Question

I came across a code which calls setdefaultcloseoperation() without reference to any object. I read that methods are called with reference to object. Here is the code

public class Mainpage extends JFrame implements ActionListener{
    JFrame f = new JFrame("Mainpage");

    public Mainpage() {
        super("Mainpage");
        f.setSize(1000,6000);
        f.getContentPane().setLayout(null);
        f.setVisible(true);
        setDefaultCloseOperation(EXIT_ON_CLOSE); // how is this happening?
}

I want to know how setDefaultCloseOperation(EXIT_ON_CLOSE); is working. Thanks.

Answer 1

Well you extended JFrame. So in essence you are doing this.setDefaultOperation(EXIT_ON_CLOSE).

Also it doesn’t make sense that you are creating a JFrame inside a JFrame.. unless this was an experiment of yours. Simple answer would be don’t extend JFrame, and use
f.setDefaultOperation(EXIT_ON_CLOSE).