Question

I want a hard reference class in my Java code, but, of course, there isn’t one. Is there some other way to do what I want, or should I make my own class?

This comes up with anonymous classes in methods where I want the anonymous class to set the return value for the method.

For example, given

interface Greeting {
    void greet();
}

I want code like the following:

// Does not compile
static void hello(final String who) {
    String returnValue;

    Greeting hello = new Greeting() {
        public void greet() {
            returnValue = "hello" + who;
        }
    };
    hello.greet();
    System.out.println(returnValue);
}

I can fake it using a list:

static void hello(final String who) {
    final List<String> returnValue = new ArrayList<String>();
    Greeting hello = new Greeting() {
        public void greet() {
            returnValue.add("hello" + who);
        }
    };

    hello.greet();
    System.out.println(returnValue.iterator().next());
}

But I want to not use a list. I can write a StrongReference class that solves this:

static class StrongReference<T> {
    private T referent;
    public void set(T referent) {
        this.referent = referent;
    }
    public T get() {
        return referent;
    }
}

which makes my method clearer:

static void hello(final String who) {
    final StrongReference<String> returnValue = new StrongReference<String>();
    Greeting hello = new Greeting() {
        public void greet() {
            returnValue.set("hello" + who);
        }
    };

    hello.greet();
    System.out.println(returnValue.get());
}

For my contrived example, I could have greet() return a String, but I’m working with much more complex classes, where the setting is deep within a database call that the base class manages. The instances have many different types they want to return, so I’ve just been using the List trick.

My questions are: Is there a better way to do this? What’s wrong with my StrongReference class? Has anyone written a StrongReference in a library somewhere?

Answer 1

If you want something from the standard API, perhaps an AtomicReference would do?

It has void set(V value) and a V get() methods. Unless you have multiple threads involved, just see the synchronization mechanism as a bonus 😉