How does one stretch a JButton to the full height of a JPanel

To summarize my problem, I am developing an interface for an application in Java. I have run into a problem I haven't previously faced yet. To create a sort of "navigation bar" i created a thin JPanel at the top of the JFrame. Then I wanted to add a JButton to it to represent the "home" button which leads to the starting content of the application. When I did that and made width as well as height of the Button equal to the height of the Panel, the Button had a little gap of approximately 3 pixels to the top of the Panel. I don't know why that is, there is no other element inside the Panel. I am not using any layout, however i have tried to fix that problem using a FlowLayout but nothing really changed.

Here is the full code for the interface, i marked the menu panel with red color for testing purpose only. How do I get rid of that small gap to the top?

package UserInterface;

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Toolkit;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class Interface {

private JFrame frame;
private JPanel menu;
private JPanel content;

public JButton home;

public Interface() {

    Dimension dim = new Dimension(50, 50);

    frame = new JFrame();
    menu = new JPanel();
    content = new JPanel();

    frame.setSize(1600, 900);
    menu.setSize(1600, 50);
    content.setSize(1600, 850);

    frame.setTitle("Tierland");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setLocation(
            (int)((Toolkit.getDefaultToolkit().getScreenSize().getWidth() / 2) - (frame.getWidth() / 2)),
            (int)((Toolkit.getDefaultToolkit().getScreenSize().getHeight() / 2) - (frame.getHeight() / 2))
    );

    frame.setSize(1600, 900);
    menu.setSize(1600, 50);
    content.setSize(1600, 850);

    menu.setBackground(Color.RED);

    home = new JButton();
    home.setPreferredSize(dim);
    home.setMinimumSize(dim);
    home.setMaximumSize(dim);
    home.setSize(dim);


    frame.add(menu);
    frame.add(content);

    frame.setVisible(true);
    menu.setVisible(true);
    content.setVisible(true);

}

}

Solution

I am not using any layout,

Yes you are. By default a JPanel uses a FlowLayout.

however i have tried to fix that problem using a FlowLayout but nothing really changed.

That is because the default FlowLayout uses a 5 pixel gap around any component added to the panel.

You should be using:

menu.setLayout( new FlowLayout(...) );

Read the FlowLayout API for the appropriate constructor that will allow you to use 0, for the horizontal and/or vertical gap.

frame.add(menu);
frame.add(content);

Also, by default, the content pane of the frame uses a BorderLayout, so you should be using:

frame.add(menu, BorderLayout.PAGE_START);
frame.add(content, BorderLayout.CENTER);

You should not be attempting to control the size of the button:

//home.setPreferredSize(dim);
//home.setMinimumSize(dim);
//home.setMaximumSize(dim);

Just set the text of the button or add an Icon to the button and the button will determine its own preferred size.

Also, you really should call your "menu" panel a "toolbar" panel. Swing already supports menus and there is a special method to add a menu bar to the frame. Read the section from the Swing tutorial on How to Use Menus for more information.