Cover by Patrick Niemeyer, Daniel Leuck

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GridBagLayout

GridBagLayout is a very flexible layout manager that allows you to position components relative to one another using constraints. With GridBagLayout (and a fair amount of effort), you can create almost any imaginable layout. Components are arranged at logical coordinates on an abstract grid. We call them “logical” coordinates because they designate positions in the space of rows and columns formed by the set of components. Rows and columns of the grid stretch to different sizes, based on the sizes and constraints of the components they hold.

A row or column in a GridBagLayout expands to accommodate the dimensions and constraints of the largest component it contains. Individual components may also be told to span more than one row or column. Components that aren’t as large as their grid cell can be anchored (positioned to one side) within their cell. They can also be set to fill or expand their size in either dimension. Extra area in the grid rows and columns can be parceled out according to the weight constraints of the components. In this way, you can control how various components will grow and stretch when a window is resized.

GridBagLayout is much easier to use in a graphical WYSIWYG GUI builder environment. That’s because working with GridBag is kind of like messing with the old rabbit-ears antennae on your television. It’s not particularly difficult to get the results that you want through trial and error, but writing out hard and fast rules for how to go about it is difficult. In short, GridBagLayout is complex and has some quirks. It is also simply a bit ugly both in model and implementation. Remember that you can do a lot with nested panels and by composing simpler layout managers within one another. If you look back through this chapter, you’ll see some examples of composite layouts; it’s up to you to determine how far you should go before making the break from simpler layout managers to a more complex all-in-one layout manager like GridBagLayout.

The GridBagConstraints Class

Having stated that GridBagLayout is complex and a bit ugly, we’re going to contradict ourselves a little and say that its API is surprisingly simple. There is only one constructor with no arguments—GridBagLayout()—and there aren’t a lot of fancy methods to control how the display works.

The appearance of a grid bag layout is controlled by sets of GridBagConstraints, and that’s where things get hairy. Each component that is managed by a GridBagLayout is associated with a GridBagConstraints object. GridBagConstraints holds the following variables, which we’ll describe in detail shortly:

int gridx
int gridy

Controls the position of the component on the layout’s grid

int weightx
int weighty

Controls how additional space in the row or column is allotted to the component

int fill

Controls whether the component expands to fill the allotted space

int gridheight
int gridwidth

Controls the number of rows or columns the component spans

int anchor

Controls the position of the component if there is extra room within the allotted space

int ipadx
int ipady

Controls padding between the component and the borders of its area

Insets insets

Controls padding between the component and neighboring components

To make a set of constraints for a component or components, create a new instance of GridBagConstraints and set these public variables to the appropriate values. (There is also a large constructor that takes all 11 arguments.)

The easiest way to associate a set of constraints with a component is to use the version of add() that takes both a component object and a layout object as arguments. This puts the component in the container and associates the GridBagConstraints object with it:

    Container content = getContentPane();
    JComponent component = new JLabel("constrain me, please...");
    GridBagConstraints constraints = new GridBagConstraints();
    constraints.gridx = x;
    constraints.gridy = y;
    ...
    content.add(component, constraints);

You can also add a component to a GridBagLayout using the single argument add() method and then calling the layout’s setConstraints() method directly to pass it the GridBagConstraints object for that component:

    add(component);
    ...
    myGridBagLayout.setConstraints(component, constraints);

In either case, the set of constraints is copied when it is applied to the component. It’s the individual constraints that apply to the component, not the GridBagConstraints object. Therefore, you’re free to create a single set of GridBagConstraints, modify it as needed, and apply it as needed to different objects. You might want to create a helper method that sets the constraints appropriately, then adds the component with its constraints to the layout. That’s the approach we’ll take in our examples; our helper method is called addGB(), and it takes a component plus a pair of coordinates as arguments. These coordinates become the gridx and gridy values for the constraints. We could expand upon this later and overload addGB() to take more parameters for other constraints that we often change from component to component.

Grid Coordinates

One of the biggest surprises in the GridBagLayout is that there’s no way to specify the size of the grid. There doesn’t have to be. The grid size is determined implicitly by the constraints of all the objects; the layout manager picks dimensions large enough so that everything fits. Thus, if you put one component in a layout and set its gridx and gridy constraints each to 25, the layout manager creates a virtual 25 × 25 grid, with rows and columns numbered from 0 to 24. If you then add a second component with a gridx of 30 and a gridy of 13, the virtual grid’s dimensions change to 30 × 25. You don’t have to worry about setting up an appropriate number of rows and columns. The layout manager does it automatically as you add components.

