Java Utility Library

Java.util.HashMap Class

Java HashMap Class

A Hashtable models and implements the Map interface. This implementation provides all of the optional map operations, and permits null values and the null key. A 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. It does not guarantee that the order will remain constant over time.

Class declaration

The declaration of java.util.HashMap class is:

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

Here, K and V are the type of key and value respectively maintained by the container.

Class Constructors

S.N	Constructors & Description
1.	HashMap() Creates an empty HashMap with the default initial capacity (16) and the default load factor (0.75).
2.	HashMap(int initialCapacity) Creates an empty HashMap with the specified initial capacity and the default load factor (0.75).
3.	HashMap(int initialCapacity, float loadFactor) Creates an empty HashMap with the specified initial capacity and load factor.
4.	HashMap(Map<? extends K,? extends V> m) Creates a new HashMap with the same mappings as the specified Map.

java.util.HashMap Methods

The java.util.HashMap class has a number of methods which are listed below:

Member Methods

S.N	Methods & Description
1.	void clear() Clears all mappings of the map.
2.	Object clone() Returns a shallow copy of this HashMap instance: the keys and values themselves are not cloned.
3.	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).
4.	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.
5.	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.
6.	boolean containsKey(Object key) Returns true if this map contains a mapping for the specified key.
7.	boolean containsValue(Object value) Returns true if this map maps one or more keys to the specified value.
8.	Set<Map.Entry<K,V>> entrySet() Returns a Set view of the mappings contained in this map.
9.	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.
10.	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.
11.	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.
12.	boolean isEmpty() Checks whether the map is empty or not.
13.	Set<K> keySet() Returns a Set view of the keys contained in this map.
14.	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.
15.	V put(K key, V value) Associates the specified value with the specified key in the map.
16.	void putAll(Map<? extends K,? extends V> m) Copies all of the mappings from the specified map to this map.
17.	V putIfAbsent(K key, V value) Returns null and associate the specified key with the given value if the specified key is not already associated with a value (or is mapped to null), else returns the current value.
18.	V remove(Object key) Removes the mapping for the specified key from this map if present.
19.	boolean remove(Object key, Object value) Removes the entry for the specified key only if it is currently mapped to the specified value.
20.	V replace(K key, V value) Replaces the entry for the specified key only if it is currently mapped to some value.
21.	boolean replace(K key, V oldValue, V newValue) Replaces the entry for the specified key only if currently mapped to the specified value.
22.	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.
23.	int size() Returns the number of key-value pairs in the map.
24.	Collection<V> values() Returns a Collection view of the values contained in this map.

Methods inherited

This class inherits the methods of following class:

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