java - Why does the size of the JFrame exceed the size of its ContentPane, even though it has a defined size?

Question

I noticed a strange behavior between containers in the swing.

To exemplify the test, I created a JFrame and a JPanel, and set the panel as contentPane. I defined the preferred and maximum JPanel size to 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 is:

To my surprise, the output at the terminal is:

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

I did not understand why the frame added this extra space, even having nothing in the component that forces the frame to resize.

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

The problem with this is that java is giving me a false info, since the panel is scaling in the size of the frame, and from the above result, we have seen that the two are not the same size.

Are the two measures being reported correctly? Why does this occur?

Answer 1

You may find it helpful to compare the result from a similar program on a different platform. In particular,

• The red Border is inside the panel; your usage conforms to the API recommendation, "we recommend that you put the component in a JPanel and set the border on the JPanel."

• The frame width matches the panel width on Mac OS X because the heavyweight peer used for the top-level container has no vertical border on Mac OS X; the Windows peer has its own border.

• The frame height includes the drag bar and frame border: 38 pixels on Widows, 22 on Mac OS X.

• Don't use setPreferredSize() when you really mean to override getPreferredSize().

• As an aside, the frame's add() method forwards to the content pane.

panel size: [400,300]
frame size: [400,322]

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() {
            @Override
            public Dimension getPreferredSize() {
                return new Dimension(400, 300);
            }
        };
        panel.setBorder(BorderFactory.createLineBorder(Color.RED, 1));
        frame.add(panel);
        frame.pack();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setLocationRelativeTo(null);
        frame.setResizable(false);
        frame.setVisible(true);

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

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