why when we declare string in java we can't use == to compare this string and always will turn to false while if we initialize the string from the beginning it will be true . for example :

import java.util.Scanner;

public class MyString {

/**
 * @param args
 */
public static void main(String[] args) {
    // TODO Auto-generated method stub
            Scanner input = new Scanner(System.in);
    String s = input.nextLine();
            if(s=="Hello")
            system.out.println("Hello");

            String d = "Hello";
            if(d=="Hello")
            system.out.println("Hello");
}

}

I need an explanation please ??