With this knowledge, we’re ready to create some simple displays. We’ll start by arranging a group of components in a cross shape. We maintain explicit x and y local variables, setting them as we add the components to our grid. This is partly for clarity, but it can be a handy technique when you want to add a number of components in a row or column. You can simply increment gridx or gridy before adding each component. This is a simple and problem-free way to achieve relative placement. (Later, we’ll describe GridBagConstraints’s RELATIVE constant, which performs relative placement automatically.) The following code shows the first layout (see Figure 19-7):

    //file: GridBag1.java
    import java.awt.*;
    import java.awt.event.*;
    import javax.swing.*;

    public class GridBag1 extends JPanel {
      GridBagConstraints constraints = new GridBagConstraints();

      public GridBag1() {
        setLayout(new GridBagLayout());
        int x, y;  // for clarity
        addGB(new JButton("North"),  x = 1, y = 0);
        addGB(new JButton("West"),   x = 0, y = 1);
        addGB(new JButton("Center"), x = 1, y = 1);
        addGB(new JButton("East"),   x = 2, y = 1);
        addGB(new JButton("South"),  x = 1, y = 2);
      }

      void addGB(Component component, int x, int y) {
        constraints.gridx = x;
        constraints.gridy = y;
        add(component, constraints);
      }

      public static void main(String[] args) {
        JFrame frame = new JFrame("GridBag1");
        frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
        frame.setSize(225, 150);
        frame.setLocation(200, 200);
        frame.setContentPane(new GridBag1());
        frame.setVisible(true);
      }
    }
A simple GridBagLayout

Figure 19-7. A simple GridBagLayout

The buttons in this example are “clumped” together in the center of their display area. Each button is displayed at its preferred size, without stretching to fill the available space. This is how the layout manager behaves when the “weight” constraints are left unset. We’ll talk more about weights in the next two sections.

The fill Constraint

Let’s make the buttons expand to fill the entire JFrame window. To do so, we must take two steps: we must set the fill constraint for each button to the value BOTH, and we must set the weightx and weighty to nonzero values, as shown in this example:

    //file: GridBag2.java
    import java.awt.*;
    import java.awt.event.*;
    import javax.swing.*;

    public class GridBag2 extends JPanel {
      GridBagConstraints constraints = new GridBagConstraints();

      public GridBag2() {
        setLayout(new GridBagLayout());
        constraints.weightx = 1.0;
        constraints.weighty = 1.0;
        constraints.fill = GridBagConstraints.BOTH;
        int x, y;  // for clarity
        addGB(new JButton("North"),  x = 1, y = 0);
        addGB(new JButton("West"),   x = 0, y = 1);
        addGB(new JButton("Center"), x = 1, y = 1);
        addGB(new JButton("East"),   x = 2, y = 1);
        addGB(new JButton("South"),  x = 1, y = 2);
      }

      void addGB(Component component, int x, int y) {
        constraints.gridx = x;
        constraints.gridy = y;
        add(component, constraints);
      }

      public static void main(String[] args) {
        JFrame frame = new JFrame("GridBag2");
        frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
        frame.setSize(225, 150);
        frame.setLocation(200, 200);
        frame.setContentPane(new GridBag2());
        frame.setVisible(true);
      }
    }

Figure 19-8 shows the resulting layout.

Making buttons fill the available space

Figure 19-8. Making buttons fill the available space

BOTH is one of the constants of the GridBagConstraints class; it tells the component to fill the available space in both directions. Here are the constants you can use to set the fill field:

HORIZONTAL

Fill the available horizontal space.

VERTICAL

Fill the available vertical space.

BOTH

Fill the available space in both directions.

NONE

Don’t fill the available space; display the component at its preferred size.

We set the weight constraints to 1.0; in this example, it doesn’t matter what they are, provided each component has the same nonzero weight. Filling doesn’t occur if the component’s weight in the direction you’re filling is 0, which is the default value.

Spanning Rows and Columns

One of the most important features of GridBaglayout is that it lets you create arrangements in which components span two or more rows or columns. To do so, set the gridwidth and gridheight variables of the GridBagConstraints. The following example creates such a display; button one spans two columns vertically and button four spans two horizontally. Figure 19-9 shows the resulting layout.

