Why does the Jframe size exceed the size of your Contentpane, even though it has a defined size?

Question

I noticed a strange behavior between containers in the swing.

To illustrate the test, I created a JFrame and a JPanel, and defined the panel as contentPane of JFrame. Set the preferred and maximum size of the JPanel as 400,300. All this can be seen in the example below:

import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;

import javax.swing.BorderFactory;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class ExcessiveSpacePanelTest {
    JFrame frame;
    JPanel panel;

    public void initGUI(){

        frame =  new JFrame();

        panel = new JPanel();
        panel.setBorder(BorderFactory.createLineBorder(Color.RED, 1));
        panel.setPreferredSize(new Dimension(400, 300));
        panel.setMaximumSize(new Dimension(400, 300));


        frame.setContentPane(panel);
        frame.pack();

        System.out.println("panel size: [" + panel.getSize().width + "," + panel.getSize().height +"]");
        System.out.println("frame size: [" + frame.getSize().width + "," + frame.getSize().height+"]");

        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setLocationRelativeTo(null);
        frame.setResizable(false);
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        EventQueue.invokeLater(() ->{
            new ExcessiveSpacePanelTest().initGUI();
        });
    }
}

The result of the screen is:

To my surprise, the exit at the terminal is:

panel size: [400,300]
frame size: [416,338]

I don’t understand why the frame added this extra space, even though it has nothing in the component that forces the frame to resize.

I tried to set zeroed edges on the panel by adding the line panel.setBorder(BorderFactory.createEmptyBorder(0, 0, 0, 0)); before the pack()but still the result is the same.

The problem with this is that java is giving me a false information, since the panel is sizing in the frame size, and by the result above, we saw that the two do not have the same size.

Are the two measurements being correctly informed? Why does this occur?

Answer 1

That’s right. This extra space is referenced the edges of the window, these edges count in size.

For example, in Windows 10, windows have a 1px border on each side. Other than that, there is another invisible 7px edge to hold the mouse pointer when changing the window size. Adding up these edges, there are 16, which is exactly the difference between the usable area of the Frame the actual size of it in my example.

You can capture the size of Jframe’s "usable" area using getContentPane() even without pre-defining a component to be the ContentPane. I mean, there’s already a Panel which occupies the maximum "usable" window size.

Take an example

public void initGUI(){
    frame =  new JFrame();

    frame.setPreferredSize(new Dimension(400, 300));
    frame.pack();

    System.out.println("Content pane size: [" + frame.getContentPane().getSize().width + ", " + frame.getContentPane().getSize().height + "]");
    System.out.println("frame size: [" + frame.getSize().width + "," + frame.getSize().height+"]");

    frame.setVisible(true);
}

The exit is:

Content pane size: [384, 261]
frame size: [400,300]