Java HashMap

HashMap Class In Java

Java HashMap class implements the Map interface by using a hashtable.
It inherits AbstractMap class and implements the Map interface.
The important points about the Java HashMap class are given as follows -

It contains only unique elements.
It may have one null key and multiple null values.
The HashMap class extends AbstractMap and implements the Map interface.
It uses a hash table to store the map.
This allows the execution time of get( ) and put( ) to remain constant even for large sets.
It maintains no order.
A HashMap contains values based on the key.

HashMap is a generic class that has this declaration -

class HashMap<M,N>

Here, M specifies the type of keys, and N specifies the type of values.

Example -

package javaLearnings;

import java.util.*;

// Demonstrate LinkedList.
public class Main {

public static void main(String args[]) {
// Create a hash map.
HashMap hm = new HashMap();
// Put elements to the map
hm.put("Mukul Agrawal", new Double(4737.23));
hm.put("Technical content writter ", new Double(234.545));
hm.put("Web desinging house", new Double(2342.23));
hm.put("Online Promotion House", new Double(33.123));
hm.put("Hello world", new Double(-435.32));
// Get a set of the entries.
Set> set = hm.entrySet();
// Display the set.
for(Map.Entry me : set) {
System.out.print(me.getKey() + ": ");
System.out.println(me.getValue());
}
System.out.println();
// Deposit 1000 into John Doe's account.
double balance = hm.get("Mukul Agrawal");
hm.put("Mukul agrawal", balance + 1000);
System.out.println("Mukul agrawals new balance: " +
hm.get("Mukul agrawal"));

}
}

Output -

Methods -

We can find the HashMap size using size() method, ex - hm.size().
As we have seen we can add elements using put() method.
To remove all items clear() is used
To remove a particular item remove() is used.