Question

I am going through Hello Android (Android PDF/tutorial) and have now seen this syntax a couple of times. Can someone please explain to me what Java syntax is used when run Runnable is defined?

private class AndroidBridge {

    public void callAndroid(final String arg) { // must be final
        handler.post(new Runnable() {
            public void run() {
                Log.d(TAG, "callAndroid(" + arg + ")" );
                textView.setText(arg);
            }
            ...

Is the code defining a Runnable object and overriding it’s run method?

Answer 1

As Dave Newton indicated, this is an anonymous inner class implementing the Runnable interface.

As to why one would want to use this, it could be thought of as syntactic sugar of sorts. You’ll notice that in your example, the code in run() has access to the same scope as where the anonymous inner class itself is defined.

This simplifies access to those members, as if you defined the class externally, you’d have to pass in a reference to any object whose members you wanted to invoke/use.

In fact, IIRC, this is actually what happens when Java compiles the anonymous inner class; if there are references to the outer containing class, the compiler will create a constructor that passes in a reference to the outer containing class.