Learning Java, 4th Edition

BorderLayout

BorderLayout is a little more interesting. It tries to arrange objects in one of five geographical locations, represented by constants in the BorderLayout class: NORTH, SOUTH, EAST, WEST, and CENTER, optionally with some padding between. BorderLayout is the default layout for the content panes of JWindow and JFrame objects. Because each component is associated with a direction, BorderLayout can manage at most five components; it squashes or stretches those components to fit its constraints. As we’ll see in the second example, this means that you often want to have BorderLayout manage sets of components in their own panels.

When we add a component to a container with a border layout, we need to specify both the component and the position at which to add it. To do so, we use an overloaded version of the container’s add() method that takes an additional argument as a constraint. The constraint specifies the name of a position within the BorderLayout.

The following application sets a BorderLayout and adds our five buttons again, named for their locations:

    //file: Border1.java
    import java.awt.*;
    import java.awt.event.*;
    import javax.swing.*;

    public class Border1 extends JPanel {

      public Border1() {
        setLayout(new BorderLayout());
        add(new JButton("North"), BorderLayout.NORTH );
        add(new JButton("South"), BorderLayout.SOUTH );
        add(new JButton("East"), BorderLayout.EAST );
        add(new JButton("West"), BorderLayout.WEST );
        add(new JButton("Center"), BorderLayout.CENTER );
      }

      public static void main(String[] args) {
        JFrame frame = new JFrame("Border1");
        frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
        frame.setSize(300, 300);
        frame.setLocation(200, 200);
        frame.setContentPane(new Border1());
        frame.setVisible(true);
      }
    }

The result is shown in Figure 19-4.

Figure 19-4. A border layout

So, how exactly is the area divided up? Well, the objects at NORTH and SOUTH get their preferred height and fill the display area horizontally. EAST and WEST components, on the other hand, get their preferred width and fill the remaining area between NORTH and SOUTH vertically. Finally, the CENTER object takes all the rest of the space. As you can see in Figure 19-4, our buttons get distorted into interesting shapes.

What if we don’t want BorderLayout messing with the sizes of our components? One option would be to put each button in its own JPanel. The default layout for a JPanel is FlowLayout, which respects the preferred size of components. The preferred sizes of the panels are effectively the preferred sizes of the buttons, but if the panels are stretched, they won’t pull their buttons with them. The following application illustrates this approach:

    //file: Border2.java
    import java.awt.*;
    import java.awt.event.*;
    import javax.swing.*;

    public class Border2 extends JPanel {

      public Border2() {
        setLayout(new BorderLayout());
        JPanel p = new JPanel();
        p.add(new JButton("North"));
        add(p, BorderLayout.NORTH);
        p = new JPanel();
        p.add(new JButton("South"));
        add(p, BorderLayout.SOUTH);
        p = new JPanel();
        p.add(new JButton("East"));
        add(p, BorderLayout.EAST);
        p = new JPanel();
        p.add(new JButton("West"));
        add(p, BorderLayout.WEST);
        p = new JPanel();
        p.add(new JButton("Center"));
        add(p, BorderLayout.CENTER);
      }

      public static void main(String[] args) {
        JFrame frame = new JFrame("Border2");
        frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
        frame.setSize(225, 150);
        frame.setLocation(200, 200);
        frame.setContentPane(new Border2());
        frame.setVisible(true);
      }
    }

The result is shown in Figure 19-5.

In the example, we create a number of panels, put our buttons inside the panels, and put the panels into the frame window, which has the BorderLayout manager. Now, the JPanel for the CENTER button soaks up the extra space that comes from the BorderLayout. Each JPanel’s FlowLayout centers the button in the panel and uses the button’s preferred size. In this case, it’s all a bit awkward. We’ll see how we could accomplish this more directly using GridBagLayout shortly.

Figure 19-5. Another border layout