Container Insets

Every component has a set of insets. Insets specify the widths of the component's margins. Insets include:

For example, a frame's insets include a top inset that compensates for the frame's titlebar, and insets all around that compensate for the border (which we can use to resize the frame).

A panel contained within a frame has its own insets. Indeed, any component has its own insets. Usually, however, you only have to worry about insets for containers -- unless you're designing a new GUI component.

Layout managers are supposed to respect a container's insets and never lay out a component in the inset margins. The layout managers supplied with the JDK all respect this. If you write a layout manager, you should write it to respect insets, too.

Note: It seems that the use of insets is now deprecated, in favor of borders. We'll discuss borders, which are much richer and provide much greater flexibility, shortly.

package swingExamples;

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Container;
import java.awt.Insets;

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

class BorderButtonsInsetsPanel extends JPanel
{
  public BorderButtonsInsetsPanel()
  {
    setBackground(Color.GREEN);
    setLayout(new BorderLayout());
    add(m_north, BorderLayout.NORTH);
    add(m_south, BorderLayout.SOUTH);
    add(m_east, BorderLayout.EAST);
    add(m_west, BorderLayout.WEST);
    add(m_center, BorderLayout.CENTER);
  }
  
  // Override this method to specify the panel's
  // insets (top, left, bottom, right).
  public Insets getInsets()
  {
    return new Insets(5, 10, 15, 20);
  }
  
  ////////////// Data //////////////////
  private JButton m_north = new JButton("North");
  private JButton m_south = new JButton("South");
  private JButton m_east = new JButton("East");
  private JButton m_west = new JButton("West");
  private JButton m_center = new JButton("Center");
}

class BorderButtonsInsetsFrame extends JFrame
{
  public BorderButtonsInsetsFrame()
  {
    setTitle("BorderButtonsInsets");
    setSize(300, 200);
    setDefaultCloseOperation(EXIT_ON_CLOSE);
    Container contentPane = getContentPane();
    contentPane.add( new BorderButtonsInsetsPanel() );
  }
}

public class BorderButtonsInsets
{
  public static void main(String[] args)
  {
    BorderButtonsInsetsFrame frame = 
                  new BorderButtonsInsetsFrame();
    frame.setVisible(true);
  }
}

Note that the inset margins do not have to be the same all the way around. I've emphasized this in the above example, which makes it look a little strange. However, this can be used to advantage in some situations to improve the look of a GUI application.