# Interface Map<K,V>

All Known Subinterfaces:
  `NavigableMap<K,V>, SortedMap<K,V>`

All Known Implementing Classes:
  `AbstractMap, HashMap, Hashtable, IdentityHashMap, LinkedHashMap, Properties, TreeMap, WeakHashMap`

public interface Map<K,V>

A `Map` is a data structure consisting of a set of keys and values in which each key is mapped to a single value. The class of the objects used as keys is declared when the `Map` is declared, as is the class of the corresponding values.

A `Map` provides helper methods to iterate through all of the keys contained in it, as well as various methods to access and update the key/value pairs.

- ## Nested Class Summary

  Nested Classes
  Modifier and Type
  Interface
  Description
  `static interface`
  `Map.Entry<K,V>`
  `Map.Entry` is a key/value mapping contained in a `Map`.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `void`
  `clear()`
  Removes all elements from this `Map`, leaving it empty.
  `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).
  `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`.
  `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.
  `boolean`
  `containsKey(Object key)`
  Returns whether this `Map` contains the specified key.
  `boolean`
  `containsValue(Object value)`
  Returns whether this `Map` contains the specified value.
  `Set<Map.Entry<K,V>>`
  `entrySet()`
  Returns a `Set` containing all of the mappings in this `Map`.
  `boolean`
  `equals(Object object)`
  Compares the argument to the receiver, and returns `true` if the specified object is a `Map` and both `Map`s contain the same mappings.
  `default void`
  `forEach(BiConsumer<? super K, ? super V> action)`
  Performs the given action for each entry in this map.
  `V`
  `get(Object key)`
  Returns the value of the mapping with the specified 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.
  `int`
  `hashCode()`
  Returns an integer hash code for the receiver.
  `boolean`
  `isEmpty()`
  Returns whether this map is empty.
  `Set<K>`
  `keySet()`
  Returns a set 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.
  `V`
  `put(K key, V value)`
  Maps the specified key to the specified value.
  `void`
  `putAll(Map<? extends K, ? extends V> map)`
  Copies every mapping in the specified `Map` to this `Map`.
  `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.
  `V`
  `remove(Object key)`
  Removes a mapping with the specified key from this `Map`.
  `default boolean`
  `remove(Object key, Object value)`
  Removes the entry for the specified key only if it is currently mapped to the specified value.
  `default V`
  `replace(K key, V value)`
  Replaces the entry for the specified key only if it is currently mapped to some value.
  `default boolean`
  `replace(K key, V oldValue, V newValue)`
  Replaces the entry for the specified key only if currently mapped to the specified value.
  `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.
  `int`
  `size()`
  Returns the number of mappings in this `Map`.
  `Collection<V>`
  `values()`
  Returns a `Collection` of the values contained in this `Map`.

- ## Method Details

  - ### clear

    void clear()

    Removes all elements from this `Map`, leaving it empty.

    ###### Throws

    - `UnsupportedOperationException`: if removing elements from this `Map` is not supported.

    ###### See also

    - #isEmpty()

    - #size()

  - ### containsKey

    boolean containsKey(Object key)

    Returns whether this `Map` contains the specified key.

    ###### Parameters

    - `key`: the key to search for.

    ###### Returns

    Returns:
      `true` if this map contains the specified key, `false` otherwise.

  - ### containsValue

    boolean containsValue(Object value)

    Returns whether this `Map` contains the specified value.

    ###### Parameters

    - `value`: the value to search for.

    ###### Returns

    Returns:
      `true` if this map contains the specified value, `false` otherwise.

  - ### entrySet

    Set<Map.Entry<K,V>> entrySet()

    Returns a `Set` containing all of the mappings in this `Map`. Each mapping is an instance of `Map.Entry`. As the `Set` is backed by this `Map`, changes in one will be reflected in the other.

    ###### Returns

    a set of the mappings

  - ### equals

    boolean equals(Object object)

    Compares the argument to the receiver, and returns `true` if the specified object is a `Map` and both `Map`s contain the same mappings.

    ###### Parameters

    - `object`: the `Object` to compare with this `Object`.

    ###### Returns

    Overrides:
      `equals` in class `Object`
    Returns:

      boolean `true` if the `Object` is the same as this `Object` `false` if it is different from this `Object`.

      ###### See also

      - #hashCode()

      - #entrySet()

  - ### get

    V get(Object key)

    Returns the value of the mapping with the specified key.

