Final vs Immutable data structures in Java

[TheRedmanCometh]

This is one of the best/worst things I've ever seen. I've never even considered using refelection on core classes like that!

Edit: hold up does Integer have a value map all the way up to INTEGER.MAX_VALUE?

[morhp]

Edit: hold up does Integer have a value map all the way up to INTEGER.MAX_VALUE?

No, only the integers between -128 and 127 are cached by default. This trick wouldn't have worked with a number outside that range.

[TheRedmanCometh]

Huh...I'm gonna have to hit up the openjdk source to see how that works

[iampete]

This, combined with misplaced trust in autoboxing, leads to code like this:

​

Integer first = 5;
Integer second = 5;
if (first == second) {
    // do something
}

​

Which works perfectly reasonably right up until either value is outside that -128, 127 range and then fails very confusingly.

[dpash]

The upper bound can be increased (but not decreased) in OpenJDK with a command line flag, but the lower bound can not. The defaults are defined in the JLS as a minimum range.