java - Why program will print "true" "true"?

Possible Duplicate:
Why are these == but not `equals()`?

why this code will print

true

true

public class Test {
        public static void main(String[] args){
            String st1 = "abc";
            String st2 = "abc";


            Integer k1 = 100;
            Integer k2 = 100;

            System.out.println(st1 == st2);
            System.out.println(k1 == k2);
        }
    }

To compare objects we use method equals(). But why it is ok in this way?