    //file: GridBag3.java
    import java.awt.*;
    import java.awt.event.*;
    import javax.swing.*;

    public class GridBag3 extends JPanel {
      GridBagConstraints constraints = new GridBagConstraints();

      public GridBag3() {
        setLayout(new GridBagLayout());
        constraints.weightx = 1.0;
        constraints.weighty = 1.0;
        constraints.fill = GridBagConstraints.BOTH;
        int x, y;  // for clarity
        constraints.gridheight = 2; // span two rows
        addGB(new JButton("one"),   x = 0, y = 0);
        constraints.gridheight = 1; // set it back
        addGB(new JButton("two"),   x = 1, y = 0);
        addGB(new JButton("three"), x = 2, y = 0);
        constraints.gridwidth = 2; // span two columns
        addGB(new JButton("four"),  x = 1, y = 1);
        constraints.gridwidth = 1; // set it back
      }

      void addGB(Component component, int x, int y) {
        constraints.gridx = x;
        constraints.gridy = y;
        add(component, constraints);
      }

      public static void main(String[] args) {
        JFrame frame = new JFrame("GridBag3");
        frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
        frame.setSize(200, 200);
        frame.setLocation(200, 200);
        frame.setContentPane(new GridBag3());
        frame.setVisible(true);
      }
    }
Making components span rows and columns

Figure 19-9. Making components span rows and columns

The size of each element is controlled by the gridwidth and gridheight values of its constraints. For button one, we set gridheight to 2; therefore, it is two cells high. Its gridx and gridy positions are both 0, so it occupies cell (0,0) and the cell directly below it, (0,1). Likewise, button four has a gridwidth of 2 and a gridheight of 1, so it occupies two cells horizontally. We place this button in cell (1,1), so it occupies that cell and its neighbor, (2,1).

In this example, we set the fill to BOTH and weightx and weighty to 1 for all components. By doing so, we tell each button to occupy all the space available and give them all equal weighting. Strictly speaking, this isn’t necessary. However, it makes it easier to see exactly how much space each button occupies.

Weighting

The weightx and weighty variables of a GridBagConstraints object determine how “extra” space in the container is distributed among the columns or rows in the layout. As long as you keep things simple, the effect these variables have is fairly intuitive: the larger the weight, the greater the amount of space allocated to the component, relative to its peers. Figure 19-10 shows what happens if we vary the weightx constraint from 0.1 to 1.0 as we place three buttons in a row.

Here’s the code:

    //file: GridBag4.java
    import java.awt.*;
    import java.awt.event.*;
    import javax.swing.*;

    public class GridBag4 extends JPanel {
      GridBagConstraints constraints = new GridBagConstraints();

      public GridBag4() {
        setLayout(new GridBagLayout());
        constraints.fill = GridBagConstraints.BOTH;
        constraints.weighty = 1.0;
        int x, y; // for clarity
        constraints.weightx = 0.1;
        addGB(new JButton("one"),   x = 0, y = 0);
        constraints.weightx = 0.5;
        addGB(new JButton("two"),   ++x,   y);
        constraints.weightx = 1.0;
        addGB(new JButton("three"), ++x,   y);
      }

      void addGB(Component component, int x, int y) {
        constraints.gridx = x;
        constraints.gridy = y;
        add(component, constraints);
      }

      public static void main(String[] args) {
        JFrame frame = new JFrame("GridBag4");
        frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
        frame.setSize(300, 100);
        frame.setLocation(200, 200);
        frame.setContentPane(new GridBag4());
        frame.setVisible(true);
      }
    }
Using weight to control component size

Figure 19-10. Using weight to control component size

The specific values of the weights are not meaningful; it is only their relative proportions that matter. After the preferred sizes of the components (including padding and insets—see the next section) are determined, any extra space is doled out in proportion to the component’s weights. For example, if each of our three components had the same weight, each would receive a third of the extra space. To make this more obvious, you may prefer to express the weights for a row or column as fractions totaling 1.0—for example: 0.25, 0.25, 0.50. Components with a weight of 0 receive no extra space.

The situation is a bit more complicated when there are multiple rows or columns and when there is even the possibility of components spanning more than one cell. In the general case, GridBagLayout calculates an effective overall weight for each row and column and then distributes the extra space to them proportionally. Note that the previous single-row example is just a special case where the columns each have one component. The gory details of the calculations follow.

