JLabel.setBackground(Color color) doesn't work?

Question

In this SSCCE code:

This method work

label.setForeground(Color.GREEN);

But this next method doesn't work!

label.setBackground(Color.BLUE);

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

public class LabelColorTest extends JPanel
{

    static JLabel label;
    JPanel panel;

    public LabelColorTest()
    {
        label = new JLabel();
        label.setVerticalAlignment(JLabel.CENTER);
        label.setHorizontalAlignment(JLabel.CENTER);
        label.setText("Hello world");

        panel = new JPanel();
        panel.setLayout(new BorderLayout());
        panel.add(label, BorderLayout.CENTER);

        label.setForeground(Color.GREEN);  //HERE
        label.setBackground(Color.BLUE);  //HERE


        this.setLayout(new BorderLayout());
        this.add(panel);
    }

    private static void createAndShowGUI()
    {
        JFrame frame = new JFrame("Hellow world");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(400, 250);
        frame.add(new LabelColorTest(), BorderLayout.CENTER);
        frame.setVisible(true);
    }

    public static void main(String[] args)
    {
        SwingUtilities.invokeLater(new Runnable()
        {
            public void run()
            {
                UIManager.put("swing.boldMetal", Boolean.FALSE);
                createAndShowGUI();

            }
        });
    }
}

Answer 1

组件必须是不透明的，因为它的背景有效，JLabel的默认值为false，因此您必须设置它：

label.setOpaque(true);

Answer 2

You will need to set the label to opaque first. Else, it wont be painted. Look at: How do I set a JLabel's background color?