String Pool behavior

Question

I read this Questions about Java's String pool and understand the basic concept of string pool but still don\'t understand the behavior.

First: it works if you

Answer 1

The compiler can perform constant evaluation but not in the case where you modify the values

Try instead following and see what happens if you drop final from either variable.

final String s1 = "abc";
final String s2 = "abc";
System.out.println("s1 == s2? " + (s1 == s2));

String s3 = s1 + "d";                  
String s4 = s2 + "d";
System.out.println("s3 == s4? " + (s3 == s4));