Package java.util

## Interface Map<K,​V>

- Type Parameters:
    `K` - the type of keys maintained by this map
    `V` - the type of mapped values
  All Known Implementing Classes:
    `AbstractMap`, `HashMap`, `Hashtable`, `WeakHashMap`, `WeakHashtable`

      public interface Map<K,​V>

  An object that maps keys to values. A map cannot contain duplicate keys; each key can map to at most one value.

  This interface takes the place of the `Dictionary` class, which was a totally abstract class rather than an interface.

  The `Map` interface provides three collection views, which allow a map's contents to be viewed as a set of keys, collection of values, or set of key-value mappings. The order of a map is defined as the order in which the iterators on the map's collection views return their elements. Some map implementations, like the `TreeMap` class, make specific guarantees as to their order; others, like the `HashMap` class, do not.

  Note: great care must be exercised if mutable objects are used as map keys. The behavior of a map is not specified if the value of an object is changed in a manner that affects `equals` comparisons while the object is a key in the map. A special case of this prohibition is that it is not permissible for a map to contain itself as a key. While it is permissible for a map to contain itself as a value, extreme caution is advised: the `equals` and `hashCode` methods are no longer well defined on such a map.

  All general-purpose map implementation classes should provide two "standard" constructors: a void (no arguments) constructor which creates an empty map, and a constructor with a single argument of type `Map`, which creates a new map with the same key-value mappings as its argument. In effect, the latter constructor allows the user to copy any map, producing an equivalent map of the desired class. There is no way to enforce this recommendation (as interfaces cannot contain constructors).

  The "destructive" methods contained in this interface, that is, the methods that modify the map on which they operate, are specified to throw `UnsupportedOperationException` if this map does not support the operation. If this is the case, these methods may, but are not required to, throw an `UnsupportedOperationException` if the invocation would have no effect on the map. For example, invoking the `putAll(Map)` method on an unmodifiable map may, but is not required to, throw the exception if the map whose mappings are to be "superimposed" is empty.

  Some map implementations have restrictions on the keys and values they may contain. For example, some implementations prohibit null keys and values, and some have restrictions on the types of their keys. Attempting to insert an ineligible key or value throws an unchecked exception, typically `NullPointerException` or `ClassCastException`. Attempting to query the presence of an ineligible key or value may throw an exception, or it may simply return false; some implementations will exhibit the former behavior and some will exhibit the latter. More generally, attempting an operation on an ineligible key or value whose completion would not result in the insertion of an ineligible element into the map may throw an exception or it may succeed, at the option of the implementation. Such exceptions are marked as "optional" in the specification for this interface.

  This interface is a member of the Java Collections Framework

  Many methods in Collections Framework interfaces are defined in terms of the `equals` method. For example, the specification for the `containsKey(Object key)` method says: "returns `true` if and only if this map contains a mapping for a key `k` such that `(key==null ? k==null : key.equals(k))`." This specification should not be construed to imply that invoking `Map.containsKey` with a non-null argument `key` will cause `key.equals(k)` to be invoked for any key `k`. Implementations are free to implement optimizations whereby the `equals` invocation is avoided, for example, by first comparing the hash codes of the two keys. (The `Object.hashCode()` specification guarantees that two objects with unequal hash codes cannot be equal.) More generally, implementations of the various Collections Framework interfaces are free to take advantage of the specified behavior of underlying `Object` methods wherever the implementor deems it appropriate.

  See Also:
    `HashMap`, `Hashtable`, `Collection`, `Set`

- - ### Nested Class Summary

    Nested Classes
    Modifier and Type | Interface | Description
    --- | --- | ---
    `static interface` | `Map.Entry<K,​V>` | A map entry (key-value pair).

  - ### Method Summary

    All Methods Instance Methods Abstract Methods
    Modifier and Type | Method | Description
    --- | --- | ---
    `void` | `clear()` | Removes all of the mappings from this map (optional operation).
    `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<Map.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.
    `Collection<V>` | `values()` | Returns a `Collection` view of the values contained in this map.

