摆动组件未显示在框架上

Question

I don't understand why the RackBuilder object I added to the frame does not get displayed.

The code runs and the frame gets generated. I expect to see a panel with 42 rows, each row containing a JLabel "test". Is there something incorrect/missing from my constructor?

public class RackBuilderTool extends JPanel{

    public RackBuilderTool() {
        super(new GridLayout(42, 0));
        for (int i = 0; i < 42; i++) {
            add(new JLabel("test"));
        }
    }
    private static void createAndShowGUI() {
        JFrame frame = new JFrame("Rack Builder Tool");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        RackBuilderTool rackBuilder = new RackBuilderTool();
        rackBuilder.setOpaque(true);
        frame.setContentPane(rackBuilder);

        frame.pack();
        frame.setVisible(true);
    }

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

Thanks!

Answer 1

import javax.swing.*;
import java.awt.*;
public class RackBuilderTool extends JPanel{

    public RackBuilderTool() {
        super(new GridLayout(42, 0));
        for (int i = 0; i < 42; i++) {
            add(new JLabel("test"));
        }
    }
    private static void createAndShowGUI() {
        JFrame frame = new JFrame("Rack Builder Tool");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        RackBuilderTool rackBuilder = new RackBuilderTool();
        rackBuilder.setOpaque(true);
        frame.setContentPane(rackBuilder);

        frame.pack();
        frame.setVisible(true);
    }

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

now it will show you 42 rows of your label.

Answer 2

Realized that the "run" button on Netbeans IDE is to run the entire project. As a result it was running another java file under the same project.

Once I right clicked on the java file I wanted to compile and clicked run it worked.

Thanks everyone for the help.