Java.util.HashMap Class


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Introduction

The java.util.HashMap class is the Hash table based implementation of the Map interface.Following are the important points about HashMap:

  • This class makes no guarantees as to the iteration order of the map; in particular, it does not guarantee that the order will remain constant over time.

  • This class permits null values and the null key.

Class declaration

Following is the declaration for java.util.HashMap class:

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

Parameters

Following is the parameter for java.util.HashMap class:

  • K -- This is the type of keys maintained by this map.

  • V -- This is the type of mapped values.

Class constructors

S.N. Constructor & Description
1

HashMap()

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

2

HashMap(Collection<? extends E> c)

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

3

HashMap(int initialCapacity, float loadFactor)

This constructs an empty HashMap with the specified initial capacity and load factor.

4

HashMap(Map<? extends K,? extends V> m)

This constructs a new HashMap with the same mappings as the specified Map.

Class methods

S.N. Method & Description
1

void clear()

This method removes all of the mappings from this map.

2

Object clone()

This method returns a shallow copy of this HashMap instance, the keys and values themselves are not cloned.

3

boolean containsKey(Object key)

This method returns true if this map contains a mapping for the specified key.

4

boolean containsValue(Object value)

This method returns true if this map maps one or more keys to the specified value.

5

Set<Map.Entry<K,V>> entrySet()

This method returns a Set view of the mappings contained in this map.

6

V get(Object key)

This method returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.

7

boolean isEmpty()

This method returns true if this map contains no key-value mapping.

8

Set<K> keySet()

This method returns a Set view of the keys contained in this map.

9

V put(K key, V value)

This method associates the specified value with the specified key in this map.

10

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

This method copies all of the mappings from the specified map to this map.

11

V remove(Object key)

This method removes the mapping for the specified key from this map if present.

12

int size()

This method returns the number of key-value mappings in this map.

13

Collection<V> values()

This method returns a Collection view of the values contained in this map.

Methods inherited

This class inherits methods from the following classes:

  • java.util.AbstractMap
  • java.util.Object
  • java.util.Map


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