Question

I have this class, let’s say, Foo. It extends JFrame and is a singleton. That being said, it has two static fields: 1) an instance of Foo and 2) a Color.

Here’s a code snippet of Foo:

public class Foo extends JFrame{
    private static final Color FOO_RED = new Color(155, 64, 69);
    private static final Foo INSTANCE = new Foo();

    private Foo(){
        //do stuff
    }

    public static Foo getInstance(){
        return INSTANCE;
    }
}

I also have another class, let’s say, Launcher. This is the main class that is responsible for launching the application. It’s a simple class in that its only job is to delegate the task of constructing Foo to the EDT.

Here’s a code snippet of Launcher:

public class Launcher{
    public static void main(String[] args){
        SwingUtilities.invokeLater((new Runnable(){
            @Override
            public void run()
            {
                Foo.getInstance();
            }
        }));
    }
}

Now, this all works just fine. But, when I switch the ordering of Foo‘s fields (See below), the components that use FOO_RED are no longer painted this color.

public class Foo extends JFrame{
    private static final Foo INSTANCE = new Foo(); //declared before Color
    private static final Color FOO_RED = new Color(155, 64, 69);

    private Foo(){
        //do stuff
    }

    public static Foo getInstance(){
        return INSTANCE;
    }
}

So, this begs the question, does the ordering of static fields matter when it comes to Swing?

Answer 1

As already mentioned, ordering of static fields does matter. Executed in the order they appear.

I would make another change to this example. This would make you static field order less important.

UPDATE: Use IDOH (Initialization on Demand Holder) pattern to make singleton thread safe.

private static class FooHolder {
    private static final Foo INSTANCE = new Foo();
}

public static Foo getInstance(){
    return FooHolder.INSTANCE;
}