JPanel带有3个复选框，可在JFrame的中心设置背景色

Question

I'm designing a JPanel with three check boxes labelled “red”, “green”, and “blue” that add a red, green, or blue component to the background color of a JPanel in the centre of the GUI. Initially, the JPanel has the “blue” JCheckBox ticked and so the JPanel is colored in blue. As different check boxes are ticked, the appropriate color combination appears on the JPanel which can display a total of eight color combinations.

It should be like the pictures below:

The codes for the program is:

import java.awt.*;
import javax.swing.*;
import java.awt.event.*;

public class ColorDemo extends JFrame implements ItemListener
{
    int r=0,g=0,b=0;
    JCheckBox red,green,blue;
    JPanel P = new JPanel();
    JPanel cpanel = new JPanel();
    Container pane = getContentPane();


    ColorDemo(String cd){
        super(cd);
        red = new JCheckBox("red");
        red.addItemListener(this);

        green = new JCheckBox("green");
        green.addItemListener(this);

        blue = new JCheckBox("blue");
        blue.setSelected(true);
        blue.addItemListener(this);        

        cpanel.add(red);
        cpanel.add(green);
        cpanel.add(blue);

        getContentPane().add(cpanel,"North");
        setSize(400,400);
        setVisible(true);        

        getContentPane().add(P);
        P.setAlignmentX(JComponent.CENTER_ALIGNMENT);
        setVisible(true);

    }

    public static void main(String[] args)
    {
        ColorDemo cd = new ColorDemo("Color Check Box");
        cd.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    }

    public void itemStateChanged(ItemEvent ie){

        if(ie.getItem() == red)
        if(red.isSelected()) r=255; else r=0;
            if(ie.getItem() == green)
        if(green.isSelected()) g=255; else g=0;
            if(ie.getItem() == blue)
        if(blue.isSelected()) b=255; else b=0;

        P.setBackground(new Color(r,g,b));            

    }

}

Now the issue I'm having is:

*** It's showing the color as background, not boxed in center. See the pic:

*** Also as per the requirement, the blue checkbox is initially checked but not showing the color on the JFrame.

I'm bit confused (as I'm very much new to Programming), Any suggestions would be highly appreciated. Thanks.

Answer 1

Directly before your setVisible(true) change the code to this:

 panel = new JPanel();
    getContentPane().add(panel, BorderLayout.CENTER);
    panel.setLayout(new CardLayout(100, 100));
    panel.add(P, "name");
    P.setAlignmentX(JComponent.CENTER_ALIGNMENT);
    P.setBackground(new Color(0,0,255));

This will stop the color from being on the whole frame. This also makes the default color blue instead of the original grey default.

What I did was create a JPanel with your panel inside it with a card layout with a gap of 100 for horizontal and vertical.

Answer 2

Try using a Layout Manager

https://docs.oracle.com/javase/tutorial/uiswing/layout/layoutlist.html

And you might perhaps need to add an empty border depending on the layout you use.