string - Using "==" in Java

public class Test { 
    public static void main(String[] args) 
    { 

        String s1 = "HELLO";
        String s2 = "HELLO";

        System.out.println(s1 == s2);  // true
    }  
} 

But when I use :

public class Test { 
    public static void main(String[] args) 
    { 
        String s1 = new String("HELLO");
        String s2 = new String("HELLO");

        System.out.println(s1 == s2); // false
    } 
} 

Can anybody please explain the difference here? Thankyou!