Post increment within a loop

Question

I would really appreciate it if someone could properly explain why j++ does not increase in value (++j works though). After I run the program, it outputs 0 instead of 100.

public static void calculate() {
    int j = 0;

    for(int i=0; i<100; i++){
        j=j++;
    }

    System.out.println(j);
}

Thanks.