What prevents java swing components from being painted when added to a JComponent?

Question

JTextField, JSlider, JComboBox, etc added to a JComponent are not displayed in the JFrame containing the JComponent. It seems only drawing by the Graphics parameter allows painting. The included test program compares using JPanel to JComponent in my efforts to discover how to display components added to a JComponent. Is there any way to get such components displayed?

public class TestPaints {
    public static void main(String[] args) {
        new TestPaints();
    }

    JTextField _text1;
    JLabel _label1 = new JLabel("Text1");
    JTextField _text2;
    JLabel _label2 = new JLabel("Text2");

TestPaints() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                JFrame frame = new JFrame("Paint a Widget");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setLayout(new GridLayout(3, 2));
                GridBagConstraints  grid = new GridBagConstraints();
                frame.add(new JLabel("TextField in JComponent   "));
                grid.gridx = 2;
                frame.add(new JLabel("TextField in JPanel"), grid);
                grid.gridy = 2;
                grid.gridx = 1;
                frame.add(new TestJComponent(), grid);
                grid.gridx = 2;
                frame.add(new TestJPanel(), grid);
                grid.gridy = 3;
                grid.gridx = 1;
     /* tabbing between the two TextFields shows that keystrokes are seen */
                frame.add(_label1, grid);
                grid.gridx = 2;
                frame.add(_label2, grid);
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
}

public class TestJComponent extends JComponent {
    public TestJComponent() {
        setPreferredSize(new Dimension(100, 30));
        _text1 = new JTextField(6);
        _text1.addKeyListener(new KeyAdapter() {
            public void keyPressed(KeyEvent e) {
                _label1.setText(_text1.getText());
                _label1.repaint();
            }
        });
        _text1.setOpaque(true);
        _text1.setVisible(true);
        add(_text1);
/* This doesn't work            
        JPanel panel = new JPanel();
        panel.add(_text1);
        add(panel);    */
        setOpaque(true);
        setVisible(true);
        setBackground(Color.green);
    }
    public void paint(Graphics g) {
        super.paint(g);  // did not do background.                          Rectangle r = g.getClipBounds(); // needs this
        g.setColor(getBackground());
        g.fillRect(r.x, r.y, r.width, r.height);
            /* Variations such as these don't work */       
        _text1.setOpaque(true);
        _text1.setVisible(true);
        _text1.paintComponents(g);
    }
}

class TestJPanel extends JPanel {
    TestJPanel() {
        _text2 = new JTextField(6);
        _text2.addKeyListener(new KeyAdapter() {
            public void keyPressed(KeyEvent e) {
                _label2.setText(_text2.getText());
                _label2.repaint();
            }
        });
        add(_text2);
        setBackground(Color.blue);          
    }
  }
}

Answer 1

Edit: you need to give your JComponent a layout such as FlowLayout for components to show properly since it does not have a default layout like JPanel has. So add setLayout(new FlowLayout()) into your JComponent's constructor

You have:

frame.setLayout(new GridLayout(3, 2));

and then try to add components to the JFrame's contentPane using GridBagConstraints, and this doesn't make sense. If you want to use these constraints, then the container needs to use GridBagLayout, not GridLayout.

Also this is dangerous code:

public void paint(Graphics g) {
    super.paint(g);  // did not do background.                          Rectangle r = g.getClipBounds(); // needs this
    g.setColor(getBackground());
    g.fillRect(r.x, r.y, r.width, r.height);
        /* Variations such as these don't work */       
    _text1.setOpaque(true);
    _text1.setVisible(true);
    _text1.paintComponents(g);
}

You should be overriding JComponent's paintComponent method, not its paint method (call super.paintComponent) and should not be setting component visibility or calling a component's paintComponents method directly within any painting method.

Another issue: don't use KeyListeners within Swing text components but rather add a DocumentListener to the component's Document. Otherwise you risk breaking some of the functionality of the text component, and also your listener won't work for copy/paste, while the DocumentListener will.

And another issue, your main issue: you need to give the JComponent a layout. It does not default to FlowLayout like a JPanel does. This is why the added components are not showing within it.

For example:

import java.awt.Color;
import java.awt.FlowLayout;
import java.awt.Graphics;
import java.awt.GridLayout;

import javax.swing.BorderFactory;
import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JTextField;
import javax.swing.SwingUtilities;
import javax.swing.event.DocumentEvent;
import javax.swing.event.DocumentListener;
import javax.swing.text.BadLocationException;
import javax.swing.text.Document;

public class TestPaints2 {
    private JTextField textField1 = new JTextField(8);
    private JTextField textField2 = new JTextField(8);
    private JLabel label1 = new JLabel("Text1");
    private JLabel label2 = new JLabel("Text2");

    public TestPaints2() {
        textField1.getDocument().addDocumentListener(new MyDocListener(label1));
        textField2.getDocument().addDocumentListener(new MyDocListener(label2));

        TestJComponent2 jComponent = new TestJComponent2();
        jComponent.add(textField1);

        TestJPanel2 jPanel = new TestJPanel2();
        jPanel.add(textField2);

        JPanel mainPanel = new JPanel(new GridLayout(0, 2));
        mainPanel.add(new JLabel("JComponent"));
        mainPanel.add(new JLabel("JPanel"));
        mainPanel.add(jComponent);
        mainPanel.add(jPanel);
        mainPanel.add(label1);
        mainPanel.add(label2);

        JFrame frame = new JFrame("Test");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.add(mainPanel);
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }

    private class MyDocListener implements DocumentListener {
        private JLabel label;

        public MyDocListener(JLabel label) {
            this.label = label;
        }

        @Override
        public void changedUpdate(DocumentEvent e) {
            updateLabel(e);
        }

        @Override
        public void insertUpdate(DocumentEvent e) {
            updateLabel(e);
        }

        @Override
        public void removeUpdate(DocumentEvent e) {
            updateLabel(e);
        }

        private void updateLabel(DocumentEvent e) {
            Document doc = e.getDocument();
            int offset = doc.getLength();
            try {
                String text = doc.getText(0, offset);
                label.setText(text);
            } catch (BadLocationException e1) {
                e1.printStackTrace();
            }
        }
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> new TestPaints2());
    }
}

class TestJComponent2 extends JComponent {
    private static final Color BG = Color.GREEN;
    private static final int GAP = 5;

    public TestJComponent2() {
        setOpaque(true);
        setBackground(BG);
        setLayout(new FlowLayout());
        setBorder(BorderFactory.createEmptyBorder(GAP, GAP, GAP, GAP));
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.setColor(getBackground());
        g.fillRect(0, 0, getWidth(), getHeight());
    }
}

class TestJPanel2 extends JPanel {
    private static final Color BG = Color.BLUE;
    private static final int GAP = 5;

    public TestJPanel2() {
        setBackground(BG);
        setBorder(BorderFactory.createEmptyBorder(GAP, GAP, GAP, GAP));
    }
}