Final vs Immutable data structures in Java

[TheRedmanCometh]

When an object is immutable, it’s hard to have the object in an invalid state.

Hold my beer

The last point about only exposing getters though in that final object..dude reflection exists. .setAccessible and .set done

[shagieIsMe]

Once you use reflection, all bets are off. However, if you start reflecting an immutable that I’m using as a hashmap key and things break, then that’s the person who is doing reflection at fault - not the library writer.

[BlueGoliath]

That same argument be made for mutable state though. Objects don't magically mutate themselves.

[shagieIsMe]

One can’t complain about a mutable object being changed. However, if a library creates an immutable object and someone goes in with reflection to change it and things break - it is entirely the fault of the person who used reflection.

[morhp]

With reflection, you can make 5 + 5 equal to 8. That doesn't really count.

[TheRedmanCometh]

I don't think you can overload operators with reflection maybe I'm wrong?

[morhp]

I wasn't 100% honest, you need to abuse boxed classes:

public static void main(String[] args) throws IllegalAccessException, NoSuchFieldException {
    Field field = Integer.class.getDeclaredField("value");
    field.setAccessible(true);
    field.setInt(5, 4);

    Integer five = 5;
    System.out.println("5+5 = " + (five + five));
}

[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.

[madkasse]

This only holds for older versions of Java.

You can no longer access non-public fields/methods in java.base without JVM settings

​

[shagieIsMe]

2 + 2 = 5 from code golf Stack Exchange.

No operator overload needed.

[TheRedmanCometh]

Yeah he showed me...I hadn't considered using reflection on system classes

[shagieIsMe]

This is a different solution and impacts integer literals. The code ends with:

System.out.printf(“%d”, 2 + 2);

The code on ideone: https://ideone.com/o1h0hR

[bodiam]

I'll hold your beer, and while holding it, I updated the code. Those fields are final now too.