if the value was removed

remove

Added in API level 1

public V remove (Object key)

Removes the mapping for the specified key from this map if present.

Parameters
`key`	`Object`: key whose mapping is to be removed from the map

Returns
`V`	the previous value associated with `key`, or `null` if there was no mapping for `key`. (A `null` return can also indicate that the map previously associated `null` with `key`.)

replace

Added in API level 24

public boolean replace (K key, 
                V oldValue, 
                V newValue)

Replaces the entry for the specified key only if currently mapped to the specified value (optional operation).

Parameters
`key`	`K`: key with which the specified value is associated
`oldValue`	`V`: value expected to be associated with the specified key
`newValue`	`V`: value to be associated with the specified key

Returns
`boolean`	`true` if the value was replaced

replace

Added in API level 24

public V replace (K key, 
                V value)

Replaces the entry for the specified key only if it is currently mapped to some value (optional operation).

Parameters
`key`	`K`: key with which the specified value is associated
`value`	`V`: value to be associated with the specified key

Returns
`V`	the previous value associated with the specified key, or `null` if there was no mapping for the key. (A `null` return can also indicate that the map previously associated `null` with the key, if the implementation supports null values.)

replaceAll

Added in API level 24

public void replaceAll (BiFunction<? super K, ? super V, ? extends V> function)

Replaces each entry's value with the result of invoking the given function on that entry until all entries have been processed or the function throws an exception (optional operation). Exceptions thrown by the function are relayed to the caller.

Parameters
`function`	`BiFunction`: the function to apply to each entry

size

Added in API level 1

public int size ()

Returns the number of key-value mappings in this map.

Returns
`int`	the number of key-value mappings in this map

values

Added in API level 1

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. If the map is modified while an iteration over the collection is in progress (except through the iterator's own remove operation), the results of the iteration are undefined. The collection supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Collection.remove, removeAll, retainAll and clear operations. It does not support the add or addAll operations.

Returns
`Collection<V>`	a view of the values contained in this map

Content and code samples on this page are subject to the licenses described in the Content License. Java and OpenJDK are trademarks or registered trademarks of Oracle and/or its affiliates.

Last updated 2026-03-26 UTC.

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java.lang.Object
↳	java.util.AbstractMap<K, V>
	↳	java.util.HashMap<K, V>