在JFrame上看不到JPanel

Question

I am trying to add a JPanel onto the JFrame but I cannot see the panel. I also am trying to add a JTable to the panel but after I can get this working. Here is my code:

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

public class table
{

    public table(){

        JFrame frame = new JFrame();
        frame.setLayout(new BorderLayout());
        frame.setSize(500,300);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        JPanel panel = new JPanel();
        frame.add(panel, BorderLayout.CENTER);
        frame.setVisible(true);

    }
}

In another class, I have the code:

public class Main {

    public static void main(String[] args){
        new table();
    }

}

Here is a screenshot of the JFrame with no JPanel (I think).

Answer 1

If you take a look at the docs for Window.setVisible() , you might notice a note about the input parameter that says

The Window will be validated prior to being made visible.

Validation is the key to making new components show up when you add them to a window. You have two options:

1. Move the call to setVisible to the end of your code, once everything is added to the window:

   \nJFrame frame = new JFrame(); \nframe.setLayout(new BorderLayout()); \nframe.setSize(500,300); \nframe.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); \n\nJPanel panel = new JPanel(); \nframe.add(panel, BorderLayout.CENTER); \n \n

2. Re-validate the frame yourself after adding the panel:

   \nJFrame frame = new JFrame(); \nframe.setLayout(new BorderLayout()); \nframe.setSize(500,300); \nframe.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); \nframe.setVisible(true); \nJPanel panel = new JPanel(); \nframe.add(panel, BorderLayout.CENTER); \n \n

A note from the docs for Container.validate() :

Validating the container may be a quite time-consuming operation. For performance reasons a developer may postpone the validation of the hierarchy till a set of layout-related operations completes, eg after adding all the children to the container.

Using the first option is probably better.

Clarification

Keep in mind that the background color of a JPanel is the same as the background color of the JFrame 's content pane by default. You will not be able to determine that it was added correctly visually. At @WKS's suggestion, you can change the panel's color so you can see it clearly in the frame:

JFrame frame = new JFrame();
frame.setLayout(new BorderLayout());
frame.setSize(500,300);
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
JPanel panel = new JPanel();

frame.add(panel, BorderLayout.CENTER);

Don't forget to import java.awt.Color; in this case.

Answer 2

尝试为面板提供背景颜色：

panel.setBackground(Color.BLUE);