String immutability, pop quiz

Question

There are a lot of questions concerning String immutability, but yet I haven't found an answer for why the following happens:

1) "TEST".equals("TEST")            // TRUE obviously
2) "TEST" == "TEST"                 // Also TRUE since String's are immutable
3) "TEST" == "T" + "E" + "S" + "T"  // TRUE, but why? Due to compiler optimalization?
4) "TEST" == new String("TEST");    // FALSE, because you explicitly request a new String?

Can anyone correct me if necessary and explain in further detail 3 and 4? Many thanks!

Answer 1

The concatenation happens at compile time, so the String goes to the pool, that's why you get true .

Regarding the last one, a new object is created because your'e using new keyword and comparing the references by == , so you're getting false .

And this has nothing to do with immutability.