    ###### Parameters

    - `key`: the key.

    ###### Returns

    Returns:
      the value of the mapping with the specified key, or `null` if no mapping for the specified key is found.

  - ### hashCode

    int hashCode()

    Returns an integer hash code for the receiver. `Object`s which are equal return the same value for this method.

    ###### Returns

    the receiver's hash.

    ###### See also

    - #equals(Object)
    Overrides:
      `hashCode` in class `Object`

  - ### isEmpty

    boolean isEmpty()

    Returns whether this map is empty.

    ###### Returns

    Returns:

      `true` if this map has no elements, `false` otherwise.

      ###### See also

      - #size()

  - ### keySet

    Set<K> keySet()

    Returns a set of the keys contained in this `Map`. The `Set` is backed by this `Map` so changes to one are reflected by the other. The `Set` does not support adding.

    ###### Returns

    a set of the keys.

  - ### put

    V put(K key, V value)

    Maps the specified key to the specified value.

    ###### Parameters

    - `key`: the key.

    - `value`: the value.

    ###### Returns

    Returns:

      the value of any previous mapping with the specified key or `null` if there was no mapping.

      ###### Throws

      - `UnsupportedOperationException`: if adding to this `Map` is not supported.

      - `ClassCastException`: @throws ClassCastException if the class of the key or value is inappropriate for this `Map`.

      - `IllegalArgumentException`: if the key or value cannot be added to this `Map`.

      - `NullPointerException`: @throws NullPointerException if the key or value is `null` and this `Map` does not support `null` keys or values.

  - ### putAll

    void putAll(Map<? extends K, ? extends V> map)

    Copies every mapping in the specified `Map` to this `Map`.

    ###### Parameters

    - `map`: the `Map` to copy mappings from.

    ###### Throws

    - `UnsupportedOperationException`: if adding to this `Map` is not supported.

    - `ClassCastException`: @throws ClassCastException if the class of a key or a value of the specified `Map` is inappropriate for this `Map`.

    - `IllegalArgumentException`: if a key or value cannot be added to this `Map`.

    - `NullPointerException`: @throws NullPointerException if a key or value is `null` and this `Map` does not support `null` keys or values.

  - ### remove

    V remove(Object key)

    Removes a mapping with the specified key from this `Map`.

    ###### Parameters

    - `key`: the key of the mapping to remove.

    ###### Returns

    Returns:

      the value of the removed mapping or `null` if no mapping for the specified key was found.

      ###### Throws

      - `UnsupportedOperationException`: if removing from this `Map` is not supported.

  - ### size

    int size()

    Returns the number of mappings in this `Map`.

    ###### Returns

    the number of mappings in this `Map`.

  - ### values

    Collection<V> values()

    Returns a `Collection` of the values contained in this `Map`. The `Collection` is backed by this `Map` so changes to one are reflected by the other. The `Collection` supports `Collection#remove`, `Collection#removeAll`, `Collection#retainAll`, and `Collection#clear` operations, and it does not support `Collection#add` or `Collection#addAll` operations.

    This method returns a `Collection` which is the subclass of `AbstractCollection`. The `AbstractCollection#iterator` method of this subclass returns a "wrapper object" over the iterator of this `Map`'s `#entrySet()`. The `AbstractCollection#size` method wraps this `Map`'s `#size` method and the `AbstractCollection#contains` method wraps this `Map`'s `#containsValue` method.

    The collection is created when this method is called at first time and returned in response to all subsequent calls. This method may return different Collection when multiple calls to this method, since it has no synchronization performed.

    ###### Returns

    a collection of the values contained in this map.

  - ### getOrDefault

    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.

  - ### putIfAbsent

    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.

  - ### remove

    default boolean remove(Object key, Object value)
    Removes the entry for the specified key only if it is currently mapped to the specified value.

  - ### replace

    default boolean replace(K key, V oldValue, V newValue)
    Replaces the entry for the specified key only if currently mapped to the specified value.

  - ### replace

    default V replace(K key, V value)
    Replaces the entry for the specified key only if it is currently mapped to some value.

  - ### forEach

    default void forEach(BiConsumer<? super K, ? super V> action)
    Performs the given action for each entry in this map.

  - ### replaceAll

    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.

  - ### computeIfAbsent

    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`.

  - ### computeIfPresent

    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.

  - ### compute

    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).

  - ### merge

    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. Otherwise, replaces the associated value with the result of the given remapping function, or removes if the result is `null`.