Calculating the weights of rows and columns

For a given row or column (“rank”), GridBagLayout first considers the weights of all the components contained strictly within that rank—ignoring those that span more than one cell. The greatest individual weight becomes the overall weight of the row or column. Intuitively, this means that GridBagLayout is trying to accommodate the needs of the weightiest component in that rank.

Next, GridBagLayout considers the components that occupy more than one cell and things get a little weird. GridbagLayout wants to evaluate them to see whether they affect the determination of the largest weight in a row or column. However, because these components occupy more than one cell, GridBagLayout divides their weight among the ranks (rows or columns) that they span.

GridBagLayout tries to calculate an effective weight for the portion of the component that occupies each row or column. It does this by trying to divide the weight of the component among the ranks in the same proportions that the length (or height) of the component will be shared by the ranks. But how does it know what the proportions will be before the whole grid is determined? That’s what it’s trying to calculate, after all. It simply guesses based on the row or column weights already determined. GridBagLayout uses the weights determined by the first round of calculations to split up the weight of the component over the ranks that it occupies. For each row or column, it then considers that fraction of the weight to be the component’s weight for that rank. That weight then contends for the “heaviest weight” in the row or column, possibly changing the overall weight of that row or column, as we described earlier.

Anchoring

If a component is smaller than the space available for it, it is centered by default. But centering isn’t the only possibility. The anchor constraint tells a grid bag layout how to position a component within its cell in the grid. Possible values are GridBagConstraints.CENTER, NORTH, NORTHEAST, EAST, SOUTHEAST, SOUTH, SOUTHWEST, WEST, and NORTHWEST. For example, an anchor of GridBagConstraints.NORTH centers a component at the top of its display area; SOUTHEAST places a component at the bottom-right corner of its area.

Padding and Insets

Another way to control the behavior of a component in a grid bag layout is to use padding and insets. Padding is determined by the ipadx and ipady fields of GridBagConstraints. They specify horizontal and vertical “growth factors” for the component. In Figure 19-11, the West button is larger because we have set the ipadx and ipady values of its constraints to 25. Therefore, the layout manager gets the button’s preferred size and adds 25 pixels in each direction to determine the button’s actual size. The sizes of the other buttons are unchanged because their padding is set to 0 (the default), but their spacing is different. The West button is unnaturally tall, which means that the middle row of the layout must be taller than the others.

    //file: GridBag5.java
    import java.awt.*;
    import java.awt.event.*;
    import javax.swing.*;

    public class GridBag5 extends JPanel {
      GridBagConstraints constraints = new GridBagConstraints();

      public GridBag5() {
        setLayout(new GridBagLayout
               ());
        int x, y;  // for clarity
        addGB(new JButton("North"),  x = 1, y = 0);
        constraints.ipadx = 25;  // add padding
        constraints.ipady = 25;
        addGB(new JButton("West"),   x = 0, y = 1);
        constraints.ipadx = 0;   // remove padding
        constraints.ipady = 0;
        addGB(new JButton("Center"), x = 1, y = 1);
        addGB(new JButton("East"),   x = 2, y = 1);
        addGB(new JButton("South"),  x = 1, y = 2);
      }

      void addGB(Component component, int x, int y) {
        constraints.gridx = x;
        constraints.gridy = y;
        add(component, constraints);
      }

      public static void main(String[] args) {
        JFrame frame = new JFrame("GridBag5");
        frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );

        frame.setSize(250, 250);
        frame.setLocation(200, 200);
        frame.setContentPane(new GridBag5());
        frame.setVisible(true);
      }
    }
Using padding and insets in a layout

Figure 19-11. Using padding and insets in a layout

Notice that the horizontal padding, ipadx, is added on both the left and right sides of the button. Therefore, the button grows horizontally by twice the value of ipadx. Likewise, the vertical padding, ipady, is added on both the top and the bottom.

Insets add space between the edges of the component and its cell. They are stored in the insets field of GridBagConstraints, which is an Insets object. An Insets object has four fields to specify the margins on the component’s top, bottom, left, and right. The relationship between insets and padding can be confusing. As shown in Figure 19-12, padding is added to the component itself, increasing its size. Insets are external to the component and represent the margin between the component and its cell.

The relationship between padding and insets

Figure 19-12. The relationship between padding and insets

Padding and weighting have an odd interaction with each other. If you use padding, it’s best to use the default weightx and weighty values for each component.

