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Java AWT

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7.5 CardLayout

The CardLayout layout manager is significantly different from the other layouts. Whereas the other layout managers attempt to display all the components within the container at once, a CardLayout displays only one component at a time. (That component could be a Component or another Container.) The result is similar to Netscape Navigator's Property sheets or a tabbed Dialog, without the tabs. You can flip through the cards (components) in the layout in order or jump to a specific card if you know its name. The following call to setLayout() changes the LayoutManager of the current container to CardLayout:

lm = new CardLayout();
setLayout (lm);

Unlike most other layout managers, CardLayout has a number of instance methods that programs have to call. Therefore, you usually have to retain a reference to the layout manager. In addition, you usually have some other component to control the CardLayout (i.e., select which card to view). Most simply, you could put some buttons in a panel and stick this panel in the north region of a BorderLayout; then make another panel with a CardLayout, and place that in the center. A more complex task would be to build a set of tabs to control the CardLayout.

A CardLayout allows you to assign names to the components it manages. You can use the name to jump to an arbitrary component by calling the manager's show() method. In Java 1.0, naming was optional; you could call add(Component) to put a component in the layout with a null name. A null name meant only that you couldn't flip to the component at will; you could only display the component by calling next() or previous() (or first() or last()), which cycle through all the components in order. In Java 1.1, all components added to a CardLayout must be named.

CardLayout Methods

Constructors

public CardLayout ()

This constructor creates a CardLayout using a horizontal and vertical gap of zero pixels. With CardLayout, there is no space between components because only one component is visible at a time; think of the gaps as insets.

public CardLayout (int hgap, int vgap)

This version of the constructor allows you to create a CardLayout with a horizontal gap of hgap and vertical gap of vgap to add some space around the outside of the component that is displayed. The units for gaps are pixels. Using negative gaps chops off components at the edges of the container.

Informational methods

public int getHgap () (New)

The getHgap() method retrieves the current horizontal gap setting.

public void setHgap (int hgap) (New)

The setHgap() method changes the current horizontal gap setting to hgap. After changing the gaps, you must validate() the Container.

public int getVgap () (New)

The getVgap() method retrieves the current vertical gap setting.

public void setVgap (int hgap) (New)

The setVgap() method changes the current vertical gap setting to vgap. After changing the gaps, you must validate() the Container.

LayoutManager methods

public void addLayoutComponent (String name, Component component) (Deprecated)

This version of addLayoutComponent() has been deprecated and replaced by the addLayoutComponent(Component, Object) method of the LayoutManager2 interface.

public void removeLayoutComponent (Component component)

The removeLayoutComponent() method of CardLayout removes component from the container. If component is not in the container already, nothing happens.

public Dimension preferredLayoutSize (Container target)

The preferredLayoutSize() method of CardLayout retrieves the preferred size for all the components within it. The preferredLayoutSize() method then determines the widest and tallest size of all components (not necessarily from the same one), adds the appropriate insets and gaps, and uses that as the preferred size for the layout.

public Dimension minimumLayoutSize (Container target)

The minimumLayoutSize() method of CardLayout calculates the minimum size for all the components within it. The minimumLayoutSize() method then determines the widest and tallest minimum size of all components (not necessarily from the same one), adds the appropriate insets and gaps, and uses that as the minimum size for the layout.

public void layoutContainer (Container target)

The layoutContainer() method draws target's visible components one on top of another. Initially, all components are visible. Components do not become invisible until you select one for display, by calling the first(), last(), next(), previous(), or show() methods. Where possible, CardLayout reshapes all components to fit the target container.

LayoutManager2 methods

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

This addLayoutComponent() method of CardLayout puts component into an internal table with a key of name. The name comes from the version of add() that has a constraints object as a parameter. The name allows you to refer to the component when you call other card layout methods, like show(). If you call the version of add() that only takes a Component parameter, you cannot call the show() method to flip to the specific component.

If name is not a String, the run-time exception IllegalArgumentException is thrown.

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 CardLayout doesn't support the concept of maximum size.

public abstract float getLayoutAlignmentX(Container target) (New)

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

public abstract float getLayoutAlignmentY(Container target) (New)

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

public abstract void invalidateLayout(Container target) (New)

The invalidateLayout() method of CardLayout does nothing.

CardLayout methods

This group of methods controls which component the CardLayout displays. The show() is only usable if you assigned components names when adding them to the container. The others can be used even if the components are unnamed; they cycle through the components in the order in which they were added. All of these methods require the parent Container (i.e., the container being managed by this layout manager) as an argument. If the layout manager of the parent parameter is anything other than the container using this instance of the CardLayout, the method throws the run-time exception IllegalArgumentException.

public void first (Container parent)

The first() method flips to the initial component in parent.

public void next (Container parent)

The next() method flips to the following component in parent, wrapping back to the beginning if the current component is the last.

public void previous (Container parent)

The previous() method flips to the prior component in parent, wrapping to the end if the current component is the first.

public void last (Container parent)

The last() method flips to the final component in parent.

public void show (Container parent, String name)

The show() method displays the component in parent that was assigned the given name when it was added to the container. If there is no component with name contained within parent, nothing happens.

Miscellaneous methods

public String toString ()

The toString() method of CardLayout returns the a string showing the current horizontal and vertical gap settings. The result for a typical CardLayout would be:

java.awt.CardLayout[hgap=0,vgap=0]

CardLayout Example

Figure 7.7 shows a simple CardLayout. This layout has three cards that cycle when you make a selection. The first card (A) contains some Checkbox items within a Panel, the second card (B) contains a single Button, and the third (C) contains a List and a Choice within another Panel.

Example 7.1 is the code that generated Figure 7.7.

Example 7.1: The CardExample Class

import java.awt.*;
import java.applet.*;
public class CardExample extends Applet {
    CardLayout cl = new CardLayout();
    public void init () {
        String fonts[] = Toolkit.getDefaultToolkit().getFontList();
        setLayout (cl);
        Panel pA = new Panel();
        Panel pC = new Panel ();
        p1.setLayout (new GridLayout (3, 2));
        List l = new List(4, false);
        Choice c = new Choice ();
        for (int i=0;i<fonts.length;i++) {
            pA.add (new Checkbox (fonts[i]));
            l.addItem (fonts[i]);
            c.addItem (fonts[i]);
        }
        pC.add (l);
        pC.add (c);
        add ("One", pA);
        add ("Two", new Button ("Click Here"));
        add ("Three", pC);
    }
    public boolean action (Event e, Object o) {
        cl.next(this);
        return true;
    }
}

Each panel within the CardLayout has its own layout manager. Panel A uses a GridLayout; panel C uses its default layout manager, which is a FlowLayout. When the user takes any action (i.e., clicking on a checkbox or button, or selecting an item from the List or Choice components), the system generates a call to action(), which calls the CardLayout's next() method, thus displaying the next card in the sequence.


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