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Java AWT

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7.6 GridBagLayout

The GridBagLayout is the most complex and flexible of the standard layout managers. Although it sounds like it should be a subclass of GridLayout, it's a different animal entirely. With GridLayout, elements are arranged in a rectangular grid, and each element in the container is sized identically (where possible). With GridBagLayout, elements can have different sizes and can occupy multiple rows or columns. The position and behavior of each element is specified by an instance of the GridBagConstraints class. By properly constraining the elements, you can specify the number of rows and columns an element occupies, which element grows when additional screen real estate is available, and various other restrictions. The actual grid size is based upon the number of components within the GridBagLayout and the GridBagConstraints of those objects. For example, Figure 7.8 shows a GridBagLayout with seven components, arranged on a 3x3 grid. The maximum capacity of a screen using GridBagLayout in Java 1.0 is 128 x 128 cells; in Java 1.1, the maximum size is 512 x 512 cells.

With the other layout managers, adding a component to the container requires only a call to add(). In Java 1.0, the GridBagLayout also requires you to call setConstraints() to tell the layout manager how to position the component. With Java 1.1, you use the new add() method that permits you to pass the component and its constraints in a single method call (add(Component, Object)). If no components are added with constraints (thus all using the defaults), the GridBagLayout places the components in a single row at the center of the screen and sizes them to their getPreferredSize(). This is a nice way to place a single object in the center of the screen without stretching it to take up the available space, as BorderLayout does. Figure 7.9 compares the default GridBagLayout with a BorderLayout displaying the same object in the center region.

When designing a container that will use GridBagLayout, it is easiest to plan what you want on graph paper, and then determine how the constraints should be set. The alternative, adding the components to the layout and then tweaking the constraints until you have something you like, could lead to premature baldness. Seriously, a trial-and-error approach to getting the constraints right will certainly be frustrating and will probably fail. Figure 7.10, using the same GridBagLayout used in Figure 7.8, indicates how the layout manager counts cells. The partial code used to create the screen follows in Example 7.2.

Example 7.2: Creating a GridBagLayout

public void init() {
    Button b;
    GridBagLayout gb = new GridBagLayout();
    GridBagConstraints gbc = new GridBagConstraints();
    setLayout(gb);
    try {
/* Row One - Three button */
        b = new Button ("One");
        addComponent (this, b, 0, 0, 1, 1, 
                GridBagConstraints.NONE, GridBagConstraints.CENTER);
        b = new Button ("Two");
        addComponent (this, b, 1, 0, 1, 1, 
                GridBagConstraints.NONE, GridBagConstraints.CENTER);
        b = new Button ("Three");
        addComponent (this, b, 2, 0, 1, 1, 
                GridBagConstraints.NONE, GridBagConstraints.CENTER);
/* Row Two - Two buttons */
        b = new Button ("Four");
        addComponent (this, b, 0, 1, 2, 1, 
                GridBagConstraints.NONE, GridBagConstraints.CENTER);
        b = new Button ("Five");
        addComponent (this, b, 2, 1, 1, 2,
                GridBagConstraints.NONE, GridBagConstraints.CENTER);
/* Row Three - Two buttons */
        b = new Button ("Six");
        addComponent (this, b, 0, 2, 1, 1, 
                GridBagConstraints.NONE, GridBagConstraints.CENTER);
        b = new Button ("Seven");
        addComponent (this, b, 1, 2, 1, 1, 
                GridBagConstraints.NONE, GridBagConstraints.CENTER);
    } catch (Exception e) {
        e.printStackTrace();
    }
}

Most of the work in Example 7.2 is done by the helper method addComponent(), which creates a set of constraints, applies them to a component, and adds the component to a container. The code for addComponent() appears in GridBagConstraints; its signature is:

public static void addComponent (Container container, Component component,
        int gridx, int gridy, int gridwidth, int gridheight, int fill,
        int anchor) throws AWTException ;

The top left cell in the layout has location (0,0). There's nothing very surprising about buttons one, two, three, six, and seven. They occupy a 1x1 area on the layout's 3x3 grid. Button four occupies a 2x1 area; it is placed at location (0,1), and thus occupies this cell plus the cell at (1,1). Likewise, button five occupies a 1x2 area, and takes up the cells at (2,1) and (2,2). The total size of the layout is determined entirely by the components that are placed in it and their constraints.

GridBagLayout Methods

Variables

There are a handful of instance variables for GridBagLayout. They are not initialized until the container whose layout is GridBagLayout has been validated.

public int columnWidths[]

The columnWidths[] array contains the widths of the components in the row with the most elements. The values of this array are returned by the getLayoutDimensions() method. You can access the array directly, but it is not recommended.

public int rowHeights[]

The rowHeights[] array contains the heights of the components in the column with the most elements. The values of this array are returned by the getLayoutDimensions() method. You can access the array directly, but it is not recommended.

public double columnWeights[]

The columnWeights[] array contains the weightx values of the components in the row with the most elements. The values of this array are returned by the getLayoutWeights() method. You can access the array directly, but it is not recommended.

public double rowWeights[]

The row Weights[] array contains the weighty values of the components in the column with the most elements. The values of this array are returned by the getLayoutWeights() method. You can access the array directly, but it is not recommended.

