HashMap in JAVA

Consider a scenario where we need to store data in the form of key value pairs. Which data structure will we use?

Well, The obvious answer which comes in our mind is Map. HashMap is Hash table based implementation of the Map interface which stores data in the form of key value pairs. This class provides Amortized Time complexities close to O(1) given a good hashFunction insert, delete and search operation because it employs hashing in it’s core.

class is defined in java.util package and class declaration is provided below –

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

Constructors And Description

Constructor and 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

Modifier and Type Method and 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.
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 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.

Example –

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