自定义 JPanel 未添加到 JFrame

Question

I am creating a GUI that utilizes a custom JPanel that shows up three times in a row on a JPanel that is then added to the JFrame. For some reason, the custom JPanel does not show up when added to the JFrame .

The only way I managed to get the JPanel to show is when I added the below line to the bottom of the custom JPanel class:

this.add(myPanel); // myPanel is the JPanel instance

I am sure that is not the correct way

class Driver with main method

public class Driver {
    public static void createAndShowGUI() {
        JFrame frame = new JFrame("Test");

        JPanel mainPanel = new JPanel(new GridLayout(1, 3));

        MyPanel firstPanel = new MyPanel();
        MyPanel secondPanel = new MyPanel();
        MyPanel thirdPanel = new MyPanel();

        mainPanel.add(firstPanel);
        mainPanel.add(secondPanel);
        mainPanel.add(thirdPanel);

        frame.getContentPane().add(mainPanel);

        // show the window.
        frame.setSize(MyPanel.WIDTH, MyPanel.HEIGHT);
        frame.setResizable(false);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        frame.setVisible(true);
    } // createAndShowGUI

    public static void main(String[] args) {
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                createAndShowGUI();
            } // run
        }); // Runnable
    } // main
} // Driver

Custom JPanel class

@SuppressWarnings("serial")
public class MyPanel extends JPanel {
     public MyPanel() {
        JPanel myPanel = new JPanel();

        // inner panel
        JPanel innerPanel = new JPanel();

        innerPanel.setBorder(BorderFactory.createEtchedBorder());
        innerPanel.setBackground(Color.red);
        innerPanel.setPreferredSize(new Dimension(WIDTH / 3, HEIGHT - 100));
        myPanel.add(innerPanel);

        JPanel buttonPanel = new JPanel();

        buttonPanel.setBackground(Color.MAGENTA);

        myPanel.add(buttonPanel);
    } // constructor
} // class

Some lines have been edited out

Answer 1

This class is in error:

public MyPanel() {
        JPanel myPanel = new JPanel();
        myPanel.setBorder(BorderFactory.createEtchedBorder());
        myPanel.setLayout(new BoxLayout(myPanel, BoxLayout.Y_AXIS));

        // inner panel
        JPanel innerPanel = new JPanel();

        innerPanel.setBorder(BorderFactory.createEtchedBorder());
        innerPanel.setBackground(Color.red);
        innerPanel.setPreferredSize(new Dimension(WIDTH / 3, HEIGHT - 100));

        myPanel.add(innerPanel);

        JPanel buttonPanel = new JPanel();
        buttonPanel.setBackground(Color.MAGENTA);

        myPanel.add(buttonPanel);
    } // constructor
} // class

I'm guessing that it extends JPanel, meaning that there are two JPanels being created when this class is created, a myPanel variable which gets components added to it, but which is never added to the GUI, and the MyPanel instance which has nothing added to it, and which does get added to the GUI.

Suggestion: don't have this class extend JPanel and instead make the myPanel local variable an instance field. Give the class a getMyPanel() method that returns this field and add this to the GUI.

Option 2: get rid of the myPanel local variable and instead add everything to this .