Question

Possible Duplicate:
Java this.method() vs method()

I’ve been reading some things and doing some tutorials about android java but I still dont understand what “this” means, like in the following code.

    View continueButton = this.findViewById(R.id.continue_button);
    continueButton.setOnClickListener(this);
    View newButton = this.findViewById(R.id.new_button);
    newButton.setOnClickListener(this);

Also why is it in this example that a button is not defined with Button but with View, what is the difference?

ps. Great site!! trying to learn java and got ALLOT of answers by searching here!

Answer 1

The this keyword is a reference to the current object. It is used to pass this instance of the object, and more.

For example, these two allocations are equal:

class Test{

    int a;

    public Test(){
        a = 5;
        this.a = 5;
    }

}

Sometimes you have a hidden field you want to access:

class Test{

    int a;

    public Test(int a){
        this.a = a;
    }

}

Here you assigned the field a with the value in the parameter a.

The this keyword works the same way with methods. Again, these two are the same:

this.findViewById(R.id.myid);
findViewById(R.id.myid);

Finally, say you have a class MyObject that has a method which takes a MyObject parameter:

class MyObject{

    public static void myMethod(MyObject object){
        //Do something
    }

    public MyObject(){
        myMethod(this);
    }

}

In this last example you passed a reference of the current object to a static method.

Also why is it in this example that a button is not defined with Button but with View, what is the difference?

In Android SDK, a Button is a subclass of View. You can request the Button as a View and cast the View to a Button:

Button newButton = (Button) this.findViewById(R.id.new_button);