为什么这个JLabel不在这个Java应用程序的中心？

Question

newbie Java programmer here:

I hate asking questions every time I run into a problem, but I don't see what I should use to get the green "Hello World" label go right into the center of the JPanel. Here is my code:

package game;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class Javagame extends JPanel implements ActionListener{
    protected JButton b1;
    private JLabel label;
    public Javagame() {
        b1 = new JButton("Button!");
        b1.setActionCommand("change");

        b1.addActionListener(this);
        add(b1);

        label = new JLabel("Hello World!", SwingConstants.CENTER);
        label.setFont(new Font("Arial", Font.BOLD, 20));
        label.setForeground(new Color(0x009900));
        add(label, BorderLayout.CENTER);
    }
    public void actionPerformed(ActionEvent e) {
        if ("change".equals(e.getActionCommand())) {
            label.setText("Hello Universe!");   
            b1.setActionCommand("changeBack");
        }
        if ("changeBack".equals(e.getActionCommand())) {
            label.setText("Hello World!");
            b1.setActionCommand("change");
        }
    }
    private static void createWindow(){
        JFrame frame = new JFrame("Javagame");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setPreferredSize(new Dimension(500,500));

        Javagame newContentPane = new Javagame();
        newContentPane.setOpaque(true);
        frame.setContentPane(newContentPane);

        frame.setLocationRelativeTo(null);
        frame.pack();
        frame.setVisible(true);
    }
    public static void main(String[] args) {
        createWindow();
    }
}

BorderLayout.CENTER doesn't seem to work in add() . Any help would be appreciated, thank you!

Answer 1

The JLabel is in the centered within the parent container, the text is aligned within the label.

Try...

label.setHorizontalAlignment(JLabel.CENTER);
label.setVerticalAlignment(JLabel.CENTER);

Answer 2

BorderLayout.CENTER doesn't seem to work

The default layout of a JPanel is a FlowLayout. You need to set the layout to a BorderLayout.

Also, you need to add the button to the NORTH of the BorderLayout.

Then the button will appear at the top and the label will be centered.

Answer 3

When adding the label use FlowLayout field.

//Code apove
label = new JLabel("Hello World!", SwingConstants.CENTER);
label.setFont(new Font("Arial", Font.BOLD, 20));
label.setForeground(new Color(0x009900));
add(label, FlowLayout.CENTER);//!!
//Code under

Because you haven't changed the layout in your panel, don't use BorderLayout.

If you really want to use BorderLayout rarther than FlowLayout add just setLayout() command in it.

//Code apove
label = new JLabel("Hello World!", SwingConstants.CENTER);
label.setFont(new Font("Arial", Font.BOLD, 20));
label.setForeground(new Color(0x009900));
setLayout(new BorderLayout()); //!!
add(label, BorderLayout.CENTER);
add(b1,BorderLayout.????); //Edited
//Code under

@Edit - add your button too after setLayout() method or set the layout in the beginning when you make you panel