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Showing content from https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/HashMap.html below:

HashMap (Java SE 11 & JDK 11 )

- Nested Class Summary
  - Nested classes/interfaces declared in class java.util.AbstractMapAbstractMap.SimpleEntry<K,V>, AbstractMap.SimpleImmutableEntry<K,V>
  - Nested classes/interfaces declared in interface java.util.MapMap.Entry<K,V>
- Constructor Summary Constructors Constructor Description 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.
- Method Summary All Methods Instance Methods Concrete Methods Modifier and Type Method Description 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<Map.Entry<K,V>> entrySet()
  Returns a
  Set
  view of the mappings contained in this map.
  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.
  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 remove(Object key)
  Removes the mapping for the specified key from this map if present.
  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.
  - Methods declared in class java.util.AbstractMapequals, hashCode, toString
  - Methods declared in class java.lang.Objectfinalize, getClass, notify, notifyAll, wait, wait, wait
  - Methods declared in interface java.util.Mapequals, forEach, getOrDefault, hashCode, putIfAbsent, remove, replace, replace, replaceAll

- Constructor Detail
  - HashMap
```
public HashMap(int initialCapacity,
               float loadFactor)
```
    Constructs an empty HashMap with the specified initial capacity and load factor.
    
    Parameters:
    
    initialCapacity - the initial capacity
    
    loadFactor - 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 - 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 - the map whose mappings are to be placed in this map
    
    Throws:
    
    NullPointerException - if the specified map is null
- Method Detail
  - size
```
public int size()
```
    Returns the number of key-value mappings in this map.
    
    Specified by:
    
    size in interface Map<K,V>
    
    Overrides:
    
    size in class AbstractMap<K,V>
    
    Returns:
    
    the number of key-value mappings in this map
  - isEmpty
```
public boolean isEmpty()
```
    Returns true if this map contains no key-value mappings.
    
    Specified by:
    
    isEmpty in interface Map<K,V>
    
    Overrides:
    
    isEmpty in class AbstractMap<K,V>
    
    Returns:
    
    true if this map contains no key-value mappings
  - 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.
    
    Specified by:
    
    get in interface Map<K,V>
    
    Overrides:
    
    get in class AbstractMap<K,V>
    
    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
    
    See Also:
    
    put(Object, Object)
  - containsKey
```
public boolean containsKey(Object key)
```
    Returns true if this map contains a mapping for the specified key.
    
    Specified by:
    
    containsKey in interface Map<K,V>
    
    Overrides:
    
    containsKey in class AbstractMap<K,V>
    
    Parameters:
    
    key - The key whose presence in this map is to be tested
    
    Returns:
    
    true if this map contains a mapping for the specified 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.
    
    Specified by:
    
    put in interface Map<K,V>
    
    Overrides:
    
    put in class AbstractMap<K,V>
    
    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.)
  - 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.
    
    Specified by:
    
    putAll in interface Map<K,V>
    
    Overrides:
    
    putAll in class AbstractMap<K,V>
    
    Parameters:
    
    m - mappings to be stored in this map
    
    Throws:
    
    NullPointerException - if the specified map is null
  - remove
```
public V remove(Object key)
```
    Removes the mapping for the specified key from this map if present.
    
    Specified by:
    
    remove in interface Map<K,V>
    
    Overrides:
    
    remove in class AbstractMap<K,V>
    
    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. (A null return can also indicate that the map previously associated null with key.)
  - clear
```
public void clear()
```
    Removes all of the mappings from this map. The map will be empty after this call returns.
    
    Specified by:
    
    clear in interface Map<K,V>
    
    Overrides:
    
    clear in class AbstractMap<K,V>
  - containsValue
```
public boolean containsValue(Object value)
```
    Returns true if this map maps one or more keys to the specified value.
    
    Specified by:
    
    containsValue in interface Map<K,V>
    
    Overrides:
    
    containsValue in class AbstractMap<K,V>
    
    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
  - 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.
    
    Specified by:
    
    keySet in interface Map<K,V>
    
    Overrides:
    
    keySet in class AbstractMap<K,V>
    
    Returns:
    
    a set view of the keys contained 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.
    
    Specified by:
    
    values in interface Map<K,V>
    
    Overrides:
    
    values in class AbstractMap<K,V>
    
    Returns:
    
    a view of the values contained in this map
  - entrySet
```
public 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.
    
    Specified by:
    
    entrySet in interface Map<K,V>
    
    Returns:
    
    a set view of the mappings contained in this map
  - 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.
    
    This method will, on a best-effort basis, throw a ConcurrentModificationException if it is detected that the mapping function modifies this map during computation.
    
    Specified by:
    
    computeIfAbsent in interface Map<K,V>
    
    Parameters:
    
    key - key with which the specified value is to be associated
    
    mappingFunction - the mapping function to compute a value
    
    Returns:
    
    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
```
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.
    
    This method will, on a best-effort basis, throw a ConcurrentModificationException if it is detected that the remapping function modifies this map during computation.
    
    Specified by:
    
    computeIfPresent in interface Map<K,V>
    
    Parameters:
    
    key - key with which the specified value is to be associated
    
    remappingFunction - the remapping function to compute a value
    
    Returns:
    
    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
  - 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.
    
    This method will, on a best-effort basis, throw a ConcurrentModificationException if it is detected that the remapping function modifies this map during computation.
    
    Specified by:
    
    compute in interface Map<K,V>
    
    Parameters:
    
    key - key with which the specified value is to be associated
    
    remappingFunction - the remapping function to compute a value
    
    Returns:
    
    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
  - 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.
    
    This method will, on a best-effort basis, throw a ConcurrentModificationException if it is detected that the remapping function modifies this map during computation.
    
    Specified by:
    
    merge in interface Map<K,V>
    
    Parameters:
    
    key - key with which the resulting value is to be associated
    
    value - 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 - the remapping function to recompute a value if present
    
    Returns:
    
    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
  - clone
```
public Object clone()
```
    Returns a shallow copy of this HashMap instance: the keys and values themselves are not cloned.
    
    Overrides:
    
    clone in class AbstractMap<K,V>
    
    Returns:
    
    a shallow copy of this map
    
    See Also:
    
    Cloneable

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