Using a JPanel with a null layout

问题

So I have a class called CalendarPanel that extends JPanel. It uses a null layout. How would I use CalendarPanel as a regular component? When I put it in another JPanel and then add it to a window, it disappears. It is only visible when I add it directly to a window.

EDIT: And yes, I realize using a JPanel with a null layout is bad practice. CalendarPanel is actually someone else's code, and I'm trying to use it for my purposes without having to refactor it.

回答1:

It is only visible when I add it directly to a window.

That is because a window uses a BorderLayout by default and will automatically resize the panel to fit in the window.

When I put it in another JPanel and then add it to a window, it disappears.

The is because a JPanel uses a FlowLayout by default and a flow layout respects the preferred size of the components added to it. Since you are using a null layout your panel doesn't have a preferred size so nothing gets painted.

That is why you should NOT use null layout. Instead use layout managers because they do all this extra work for you.

回答2:

NOTE: it is a very bad idea in general to use a null layout. Use a LayoutManager instead.

If you insist on using a null layout, you're going to have to position the JPanel manually as mentioned in the documentation. Here's an example.

import java.awt.BorderLayout;
import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

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

public class Test extends JFrame {
   static int defaultX = 10;
   static int defaultY = 10;
   static int defaultW = 150;
   static int defaultH = 50;

   public Test() {
      super("Test");
      setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

      // here is the outer JPanel
      final JPanel outer = new JPanel(new BorderLayout());
      JPanel inner = new JPanel(new BorderLayout());
      // here is the main component we want to see
      // when the outer panel is added to the null layout
      JButton mainComponent = new JButton("Test");
      inner.add("Center", mainComponent);
      outer.add("Center", inner);

      JPanel c = (JPanel)getContentPane();

      // This panel has a null layout!
      final JPanel nullLayoutPanel = new JPanel();
      nullLayoutPanel.setLayout(null);
      c.add("Center", nullLayoutPanel);

      // set the bounds of the outer panel manually
      // when using the null layout!
      nullLayoutPanel.add(outer);
      outer.setBounds(defaultX, defaultY, defaultW, defaultH);

      JPanel controlPanel = new JPanel(new FlowLayout(FlowLayout.CENTER, 10, 10));
      final JTextField x = new JTextField(""+defaultX, 3);
      final JTextField y = new JTextField(""+defaultY, 3);
      final JTextField w = new JTextField(""+defaultW, 3);
      final JTextField h = new JTextField(""+defaultH, 3);
      JButton b = new JButton("Resize");
      b.addActionListener(
         new ActionListener() {
            public void actionPerformed(ActionEvent e) {
               try {
                  outer.setBounds(
                        Integer.parseInt(x.getText()),
                        Integer.parseInt(y.getText()),
                        Integer.parseInt(w.getText()),
                        Integer.parseInt(h.getText())
                  );
                  outer.revalidate();
               } catch(Exception ex) {}
            }
         }
      );
      controlPanel.add(x);
      controlPanel.add(y);
      controlPanel.add(w);
      controlPanel.add(h);
      controlPanel.add(b);
      c.add("South", controlPanel);
   }

    public static void main(String[] argv) {
       SwingUtilities.invokeLater(new Runnable() {
          public void run() {
             Test p = new Test();
             p.setSize(300, 200);
             p.setVisible(true);
          }
      });
    }
}

来源：https://stackoverflow.com/questions/14982781/using-a-jpanel-with-a-null-layout