JLabel won't show with JPanel.setLayout(null).

问题:

I want to show many different labels over a map, so I'm using null layout in my panel, and calling setLocation for each label. For some reason, though, the labels don't show. If I remove the pan.setLayout(null), then the label appears in the top-center of the panel. Why isn't null layout working with setPosition?

package mapa;

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

public class Mapa extends JFrame {
  private static JPanel pan;
  private static JLabel lab;

  public Mapa() {
  }

  private static void createAndShowGUI() {
    Mapa frame = new Mapa();
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    lab = new JLabel("TEXTO");
    lab.setBackground(Color.black);
    lab.setForeground(Color.white);
    lab.setOpaque(true);
    lab.setVisible(true);

    pan = new JPanel();
    pan.setLayout(null);
    pan.setPreferredSize(new Dimension(640,480));
    pan.add(lab);
    lab.setLocation(100, 100);

    frame.getContentPane().add(pan, BorderLayout.CENTER);
    frame.pack();
    frame.setVisible(true);
  }

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

回答1:

This is the problem with absolute positioning (or null layout). It requires you to set the sizes of all your components, otherwise they will stay are their default zero-size and won't appear. That's why it's always better to use a layout manager.

回答2:

You have to set the size of the label explicitly; try using setBounds instead of setLocation. For example, lab.setBounds(100,100,200,30); Also there's no need to call setVisible(true); on the label.

Unless there's a very good reason to use a null layout and you know exactly what you're doing, using a layout manager is always where you should start.