Question

I often wanted to be able to decorate java-classes properly, that is add behaviour to them. I know mixins from ruby, and i know that they can get terribly confusing.

I came up with the theoretical idea of having a language-construct like this:

package org.test.decorator;

public decorator ListPatch<E> extends List<E> {
    public E last() {
        return this.get(this.size() - 1);
    }
}

this would give access to public members of List and the Decorator itself.

Then in a class i could use:

package org.test.decorator;

decoration org.test.decorator.ListPatch;

public MyClass {
    public void foo() {
        List<String> list = Lists.newArrayList();
        list.add("test");
        System.out.println(list.last());
    }
}

I don’t have that much knowledge about compilers, so I wondered if something like that was possible. Also if it actually would be an improvement.

Answer 1

Sure, it’s possible.

The fact that Scala compiles to bytecode (and has a fairly straight forward mapping from Scala classes to bytecode classes) and supports mixins proves this.

Here’s how your sample code looks in Scala syntax:

class ListTest {

    // ...

}

trait ListPatch {
    def last {
        // ...
    }
}

object Main {
    def main(args: Array[String]) {
        val list = new ListTest() with ListPatch;
        list.add("test")
        println(list.last)
    }
}

Scala compiles this by adding an axillary class Main$$anon$1 which composes ListTest and ListPatch.

The motto for the Java developers have always been (and will probably always be) “If in doubt, leave it out.” though.

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