java JLabel 不会与 JPanel.setLayout(null) 一起显示。为什么？
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JLabel won't show with JPanel.setLayout(null). Why?
提问by Rodrigo
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?
我想在地图上显示许多不同的标签，所以我在面板中使用空布局，并为每个标签调用 setLocation。但是，出于某种原因，标签没有显示。如果我删除 pan.setLayout(null)，则标签会出现在面板的顶部中央。为什么 null 布局不能与 setPosition 一起使用？
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by Reimeus
This is the problem with absolute positioning (or nulllayout). 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.
这是绝对定位（或null布局）的问题。它要求您设置所有组件的大小，否则它们将保持默认的零大小并且不会出现。这就是为什么使用布局管理器总是更好的原因。
回答by rtheunissen
You have to set the size of the label explicitly; try using setBoundsinstead of setLocation. For example, lab.setBounds(100,100,200,30);Also there's no need to call setVisible(true);on the label.
您必须明确设置标签的大小；尝试使用setBounds代替setLocation. 例如，lab.setBounds(100,100,200,30);也不需要调用setVisible(true);标签。
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.
除非有很好的理由使用空布局并且您确切地知道自己在做什么，否则使用布局管理器始终是您应该开始的地方。
