List boxes

List

boxes are significantly different from

boxes, and not just in appearance. While a

box drops down when you activate it, a

A

problem with a

is that the default action is double clicking, not single clicking. A single

click adds or removes elements from the selected group and a double click calls

.

One way around this is to re-educate your user, which is the assumption made in

the following program:

//: List1.java
// Using lists with action()
import java.awt.*;
import java.applet.*;
 
public class List1 extends Applet {
  String[] flavors = { "Chocolate", "Strawberry",
    "Vanilla Fudge Swirl", "Mint Chip",
    "Mocha Almond Fudge", "Rum Raisin",
    "Praline Cream", "Mud Pie" };
  // Show 6 items, allow multiple selection:
  List lst = new List(6, true);
  TextArea t = new TextArea(flavors.length, 30);
  Button b = new Button("test");
  int count = 0;
  public void init() {
    t.setEditable(false);
    for(int i = 0; i < 4; i++)
      lst.addItem(flavors[count++]);
    add(t);
    add(lst);
    add(b);
  }
  public boolean action (Event evt, Object arg) {
    if(evt.target.equals(lst)) {
      t.setText("");
      String[] items = lst.getSelectedItems();
      for(int i = 0; i < items.length; i++)
        t.appendText(items[i] + "n");
    }
    else if(evt.target.equals(b)) {
      if(count < flavors.length)
        lst.addItem(flavors[count++], 0);
    }
    else
      return super.action(evt, arg);
    return true;
  }
} ///:~ 

When

you press the button it adds items to the

of the list (because of the second argument 0 to

).

Adding elements to a

is

more reasonable than the

box because users expect to scroll a list box (for one thing, it has a built-in

scroll bar) but they don’t expect to have to figure out how to get a

drop-down list to scroll, as in the previous example.

However,

the only way for

to be called is through a double-click. If you need to monitor other activities

that the user is doing on your

(in particular, single clicks) you must take an alternative approach.

handleEvent( )

So

far we’ve been using

,

but there’s another method that gets first crack at everything:

What

if these other methods –

in particular – don’t satisfy your needs? In the case of

,

for example, what if you want to catch single mouse clicks but

responds to only double clicks? The solution is to override

for your applet, which after all is derived from

and can therefore override any non-

methods. When you override

for the applet you’re getting all the applet events before they are

routed, so you cannot just assume “This has to do with my button so I can

assume it’s been pressed,” since that’s true only for

.

Inside

it’s possible that the button has the focus and someone is typing to it.

Whether it makes sense or not, those are events that you can detect and act

upon in

.

To

modify the

example so that it will react to single mouse clicks, the button detection will

be left in

but the code to handle the

will be moved into

as follows:

//: List2.java
// Using lists with handleEvent()
import java.awt.*;
import java.applet.*;
 
public class List2 extends Applet {
  String[] flavors = { "Chocolate", "Strawberry",
    "Vanilla Fudge Swirl", "Mint Chip",
    "Mocha Almond Fudge", "Rum Raisin",
    "Praline Cream", "Mud Pie" };
  // Show 6 items, allow multiple selection:
  List lst = new List(6, true);
  TextArea t = new TextArea(flavors.length, 30);
  Button b = new Button("test");
  int count = 0;
  public void init() {
    t.setEditable(false);
    for(int i = 0; i < 4; i++)
      lst.addItem(flavors[count++]);
    add(t);
    add(lst);
    add(b);
  }
  public boolean handleEvent(Event evt) {
    if(evt.id == Event.LIST_SELECT ||
       evt.id == Event.LIST_DESELECT) {
      if(evt.target.equals(lst)) {
        t.setText("");
        String[] items = lst.getSelectedItems();
        for(int i = 0; i < items.length; i++)
          t.appendText(items[i] + "n");
      }
      else
        return super.handleEvent(evt);
    }
    else
      return super.handleEvent(evt);
    return true;
  }
  public boolean action(Event evt, Object arg) {
    if(evt.target.equals(b)) {
      if(count < flavors.length)
        lst.addItem(flavors[count++], 0);
    }
    else
      return super.action(evt, arg);
    return true;
  }
} ///:~ 

The

example is the same as before except for the addition of

.

Inside, a check is made to see whether a list selection or deselection has

occurred. Now remember,

is

being overridden for the applet, so this occurrence could be anywhere on the

form and it could be happening to another list. Thus, you must also check to

see what the target is. (Although in this case there’s only one list on

the applet so we could have made the assumption that all list events must be

about that list. This is bad practice since it’s going to be a problem as

soon as another list is added.) If the list matches the one we’re

interested in, the same code as before will do the trick.

Note

that the form for

is

similar to

:

if

you deal with a particular event you

,

but if you’re not interested in any of the other events via

you must

.

This is vital because if you don’t do this, none of the other

event-handling code will get called. For example, try commenting out the

in the code above. You’ll discover that

never gets called, certainly not what you want. For both

and

it’s important to follow the format above and always return the

base-class version of the method when you do not handle the event yourself (in

which case you should return

).

(Fortunately, these kinds of bug-prone details are relegated to Java 1.0

In

Windows, a list box automatically allows

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