String comparison in Java: what is wrong with "=="?

public class S_eaqual {
    public static void main(String[] args) {
        String s1 = "one", s2 = "two";
        if (s1 + s2 == "onetwo") {
            System.out.println("Yes..equal");
        }
    }
}

This type of comparison shows errors. Is this not the right way of comparing strings?
Two String objects can be compared using == operator. So why this is showing error?