Java GUI 不会显示 JLabel

Question

I would like to create a simple GUI in Java. I know the basics of creating JLabel , etc. However, I cannot find why my JLabel is not displayed on the screen. Here is my code:

package test;

import java.awt.Color;
import java.awt.Font;
import java.awt.GridLayout;
import java.awt.event.*;
import javax.swing.*;

public class A1Panel extends JPanel implements ActionListener { 
    JLabel firstInt;

    public void init() {
        makeComponents();
        makeLayout();
    }

    private void makeComponents() {
        firstInt = new JLabel("First argument");
        firstInt.setFont(new Font("Helvetica", Font.BOLD, 16));
        firstInt.setBackground(Color.lightGray);
        firstInt.setVisible(true);
        firstInt.setHorizontalAlignment(SwingConstants.CENTER);
    }

    private void makeLayout() {
        add(firstInt);
    }

    public void actionPerformed(ActionEvent e) {    
    }
}

I then add my JPanel to my JFrame using a different class called GUI:

import test.A1Panel;

public class GUI {
public static void main(String[] args) {       
    JFrame frame = new JFrame("Testing GUI"); 
    frame.setLayout( new GridLayout(1,3));

    JPanel panel = new A1Panel();
    panel.setBorder( BorderFactory.createRaisedBevelBorder() );
    frame.add( panel);

    frame.setSize(800,600);
    frame.setLocationRelativeTo(null);
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setVisible(true);
    frame.pack();
    }
}

When I hit compile, what I get is a simple frame with three empty panels. I do not understand why my JLabel is not in the first panel since I have added it to my frame. Am I missing something?

Answer 1

After you instance A1Panel, you haven't called A1Panel.init()

I would suggest removing init() and adding all the code to the constructor of A1Panel . If, however, you wanted to keep the init() function, you would want to call it after JPanel panel = new A1Panel()

Answer 2

The frame is not empty, the panel is. Nowhere in your code do I see a call to the methods init() or makeComponents() . In fact, I would turn your init() method into a constructor, like so:

public A1Panel() {
    makeComponents();
    makeLayout();
}

Another alternative to this would be to call panel.init() after declaring JPanel panel = new A1Panel()

Answer 3

The code to add the label was not actually called in the main, was it? So look carefully, when is init actually called?

Look at the

private void makeLayout()

method.

Answer 4

If I replace public void init() by A1Panel() , it does the job. Thank you for your help.