- - ### Method Detail

    - #### clear

          void clear()

      Removes all of the mappings from this map (optional operation). The map will be empty after this call returns.
      Throws:
        `UnsupportedOperationException` - if the `clear` operation is not supported by this map

    - #### containsKey

          boolean containsKey​(Object key)

      Returns `true` if this map contains a mapping for the specified key. More formally, returns `true` if and only if this map contains a mapping for a key `k` such that `(key==null ? k==null : key.equals(k))`. (There can be at most one such mapping.)
      Parameters:
        `key` - key whose presence in this map is to be tested
      Returns:
        `true` if this map contains a mapping for the specified key
      Throws:
        `ClassCastException` - if the key is of an inappropriate type for this map (optional)
        `NullPointerException` - if the specified key is null and this map does not permit null keys (optional)

    - #### containsValue

          boolean containsValue​(Object value)

      Returns `true` if this map maps one or more keys to the specified value. More formally, returns `true` if and only if this map contains at least one mapping to a value `v` such that `(value==null ? v==null : value.equals(v))`. This operation will probably require time linear in the map size for most implementations of the `Map` interface.
      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:
        `ClassCastException` - if the value is of an inappropriate type for this map (optional)
        `NullPointerException` - if the specified value is null and this map does not permit null values (optional)

    - #### entrySet

          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. 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:
        a set view of the mappings contained in this map

    - #### equals

          boolean equals​(@Nullable
                         Object o)

      Compares the specified object with this map for equality. Returns `true` if the given object is also a map and the two maps represent the same mappings. More formally, two maps `m1` and `m2` represent the same mappings if `m1.entrySet().equals(m2.entrySet())`. This ensures that the `equals` method works properly across different implementations of the `Map` interface.
      Overrides:
        `equals` in class `Object`
      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()`, `HashMap`

    - #### get

          @Nullable
          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.)

      If this map permits null values, then 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` - the key whose associated value is to be returned
      Returns:
        the value to which the specified key is mapped, or `null` if this map contains no mapping for the key
      Throws:
        `ClassCastException` - if the key is of an inappropriate type for this map (optional)
        `NullPointerException` - if the specified key is null and this map does not permit null keys (optional)

    - #### hashCode

          int hashCode()

      Returns the hash code value for this map. The hash code of a map is defined to be the sum of the hash codes of each entry in the map's `entrySet()` view. This ensures that `m1.equals(m2)` implies that `m1.hashCode()==m2.hashCode()` for any two maps `m1` and `m2`, as required by the general contract of `Object.hashCode()`.
      Overrides:
        `hashCode` in class `Object`
      Returns:
        the hash code value for this map
      See Also:
        `Map.Entry.hashCode()`, `Object.equals(Object)`, `equals(Object)`

    - #### isEmpty

          boolean isEmpty()

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

    - #### keySet

          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:
        a set view of the keys contained in this map

    - #### put

          @Nullable
          V put​(K key,
                V value)

      Associates the specified value with the specified key in this map (optional operation). If the map previously contained a mapping for the key, the old value is replaced by the specified value. (A map `m` is said to contain a mapping for a key `k` if and only if `m.containsKey(k)` would return `true`.)
      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`. (A `null` return can also indicate that the map previously associated `null` with `key`, if the implementation supports `null` values.)
      Throws:
        `UnsupportedOperationException` - if the `put` operation is not supported by this map
        `ClassCastException` - if the class of the specified key or value prevents it from being stored in this map
        `NullPointerException` - if the specified key or value is null and this map does not permit null keys or values
        `IllegalArgumentException` - if some property of the specified key or value prevents it from being stored in this map

    - #### putAll

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

      Copies all of the mappings from the specified map to this map (optional operation). The effect of this call is equivalent to that of calling `put(k, v)` on this map once for each mapping from key `k` to value `v` in the specified map. The behavior of this operation is undefined if the specified map is modified while the operation is in progress.
      Parameters:
        `m` - mappings to be stored in this map
      Throws:
        `UnsupportedOperationException` - if the `putAll` operation is not supported by this map
        `ClassCastException` - if the class of a key or value in the specified map prevents it from being stored in this map
        `NullPointerException` - if the specified map is null, or if this map does not permit null keys or values, and the specified map contains null keys or values
        `IllegalArgumentException` - if some property of a key or value in the specified map prevents it from being stored in this map

    - #### remove

          @Nullable
          V remove​(Object key)

      Removes the mapping for a key from this map if it is present (optional operation). More formally, if this map contains a mapping from key `k` to value `v` such that `(key==null ? k==null : key.equals(k))`, that mapping is removed. (The map can contain at most one such mapping.)

      Returns the value to which this map previously associated the key, or `null` if the map contained no mapping for the key.

      If this map permits null values, then a return value of `null` does not necessarily indicate that the map contained no mapping for the key; it's also possible that the map explicitly mapped the key to `null`.

      The map will not contain a mapping for the specified key once the call returns.

      Parameters:
        `key` - key whose mapping is to be removed from the map
      Returns:
        the previous value associated with `key`, or `null` if there was no mapping for `key`.
      Throws:
        `UnsupportedOperationException` - if the `remove` operation is not supported by this map
        `ClassCastException` - if the key is of an inappropriate type for this map (optional)
        `NullPointerException` - if the specified key is null and this map does not permit null keys (optional)

    - #### size

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

    - #### values

          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:
        a collection view of the values contained in this map