无法在JFrame中添加多个JPanel，

Question

I have a following JFrame.

public class AddActivityWindow extends JFrame {

    //declaration of components go here
     ...
    public AddActivityWindow(ActivityStore store) {
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        JPanel pnl_date = new JPanel();
        JLabel lbl_date = new JLabel("X");
        pnl_date.add(lbl_date);
        add(pnl_date);


        pnl_activities = new JPanel();
        JLabel lbl_act = new JLabel("Y");
        pnl_activities.add(lbl_act);
        add(pnl_activities);

        setLocationRelativeTo(null);
        setVisible(true);

    }
}

When I create this Frame I was expecting that it would create a frame with two panels. But, I only see the second panel (ie) I only see Y on the screen and not X . However, if I remove the code for the second panel, then I can see X .

What happens when I add the second panel. Why does the first panel not show up in the frame?

Answer 1

The default layout of a JFrame is a BorderLayout . A characteristic of the BorderLayout is that it can only contain one component in each region. If you just use the add method without specifying any constraints, it is the same as adding the component to the CENTER . So your second add call replaces the element you first added.

Possible solutions: specify constraints, eg

add(pnl_date, BorderLayout.NORTH);
//...
add(pnl_activities, BorderLayout.CENTER);

or switch to another layout manager (eg a BoxLayout or a FlowLayout ) for your JFrame . It depends on your needs. Take a look at the Visual guide to layout managers which is a good starting point for choosing a manager.

Answer 2

You should be using a different Layout Manager, Have a look at : A Visual Guide to Layout Managers to select one for you.

This works fine for me ,

import java.awt.FlowLayout;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;

public class FrameTest extends JFrame {

    public FrameTest() {
        setLayout(new FlowLayout());
        setSize(150, 150);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        JPanel pnl_date = new JPanel();
        JLabel lbl_date = new JLabel("X");
        pnl_date.add(lbl_date);
        add(pnl_date);


        JPanel pnl_activities = new JPanel();
        JLabel lbl_act = new JLabel("Y");
        pnl_activities.add(lbl_act);
        add(pnl_activities);

        setLocationRelativeTo(null);
        setVisible(true);

    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {

            @Override
            public void run() {
                new FrameTest().setVisible(true);

            }
        });
    }
}