Constructors

public GridBagLayout ()

The constructor for GridBagLayout creates an instance of GridBagLayout with default GridBagConstraints behavior. An internal table is used to keep track of the components added to the layout.

LayoutManager methods

public void addLayoutComponent (String name, Component component)

The addLayoutComponent() method of GridBagLayout does nothing. This method is not deprecated, unlike the similarly named methods in the other layout managers that implement LayoutManager2.

public void removeLayoutComponent (Component component)

The removeLayoutComponent() method of GridBagLayout does nothing.

public Dimension preferredLayoutSize (Container target)

The preferredLayoutSize() method calculates the preferred dimensions of the components of target. Sizing is based on the constraints of the various components. This task is definitely better off left to the computer.

public Dimension minimumLayoutSize (Container target)

The minimumLayoutSize() method calculates the minimum dimensions required to position the components of target. Sizing is based on the constraints of the various components.

public void layoutContainer (Container target)

The layoutContainer() method positions the components within target based upon the constraints of each component. If a component's anchor constraints are invalid, layoutContainer() throws the run-time exception IllegalArgumentException. The process of arranging the components is very complicated and beyond the scope of this book.

LayoutManager2 methods

public void addLayoutComponent (Component component, Object constraints) (New)

This addLayoutComponent() method of GridBagLayout associates the component with the given constraints object. It calls the setConstaints() method.

If name is not a GridBagConstraints, addLayoutComponent() throws the run-time exception IllegalArgumentException.

public abstract Dimension maximumLayoutSize(Container target) (New)

The maximumLayoutSize() method returns a Dimension object with a width and height of Integer.MAX_VALUE. In practice, this means that GridBagLayout doesn't support the concept of maximum size.

public abstract float getLayoutAlignmentX(Container target) (New)

The getLayoutAlignmentX() method says that GridBagLayout containers should be centered horizontally within the area available.

public abstract float getLayoutAlignmentY(Container target) (New)

The getLayoutAlignmentY() method says that GridBagLayout containers should be centered vertically within the area available.

public abstract void invalidateLayout(Container target) (New)

The invalidateLayout() method of GridBagLayout does nothing.

Constraints

public GridBagConstraints getConstraints (Component component)

The getConstraints() method returns a clone of the current constraints for component. This makes it easier to generate constraints for a component based on another component.

public void setConstraints (Component component, GridBagConstraints constraints)

The setConstraints() method changes the constraints on component to a clone of constraints. The system creates a clone() of constraints so you can change the original constraints without affecting component.

Layout

public Point getLayoutOrigin ()

The getLayoutOrigin() method returns the origin for the GridBagLayout. The origin is the top left point within the container at which the components are drawn. Before the container is validated, getLayoutOrigin() returns the Point (0,0). After validation, getLayoutOrigin() returns the actual origin of the layout. The space used by the components within a GridBagLayout may not fill the entire container. You can use the results of getLayoutOrigin() and getLayoutDimensions() to find the layout's actual size and draw a Rectangle around the objects.

public int[][] getLayoutDimensions ()

The getLayoutDimensions() method returns two one-dimensional arrays as a single two-dimensional array. Index 0 is an array of widths (columnWidths instance variable), while index 1 is an array of heights (rowHeights instance variable). Until the layout is validated, these will be empty. After validation, the first array contains the widths of the components in the row with the most elements. The second contains the heights of the components in the column with the most elements. For Figure 7.10, the results would be (38, 51, 48) for widths since the first row has three elements and (21, 21, 21) for the heights since the first (and second) column has three elements in it.

public double[][] getLayoutWeights ()

The getLayoutWeights() method returns two one-dimensional arrays as a single two-dimensional array. Index 0 is an array of column weights (columnWeights instance variable), while index 1 is an array of row weights (rowWeights instance variable). Until the layout is validated, these will be empty. After validation, the first dimension contains all the weightx values of the components in the row with the most elements. The second dimension contains all the weighty values of the components in the column with the most elements. For Figure 7.10, the results would be (0, 0, 0) for weightx since the first row has three elements and (0, 0, 0) for weighty since the first column has three elements in it.

Miscellaneous methods

public Point location (int x, int y)

The location() method returns the Point (0,0) until the container is validated. After validation, this method returns the grid element under the location (x, y), where x and y are in pixels. The results could be used as the gridx and gridy constraints when adding another component.

public String toString ()

The toString() method of GridBagLayout returns the name of the class:

java.awt.GridBagLayout


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