Strange String pool behavior

Question

I've got a question of some strange String pool behavior. I'm using == to compare equal Strings to find out whether they're in the pool or not.

public class StringPoolTest {   public static void main(String[] args) {     new StringPoolTest().run();   }    String giveLiteralString() {     return "555";   }    void run() {     String s1 = giveLiteralString() + "";     System.out.println("555" == "555" + "");     System.out.println(giveLiteralString() == giveLiteralString() + "");   } } 

The output is:

true false 

which is a big surprise for me. Could anyone explain this please? I think something about this is taking place at the compilation time. But why does adding "" to a String makes any difference at all?

Answer 1

"555" + "" 

is a compile-time constant, whereas

giveLiteralString() + "" 

isn't. Therefore the former compiles into just the string constant "555" and the latter compiles into the actual method invocation and concatenation, resulting in a fresh String instance.

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Also see JLS §3.10.5 (String Literals):

Strings computed by concatenation at run time are newly created and therefore distinct.