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Summary: Ctors | Methods | Inherited Methods

# HashMap

`public class HashMap`
`` `extends AbstractMap<K, V>` `implements Map<K, V>, Cloneable, Serializable`

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

Known direct subclasses LinkedHashMap<K, V>
LinkedHashMap<K, V> | Hash table and linked list implementation of the `Map` interface, with predictable iteration 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:

- `Object.hashCode()`
- `Collection`
- `Map`
- `TreeMap`
- `Hashtable`

## Summary

### Public constructors
`HashMap(int initialCapacity, float loadFactor)` Constructs an empty `HashMap` with the specified initial capacity and load factor.
`HashMap(int initialCapacity)` Constructs an empty `HashMap` with the specified initial capacity and the default load factor (0.75).
`HashMap()` Constructs an empty `HashMap` with the default initial capacity (16) and the default load factor (0.75).
`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).
`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`.
`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 `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.
`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.
`boolean` | `remove(Object key, Object value)` Removes the entry for the specified key only if it is currently mapped to the specified value.
`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.
`V` | `replace(K key, V value)` Replaces the entry for the specified key only if it is currently mapped to some value.
`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.
`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 timeout, int nanos)` Causes the current thread to wait until another thread invokes the `notify()` method or the `notifyAll()` method for this object, or some other thread interrupts the current thread, or a certain amount of real time has elapsed.
`final void` | `wait(long timeout)` Causes the current thread to wait until either another thread invokes the `notify()` method or the `notifyAll()` method for this object, or a specified amount of time has elapsed.
`final void` | `wait()` Causes the current thread to wait until another thread invokes the `notify()` method or the `notifyAll()` method for this object.
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).
`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.
`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 immutable `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.
`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 immutable 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 immutable map containing four mappings.
`static <K, V> Map<K, V>` | `of(K k1, V v1)` Returns an immutable 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 immutable map containing eight mappings.
`static <K, V> Map<K, V>` | `of(K k1, V v1, K k2, V v2)` Returns an immutable 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 immutable map containing three mappings.
`static <K, V> Map<K, V>` | `of()` Returns an immutable 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 immutable 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 immutable 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 immutable 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 immutable map containing ten mappings.
`static <K, V> Map<K, V>` | `ofEntries(Entry...<? extends K, ? extends V> entries)` Returns an immutable 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.
`default boolean` | `remove(Object key, Object value)` Removes the entry for the specified key only if it is currently mapped to the specified value.
`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.
`default V` | `replace(K key, V value)` Replaces the entry for the specified key only if it is currently mapped to some 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 until all entries have been processed or the function throws an exception.
`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

### HashMap

    public HashMap (int initialCapacity,
                    float loadFactor)

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

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

    public HashMap (int initialCapacity)

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

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

Throws
`IllegalArgumentException` | if the initial capacity is negative.

### HashMap

    public HashMap ()

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

### HashMap

    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

    public void clear ()

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

### clone

    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

    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). 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.

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

### computeIfAbsent

    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`.

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.

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

### computeIfPresent

    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.

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.

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

### containsKey

    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

    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

    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

    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

    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

    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

    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

    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

    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. 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)

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.

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

### put

    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

    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

    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.

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

    public boolean remove (Object key,
                    Object value)

Removes the entry for the specified key only if it is currently mapped to the specified value.

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

    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

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

Replaces the entry for the specified key only if currently mapped to the specified value.

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

    public V replace (K key,
                    V value)

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

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

    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. Exceptions thrown by the function are relayed to the caller.

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

### size

    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

    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

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