無法正確理解代碼

在下面的代碼中，我無法理解setBackground()方法爲哪個圖層設置背景。其次，當我包括這一行爲什麼在窗口中有一個洞，意味着當我在窗口之間點擊時，它最小化，因爲我已經點擊了其他地方。無法正確理解代碼

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();}}); 
} 

}

來源

2012-08-08 Insane Coder