The == with Strings Trap : Comparison Operators « Operators « SCJP

1. SCJP
2. Operators
3. Comparison Operators

The compiler optimizes storage of String literals by reusing them. 

public class MainClass {
  public static void main(String[] argv) {
    String s1 = "YES";
    String s2 = "YES";
    // The String literal "YES" appears in both lines 1 and 2,
    // but the compiler creates only one String object, referred to by both s1
    // and s2.

    if (s1 == s2){
      System.out.println("equal");

    }
    String s3 = new String("YES");
    String s4 = new String("YES");

    if (s3 == s4){
      System.out.println("s3 eq s4");

    }

  }
}