Relative Positioning

In all our grid bag layouts so far, we have specified the gridx and gridy coordinates of each component explicitly using its constraints. Another alternative is relative positioning.

Conceptually, relative positioning is simple: we just say, “put this component to the right of (or below) the previous component.” To do so, you can set gridx or gridy to the constant GridBagConstraints.RELATIVE. Unfortunately, it’s not as simple as this. Here are a couple of warnings:

  • To place a component to the right of the previous one, set gridx to RELATIVEand use the same value for gridy that you used for the previous component.

  • Similarly, to place a component below the previous one, set gridy to RELATIVEand leave gridx unchanged.

  • Setting both gridx and gridy to RELATIVE places all the components in one row, not in a diagonal line, as you might expect. (This is the default.)

In other words, if gridx or gridy is RELATIVE, you had better leave the other value unchanged. RELATIVE makes it easy to arrange a lot of components in a row or a column. That’s what it was intended for; if you try to do something else, you’re fighting against the layout manager, not working with it.

GridBagLayout allows another kind of relative positioning in which you specify where, in a row or a column, the component should be placed overall using the gridwidth and gridheight fields of GridBagConstraints. Setting either of these to the constant REMAINDER says that the component should be the last item in its row or column and, therefore, should occupy all the remaining space. Setting either gridwidth or gridheight to RELATIVE says that it should be the second to the last item in its row or column. Unfortunately, you can use these constants to create constraints that can’t possibly be met; for example, you can say that two components must be the last component in a row. In these cases, the layout manager tries to do something reasonable, but it will almost certainly be something you don’t want done.

Composite Layouts

Sometimes things don’t fall neatly into little boxes. This is true of layouts as well as life. For example, if you want to use some of GridBagLayout’s weighting features for part of your GUI, you could create separate layouts for different parts of the GUI and combine them with yet another layout. That’s how we’ll build the pocket calculator interface in Figure 19-13. We will use three grid bag layouts: one for the first row of buttons (C, %, +), one for the last (0, ., =) and one for the window itself. The master layout (the window’s) manages the text field we use for the display, the panels containing the first and last rows of buttons, and the 12 buttons in the middle.[43]

The Calculator application

Figure 19-13. The Calculator application

Here’s the code for the Calculator example. It implements only the user interface (i.e., the keyboard); it collects everything you type in the display field until you press C (clear). Figuring out how to connect the GUI to some other code that would perform the operations is up to you. One strategy would be to send an event to the object that does the computation whenever the user presses the equals sign. That object could read the contents of the text field, parse it, do the computation, and display the results.

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

    public class Calculator extends JPanel implements ActionListener {
      GridBagConstraints gbc = new GridBagConstraints();
      JTextField theDisplay = new JTextField();

      public Calculator() {
        gbc.weightx = 1.0;  gbc.weighty = 1.0;
        gbc.fill = GridBagConstraints.BOTH;
        ContainerListener listener = new ContainerAdapter() {
          public void componentAdded(ContainerEvent e) {
            Component comp = e.getChild();
            if (comp instanceof JButton)
              ((JButton)comp).addActionListener(Calculator.this);
          }
        };
        addContainerListener(listener);
        gbc.gridwidth = 4;
        addGB(this, theDisplay, 0, 0);
        // make the top row
        JPanel topRow = new JPanel();
        topRow.addContainerListener(listener);
        gbc.gridwidth = 1;
        gbc.weightx = 1.0;
        addGB(topRow, new JButton("C"), 0, 0);
        gbc.weightx = 0.33;
        addGB(topRow, new JButton("%"), 1, 0);
        gbc.weightx = 1.0;
        addGB(topRow, new JButton("+"), 2, 0 );
        gbc.gridwidth = 4;
        addGB(this, topRow, 0, 1);
        gbc.weightx = 1.0;  gbc.gridwidth = 1;
        // make the digits
        for(int j=0; j<3; j++)
            for(int i=0; i<3; i++)
                addGB(this, new JButton("" + ((2-j)*3+i+1) ), i, j+2);
        // -, x, and divide
        addGB(this, new JButton("-"), 3, 2);
        addGB(this, new JButton("x"), 3, 3);
        addGB(this, new JButton("\u00F7"), 3, 4);
        // make the bottom row
        JPanel bottomRow = new JPanel();
        bottomRow.addContainerListener(listener);
        gbc.weightx = 1.0;
        addGB(bottomRow, new JButton("0"), 0, 0);
        gbc.weightx = 0.33;
        addGB(bottomRow, new JButton("."), 1, 0);
        gbc.weightx = 1.0;
        addGB(bottomRow, new JButton("="), 2, 0);
        gbc.gridwidth = 4;
        addGB(this, bottomRow, 0, 5);
      }

