Why doesn’t == work on String?

I just started Java programming. I love it so far, but I have been stuck on this problem for a while.

When I run this code, whenever I type in “boy” it will just respond with GIRL:

import java.util.Scanner;

public class ifstatementgirlorboy {
    public static void main(String args[]) {
        System.out.println("Are you a boy or a girl?");
        Scanner input = new Scanner(System.in);
        String gender = input.nextLine();

        if(gender=="boy") { 
            System.out.println("BOY");
        } 
        else { 
            System.out.println("GIRL"); 
        }
    }
}

Why?