Java Hashmap

Java Hashmap

Java Hashmap

  • A map contains values on the basis of key i.e. key and value pair.
  • Each key and value pair is known as an entry. Map contains only unique keys.
  • Map is useful if you have to search, update or delete elements on the basis of key.

Syntax of HashMap with Generic

Map<Integer,String> map=new HashMap<Integer,String>();


package Mypkg;
import java.util.HashMap;
import java.util.Hashtable;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;
public class MAPex {
             * @param args
            public static void main(String[] args) {
                        // TODO Auto-generated method stub
                        HashMap map=new HashMap(); 
                //Adding elements to map 
                System.out.println("Map Representation :");
                System.out.println( "Value against Key- 28->" + map.get(28));
                //Traversing Map 
                Set set=map.entrySet();//Converting to Set so that we can traverse 
                Iterator itr=set.iterator(); 
                System.out.println("Map elements are");
                    //Converting to Map.Entry so that we can get key and value separately 
                    Map.Entry entry=(Map.Entry); 
                    System.out.println(entry.getKey()+" -> "+entry.getValue());                



Map Representation :

{1=Neo, 3=Rahul, 28=Jai, 31=Amit}


Value against Key- 28->Jai

Map elements are

1 -> Neo

3 -> Rahul

28 -> Jai

31 -> Amit

Methods of HashMap

Method Name Description
void clear()It is used to remove all elements from map.
boolean isEmpty()It is used to return true if this map contains no key-value mappings.
Object clone()It is used to return a shallow copy of this HashMap instance: the keys and values themselves are not cloned.
Set entrySet()It is used to return a collection view of the mappings contained in this map.
Set keySet()It is used to return a set view of the keys contained in this map.
V put(Object key, Object value)It is used to insert an entry in the map.
void putAll(Map map)It is used to insert the specified map in the map.
V putIfAbsent(K key, V value)It inserts the specified value with the specified key in the map only if it is not already specified.
V remove(Object key)It is used to delete an entry for the specified key.
boolean remove(Object key, Object value)It removes the specified values with the associated specified keys from the map.
V compute(K key, BiFunction<? super K,? super V,? extends V> remappingFunction)It is used 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)It is used to compute its value using the given mapping function, if the specified key is not already associated with a value (or is mapped to null), and enters it into this map unless null.
V computeIfPresent(K key, BiFunction<? super K,? super V,? extends V> remappingFunction)It is used to compute a new mapping given the key and its current mapped value if the value for the specified key is present and non-null.
boolean containsValue(Object value)This method returns true if some value equal to the value exists within the map, else return false.
boolean containsKey(Object key)This method returns true if some key equal to the key exists within the map, else return false.
boolean equals(Object o)It is used to compare the specified Object with the Map.
void forEach(BiConsumer<? super K,? super V> action)It performs the given action for each entry in the map until all entries have been processed or the action throws an exception.
V get(Object key)This method returns the object that contains the value associated with the key.
V getOrDefault(Object key, V defaultValue)It returns the value to which the specified key is mapped, or defaultValue if the map contains no mapping for the key.
boolean isEmpty()This method returns true if the map is empty; returns false if it contains at least one key.
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 replace(K key, V value)It replaces the specified value for a specified key.
boolean replace(K key, V oldValue, V newValue)It replaces the old value with the new value for a specified key.
void replaceAll(BiFunction<? super K,? super V,? extends V> function)It 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.
Collection values()It returns a collection view of the values contained in the map.
int size()This method returns the number of entries in the map.


Hash Table Vs HashMap

 non synchronized. It is not-thread safe and can’t be shared between many threads without proper synchronization code.synchronized. It is thread-safe and can be shared with many threads.
allows one null key and multiple null values.doesn’t allow any null key or value.
faster. Slower.
 HashMap is traversed by Iterator.Hashtable is traversed by Enumerator and Iterator.
 Iterator in HashMap is fail-fast.Enumerator in Hashtable is not fail-fast.
 HashMap inherits AbstractMap class.Hashtable inherits Dictionary class.



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