      void addGB(Container cont, Component comp, int x, int y) {
        if ((cont.getLayout() instanceof GridBagLayout) == false)
          cont.setLayout(new GridBagLayout());
        gbc.gridx = x; gbc.gridy = y;
        cont.add(comp, gbc);
      }

      public void actionPerformed(ActionEvent e) {
        if (e.getActionCommand().equals("C"))
          theDisplay.setText("");
        else
          theDisplay.setText(theDisplay.getText()
                             + e.getActionCommand());
      }

      public static void main(String[] args) {
        JFrame frame = new JFrame("Calculator");
        frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
        frame.setSize(200, 250);
        frame.setLocation(200, 200);
        frame.setContentPane(new Calculator());
        frame.setVisible(true);
      }
    }

Once again, we use an addGB() helper method to add components with their constraints to the layout. Before discussing how to build the layout, let’s take a look at addGB(). We said earlier that three layout managers are in our user interface: one for the application panel itself, one for the panel containing the first row of buttons (topRow), and one for the panel containing the bottom row of buttons (bottomRow). We use addGB() for all three layouts; its first argument specifies the container to which to add the component. Thus, when the first argument is this, we’re adding an object to the content pane of the JFrame. When the first argument is topRow, we’re adding a button to the first row of buttons. addGB() first checks the container’s layout manager and sets it to GridBagLayout if it isn’t already set properly. It sets the object’s position by modifying a set of constraints, gbc, and then uses these constraints to add the object to the container.

We use a single set of constraints throughout the example, modifying fields as we see fit. The constraints are initialized in Calculator’s constructor. Before calling addGB(), we set any fields of gbc for which the defaults are inappropriate. Thus, for the answer display, we set the grid width to 4 and add the answer display directly to the application panel (this). The add() method, which is called by addGB(), makes a copy of the constraints, so we’re free to reuse gbc throughout the application.

The first and last rows of buttons motivate the use of multiple GridBagLayout containers, each with its own grid. These buttons appear to straddle grid lines, but you really can’t accomplish this using a single grid. Therefore, topRow has its own layout manager, with three horizontal cells, allowing each button in the row to have a grid width of 1. To control the size of the buttons, we set the weightx variables so that the clear and plus buttons take up more space than the percent button. We then add the topRow as a whole to the application, with a grid width of 4. The bottom row is built similarly.

To build the buttons for the digits 1–9, we use a doubly nested loop. There’s nothing particularly interesting about this loop, except that it’s probably a bit too cute. The minus, multiply, and divide buttons are also simple: we create a button with the appropriate label and use addGB() to place it in the application. It’s worth noting that we used a Unicode constant to request a real division sign rather than wimping out and using a slash.

That’s it for the user interface; what’s left is event handling. Each button generates action events; we need to register listeners for these events. We’ll make the application panel, the Calculator, the listener for all the buttons. To register the Calculator as a listener, we’ll be clever. Whenever a component is added to a container, the container generates a ContainerEvent. We use an anonymous inner class ContainerListener to register listeners for our buttons. This means that the Calculator must register as a ContainerListener for itself and for the two panels, topRow and bottomRow. The componentAdded() method is very simple. It calls getChild() to find out what component caused the event (i.e., what component was added). If that component is a button, it registers the Calculator as an ActionListener for that button.

actionPerformed() is called whenever the user presses any button. It clears the display if the user pressed the C button; otherwise, it appends the button’s action command (in this case, its label) to the display.

Combining layout managers is an extremely useful trick. Granted, this example verges on overkill. You won’t often need to create a composite layout using multiple grid bags. Composite layouts are common, however, with BorderLayout; you’ll frequently use different layout managers for each of a border layout’s regions. For example, the CENTER region might be a ScrollPane, which has its own special-purpose layout manager; the EAST and SOUTH regions might be panels managed by grid layouts or flow layouts, as appropriate.



[43] If you’re curious, this calculator is based on the ELORG-801, encountered in an online “calculator museum”.

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