Alternatives to action

Bruce Eckel's Thinking in Java Contents | Prev | Next

Component method

When it’s called

action (Event evt, Object what)

When the “typical” event occurs for this component (for example, when a button is pushed or a drop-down list item is selected)

keyDown (Event evt, int key)

A key is pressed when this component has the focus. The second argument is the key that was pressed and is redundantly copied from evt.key.

keyUp(Event evt, int key)

A key is released when this component has the focus.

lostFocus(Event evt, Object what)

The focus has moved away from the target. Normally, what is redundantly copied from evt.arg.

gotFocus(Event evt, Object what)

The focus has moved into the target.

mouseDown(Event evt,

int x, int y)

A mouse down has occurred over the component, at the coordinates x, y.

mouseUp(Event evt, int x, int y)

A mouse up has occurred over the component.

mouseMove(Event evt, int x, int y)

The mouse has moved while it’s over the component.

mouseDrag(Event evt, int x, int y)

The mouse is being dragged after a mouseDown occurred over the component. All drag events are reported to the component in which the mouseDown occurred until there is a mouseUp.

mouseEnter(Event evt, int x, int y)

The mouse wasn’t over the component before, but now it is.

mouseExit(Event evt, int x, int y)

The mouse used to be over the component, but now it isn’t.

You can see that each method receives an Event object along with some information that you’ll typically need when you’re handling that particular situation – with a mouse event, for example, it’s likely that you’ll want to know the coordinates where the mouse event occurred. It’s interesting to note that when Component’s handleEvent( ) calls any of these methods (the typical case), the extra arguments are always redundant as they are contained within the Event object. In fact, if you look at the source code for Component.handleEvent( ) you can see that it explicitly plucks the additional arguments out of the Event object. (This might be considered inefficient coding in some languages, but remember that Java’s focus is on safety, not necessarily speed.)

To prove to yourself that these events are in fact being called and as an interesting experiment, it’s worth creating an applet that overrides each of the methods above (except for action( ), which is overridden in many other places in this chapter) and displays data about each of the events as they happen.

//: AutoEvent.java
// Alternatives to action()
import java.awt.*;
import java.applet.*;
import java.util.*;
 
class MyButton extends Canvas {
  AutoEvent parent;
  Color color;
  String label;
  MyButton(AutoEvent parent, 
           Color color, String label) {
    this.label = label;
    this.parent = parent;
    this.color = color;
  }
  public void paint(Graphics  g) {
    g.setColor(color);
    int rnd = 30;
    g.fillRoundRect(0, 0, size().width, 
                    size().height, rnd, rnd);
    g.setColor(Color.black);
    g.drawRoundRect(0, 0, size().width, 
                    size().height, rnd, rnd);
    FontMetrics fm = g.getFontMetrics();
    int width = fm.stringWidth(label);
    int height = fm.getHeight();
    int ascent = fm.getAscent();
    int leading = fm.getLeading();
    int horizMargin = (size().width - width)/2;
    int verMargin = (size().height - height)/2;
    g.setColor(Color.white);
    g.drawString(label, horizMargin, 
                 verMargin + ascent + leading);
  }
  public boolean keyDown(Event evt, int key) {
    TextField t = 
      (TextField)parent.h.get("keyDown");
    t.setText(evt.toString());
    return true;
  }
  public boolean keyUp(Event evt, int key) {
    TextField t = 
      (TextField)parent.h.get("keyUp");
    t.setText(evt.toString());
    return true;
  }
  public boolean lostFocus(Event evt, Object w) {
    TextField t = 
      (TextField)parent.h.get("lostFocus");
    t.setText(evt.toString());
    return true;
  }
  public boolean gotFocus(Event evt, Object w) {
    TextField t = 
      (TextField)parent.h.get("gotFocus");
    t.setText(evt.toString());
    return true;
  }
  public boolean 
  mouseDown(Event evt,int x,int y) {
    TextField t = 
      (TextField)parent.h.get("mouseDown");
    t.setText(evt.toString());
    return true;
  }
  public boolean 
  mouseDrag(Event evt,int x,int y) {
    TextField t = 
      (TextField)parent.h.get("mouseDrag");
    t.setText(evt.toString());
    return true;
  }
  public boolean 
  mouseEnter(Event evt,int x,int y) {
    TextField t = 
      (TextField)parent.h.get("mouseEnter");
    t.setText(evt.toString());
    return true;
  }
  public boolean 
  mouseExit(Event evt,int x,int y) {
    TextField t = 
      (TextField)parent.h.get("mouseExit");
    t.setText(evt.toString());
    return true;
  }
  public boolean 
  mouseMove(Event evt,int x,int y) {
    TextField t = 
      (TextField)parent.h.get("mouseMove");
    t.setText(evt.toString());
    return true;
  }
  public boolean mouseUp(Event evt,int x,int y) {
    TextField t = 
      (TextField)parent.h.get("mouseUp");
    t.setText(evt.toString());
    return true;
  }
}
 
public class AutoEvent extends Applet {
  Hashtable h = new Hashtable();
  String[] event = {
    "keyDown", "keyUp", "lostFocus", 
    "gotFocus", "mouseDown", "mouseUp", 
    "mouseMove", "mouseDrag", "mouseEnter", 
    "mouseExit"
  };
  MyButton 
    b1 = new MyButton(this, Color.blue, "test1"),
    b2 = new MyButton(this, Color.red, "test2");
  public void init() {
    setLayout(new GridLayout(event.length+1,2));
    for(int i = 0; i < event.length; i++) {
      TextField t = new TextField();
      t.setEditable(false);
      add(new Label(event[i], Label.CENTER)); 
      add(t);
      h.put(event[i], t);
    }
    add(b1);
    add(b2);
  }
} ///:~ 

You can see the constructor uses the technique of using the same name for the argument as what it’s assigned to, and differentiating between the two using this:

this.label = label;

The paint( ) method starts out simple: it fills a “round rectangle” with the button’s color, and then draws a black line around it. Notice the use of size( ) to determine the width and height of the component (in pixels, of course). After this, paint( ) seems quite complicated because there’s a lot of calculation going on to figure out how to center the button’s label inside the button using the “font metrics.” You can get a pretty good idea of what’s going on by looking at the method call, and it turns out that this is pretty stock code, so you can just cut and paste it when you want to center a label inside any component.



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