Unable to understand the code properly

Question

In the following code, i cannot understand setBackground() methods sets the background for which layer. Secondly, when i include this line why there becomes a hole in the window, means when i click in between the window, it minimizes as i have clicked somewhere else.

import java.awt.Color;
import java.awt.FlowLayout;
import java.awt.Graphics;
import java.awt.Graphics2D;
java.awt.GraphicsDevice;
import java.awt.GraphicsDevice.WindowTranslucency;
import java.awt.GraphicsEnvironment;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.SwingUtilities;


public class transparentWindow extends JFrame {

public transparentWindow() {
    // TODO Auto-generated constructor stub
    //JFrame jfrm=new JFrame("Transparent Window");
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setSize(300,200);
    getContentPane().setLayout(new FlowLayout());
    //setBackground(new Color(0,0,0,0));

    add(new JButton("Enter"));
    setOpacity(0.7f);
    setVisible(true);
}
public static void main(String[] args) {
    // TODO Auto-generated method stub
    GraphicsEnvironment ge=GraphicsEnvironment.getLocalGraphicsEnvironment();
    GraphicsDevice gd=ge.getDefaultScreenDevice();
    if(!gd.isWindowTranslucencySupported(WindowTranslucency.TRANSLUCENT))
    {
        System.out.println("Transparency not supported");
        System.exit(0);
    }
    JFrame.setDefaultLookAndFeelDecorated(true);
    SwingUtilities.invokeLater(new Runnable(){public void run(){new transparentWindow();}});
}

}

Answer 1

All methods that are not called on a specific object, are actually called on this, so

setBackground(new Color(0,0,0,0));

is just like

this.setBackground(new Color(0,0,0,0));

Which means that it is called on the JFrame.

Answer 2

The other issue you will find, is that setting the frame's opacity will effect all it's children equally

If you want to see a nice long discussion (& example) see how to set JFrame background transparent but JPanel or JLabel Background opaque?