Java HashMap - Java Tutorials

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Hash Map

The Map interface maps unique keys to value means it associate value to unique keys which you use to retrieve value at a later date. Some of the key points are :

Using a key and a value, you can store the value in Map object. You can retrieve it later by using it's key.
When no element exists in the invoking Map, many methods throw a 'NoSuchElementException'.
A ClassCastException is thrown when an object is incompatible with the elements in a map.
A NullPointerException is thrown if an attempt is made to use a null object and null is not allowed in the map.
An UnsupportedOperationException is thrown when an attempt is made to change an unmodifiable map.

Declaration

HashMap<String, Object> map = new HashMap<String, Object>();



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

The difference between the above two declaration is the interface they implement.
The advantage of using Map<String, Object> is that you can substitute the underlying implementation without any code compatibility problem. But this is not possible in the case of HashMap<String, Object>.

Methods	Description
void clear( )	Removes all key/value pairs from the invoking map.
boolean containsKey(Object k)	Returns true if the invoking map contains k as a key. Otherwise, returns false.
boolean containsValue(Object v)	Returns true if the map contains v as a value. Otherwise, returns false.
Set entrySet( )	Returns a Set that contains the entries in the map. The set contains objects of type Map.Entry. This method provides a set-view of the invoking map.
boolean equals(Object obj)	Returns true if obj is a Map and contains the same entries. Otherwise, returns false.
Object get(Object k)	Returns the value associated with the key k.
int hashCode( )	Returns the hash code for the invoking map.
boolean isEmpty( )	Returns true if the invoking map is empty. Otherwise, returns false.
Set keySet( )	Returns a Set that contains the keys in the invoking map. This method provides a set-view of the keys in the invoking map.
Object put(Object k, Object v)	Puts an entry in the invoking map, overwriting any previous value associated with the key. The key and value are k and v, respectively. Returns null if the key did not already exist. Otherwise, the previous value linked to the key is returned.
void putAll(Map m)	Puts all the entries from m into this map.
Object remove(Object k)	Removes the entry whose key equals k.
int size( )	Returns the number of key/value pairs in the map.
Collection values( )	Returns a collection containing the values in the map. This method provides a collection-view of the values in the map.

Example :


import java.util.*;

public class MapDemo {
	public static void main(String[] args) {
	Map m1 = new HashMap();
	m1.put("Ankit", "8");
	m1.put("Kapil", "31");
	m1.put("Saurabh", "12");
	m1.put("Apoorva", "14");
	System.out.println();
	System.out.println("Elements of Map");
	System.out.print(m1);
	}

}

Output :

C:\Program Files\Java\jdk1.6.0_18\bin>java MapDemo
Elements of Map
{Apoorva=14, Ankit=8, Saurabh=12, Kapil=31}