A
check box provides a way to make a single on-off choice; it consists of a tiny
box and a label. The box typically holds a little ‘x’ (or some
other indication that it is set) or is empty depending on whether that item was
selected.
You’ll
normally create a Checkbox
using a constructor that takes the label as an argument. You can get and set
the state, and also get and set the label if you want to read or change it
after the
Checkbox
has been created. Note that the capitalization of
Checkbox
is inconsistent with the other controls, which could catch you by surprise
since you might expect it to be “CheckBox.”
Whenever
a
Checkbox
is set or cleared an event occurs, which you can capture the same way you do a
button. The following example uses a
TextArea
to enumerate all the check boxes that have been checked:
//: CheckBox1.java
// Using check boxes
import java.awt.*;
import java.applet.*;
public class CheckBox1 extends Applet {
TextArea t = new TextArea(6, 20);
Checkbox cb1 = new Checkbox("Check Box 1");
Checkbox cb2 = new Checkbox("Check Box 2");
Checkbox cb3 = new Checkbox("Check Box 3");
public void init() {
add(t); add(cb1); add(cb2); add(cb3);
}
public boolean action (Event evt, Object arg) {
if(evt.target.equals(cb1))
trace("1", cb1.getState());
else if(evt.target.equals(cb2))
trace("2", cb2.getState());
else if(evt.target.equals(cb3))
trace("3", cb3.getState());
else
return super.action(evt, arg);
return true;
}
void trace(String b, boolean state) {
if(state)
t.appendText("Box " + b + " Set\n");
else
t.appendText("Box " + b + " Cleared\n");
}
} ///:~
The
trace( )
method sends the name of the selected
Checkbox
and its current state to the
TextArea
using appendText( )
so you’ll see a cumulative list of the checkboxes that were selected and
what their state is.
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