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Exploring Java

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Creating GUI Components
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14.6 Checkbox and CheckboxGroup

A Checkbox is a labeled toggle button. A group of such toggle buttons can be made mutually exclusive by tethering them together with a CheckboxGroup object. By now you're probably well into the swing of things and could easily master the Checkbox on your own. We'll throw out an example to illustrate a different way of dealing with the state of components and to show off a few more things about containers.

A Checkbox sends item events when it's pushed, just like a List, a Menu, or a Choice. In our last example, we caught the action events from our choice and menu selections and worked with them when they happened. For something like a checkbox, we might want to be lazy and check on the state of the buttons only at some later time, such as when the user commits an action. It's like filling out a form; you can change your choices until you submit the form.

The following applet, DriveThrough, lets us check off selections on a fast food menu, as shown in Figure 14.7. DriveThrough prints the results when we press the Place Order button. Therefore, we can ignore all the item events generated by our checkboxes and listen only for the action events generated by the button.

Figure 14.7: The DriveThrough applet

Figure 14.7
import java.awt.*;
import java.awt.event.*;
public class OrderForm extends java.applet.Applet implements ActionListener { 
    Panel condimentsPanel = new Panel();
    CheckboxGroup entreeGroup = new CheckboxGroup();
    public void init() {
        condimentsPanel.add( new Checkbox("Ketchup"));
        condimentsPanel.add( new Checkbox("Mustard"));
        condimentsPanel.add( new Checkbox("Pickles"));
        Checkbox c;
        Panel entreePanel = new Panel();
        entreePanel.add( c = new Checkbox("Beef") );
        c.setCheckboxGroup( entreeGroup );
        entreePanel.add( c = new Checkbox("Chicken") );
        c.setCheckboxGroup( entreeGroup );
        entreePanel.add( c = new Checkbox("Veggie") );
        c.setCheckboxGroup( entreeGroup );
        entreeGroup.setCurrent( c );
        Panel orderPanel = new Panel();
        Button orderButton = new Button("Place Order");
        orderButton.addActionListener( this );
        orderPanel.add( orderButton );
        setLayout( new GridLayout(3, 1) );
        add( entreePanel );
        add( condimentsPanel );
        add( orderPanel );
    }
    public void actionPerformed(ActionEvent e) {
        takeOrder();
    }
    void takeOrder() {
        Checkbox c = entreeGroup.getCurrent();
            System.out.println( c.getLabel() + " sandwich" );
        Component [] components = condimentsPanel.getComponents();
        for (int i=0; i< components.length; i++)
            if ( (c = (Checkbox)components[i]).getState() )
                System.out.println( "With " + c.getLabel() );
        System.out.println("Thank you, drive through...");
    }
}

DriveThrough lays out two panels, each containing three checkboxes. The checkboxes in the entreePanel are tied together through a single CheckboxGroup object. We call their setCheckboxGroup() methods to put them in a single CheckboxGroup that makes the checkboxes mutually exclusive. The CheckboxGroup object is an odd animal. One expects it to be a container or a component, but it isn't; it's simply a helper object that coordinates the functionality of the Checkbox objects. Because a CheckboxGroup isn't a container, it doesn't have an add() method. To put a checkbox into a group, you call the setCheckboxGroup() method of the Checkbox class.

Once a set of checkboxes has been placed in a checkbox group, only one of the boxes may be checked at a time. In this applet, the checkbox group forces you to choose a beef, chicken, or veggie entree, but not more than one. The condiment choices, however, aren't in a checkbox group, so you can request ketchup, mustard, and pickles on your chicken sandwich.

When the Place Order button is pushed, we receive an ActionEvent via our actionPerformed() method. At this point, we gather the information in the checkboxes and print it. actionPerformed() simply calls our takeOrder() method, which reads the checkboxes. We could have saved references to the checkboxes in a number of ways; this example demonstrates two. First, we find out which entree was selected. To do so, we call the CheckboxGroup's getCurrent() method. getCurrent() returns the selected Checkbox; we use getLabel() to extract the name of the entree.

To find out which condiments were selected, we use a more complicated procedure. The problem is that condiments aren't mutually exclusive, so we don't have the convenience of a CheckboxGroup. Instead, we ask the condiments Panel for a list of its components. The getComponent() method returns an array of references to the container's child components. We'll use this to loop over the components and print the results. We cast each element of the array back to Checkbox and call its getState() method to see if the checkbox is on or off. Remember that if we were dealing with different types of components, we could determine what kind of component we had with the instanceof operator.


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