7/3/00SEM107- © Kamin & ReddyClass 11 - Events - 1 Class 11 - Events r A couple of odds & ends m...

7/3/00 SEM107- © Kamin & Reddy Class 11 - Events - 1

Class 11 - Events

A couple of odds & ends Component sizes switch statement

Event typesCatching different event typesGetting information from components and

eventsDistinguishing between events of the same

type


Component sizes

Each component type has an instance method getSize() that returns its dimensions, as an object of type Dimension, e.g.

Can also get the dimensions of the entire applet:

// If b1 is a Button:Dimension b1size = b1.getSize();

Dimension appsize = getSize();


Component sizes (cont.)

From a Dimension object like b1size, you can find the width and height of the component by

These are both int values.For example, to print the height of the applet: System.out.println(getSize().height);

b1size.height // height of the buttonb1size.width // width of the button


The switch statement

The switch statement is another form of conditional statement. It executes one among a set of statements, depending upon the value of an int-valued expression.

Syntax:


switch statement syntax

switch (expression) { [case-clause]* [default-clause]}

switch-statement

case integer-constant: [statement]* break;

case-clause

Default-clause default: statement


switch statement syntax (cont.) In other words, a switch statement looks like

This behaves like:switch (expr) { case i1 : statement1; break; case i2 : statement2; break; . . . case in : statementn; break; default: statement;}


switch statement syntax (cont.)

If the default part is omitted, then if the expression does not match any of the constants, nothing happens.

int i = expr;if (i==i1) statement1;else if (i==i2) statement2; . . .else if (i==in) statementn;else statement;


The Java event model

An event is any occurrence an applet might want to respond to, e.g. user clicks mouse on button, user moves mouse, user enters text into text field.

Java event model is a method of allowing applets to respond to events.The Java event model is based on classifying events into different types.


Event typesWe will consider four types of events, and explain how to write an applet to respond to each

kind. An applet can also respond to more than one type of event.Major event types:

Action events, e.g. button click Item event, e.g. click check box Mouse event, e.g. mouse button click Mouse motion even, e.g. mouse moves in applet


Reading text from text fieldRecall how an applet was written to respond to the event

of a user entering text in a text field (and then hitting the Enter key):

class appletname extends Applet implements ActionListener { TextField t; public void init () { ... t.addActionListener(this); ... } public void actionPerformed (ActionEvent e) { ... }

declare applet listens to this type of event

“register” text field

define required method


Listening to eventsFor an applet to listen to any of the types of

events, it must do these three things: Declare itself to listen to that type of event Register any components that can trigger the

event. (If the event is a mouse event, it doesn’t need to be registered.)

Define method(s) required to listen to that type of event.

Following slides show how to do this for each type of event.


Action eventsDeclare applet: implements ActionListenerRegister component:

component.addActionListener(this);Required methods:

public void actionPerformed (ActionEvent e)

Action events are: user clicks on button; user hits enter key in text field.E.g. following applet respond to button clickby drawing a rectangle in a darker gray


Action event examplepublic class ActionApplet extends Applet implements ActionListener { Button darken = new Button(“Darken”); int red = 255, green = 255, blue = 255; public void init () { add(darken); darken.addActionListener(this); } public void paint (Graphics g) { g.setColor(new Color(red,green,blue)); g.fillRect(10,40,100,50); } public void actionPerformed (ActionEvent e) { red = red-10; green = green-10; blue = blue-10; repaint() ; }}


Item events

Declare applet: implements ItemListener Register component:

component.addItemListener(this); Required methods:

public void itemStateChanged (ItemEvent e)

Item events are: user clicks on checkbox.


Mouse motion events

Declare applet: implements MouseMotionListener

Register component: no component to register; just write: addMouseMotionListener(this);

Required methods: public void mouseMoved (MouseEvent e) public void mouseDragged (MouseEvent e)


Mouse events Declare applet: implements MouseListener Register component: no component to register;

just write: addMouseListener(this); Required methods:

public void mousePressed (MouseEvent e) public void mouseReleased (MouseEvent e) public void mouseEntered (MouseEvent e) public void mouseExited (MouseEvent e) public void mouseClicked (MouseEvent e)

Example: Darken box when mouse is clicked:


Mouse event examplepublic class MouseApplet extends Applet implements MouseListener { int red = 255, green = 255, blue = 255; public void init () { addMouseListener(this); } public void paint (Graphics g) { g.setColor(new Color(red,green,blue)); g.fillRect(10,40,100,50); } public void mouseClicked (MouseEvent e) { red = red-10; green = green-10; blue = blue-10; repaint(); } public void mousePressed (MouseEvent e) {} public void mouseReleased (MouseEvent e) {} public void mouseEntered (MouseEvent e) {} public void mouseExited (MouseEvent e) {}}


Getting information about an event

You can always get information about a component by using instance methods from the components class, e.g. in TextField: String getText()

When an event occurs, you can get information from the event object that is passed as argument e to the event’s method. The type of information depends upon the type of event:


Getting information about an event (cont.)

Action events and item events: Use e.getSource()

to find out which text field or button or check box was the source of the event.

Mouse events: Use e.getPoint()

to find the location of the mouse. (This returns a Point object; use “.x” and “.y” to find x and y coordinates.)


Another mouse event example

Place a small circle wherever mouse is clicked.public class MouseApplet2 extends Applet implements MouseListener { int x = 50, y = 50; public void init () { addMouseListener(this); } public void paint (Graphics g) { g.drawOval(x,y,20,20); } public void mouseClicked (MouseEvent e) { Point p = e.getPoint(); x = p.x; y = p.y; repaint(); } public void mousePressed (MouseEvent e) {} public void mouseReleased (MouseEvent e) {} public void mouseEntered (MouseEvent e) {} public void mouseExited (MouseEvent e) {}}


Aside: the update method In the previous example, when the mouse button is

clicked, the previous circle is erased before the new one is drawn. To prevent this, add this definition:

Here’s why: repaint calls update, which normally erases the window and then calls paint. This changes the definition of update.

public void update (Graphics g) { paint(g);}


Which component caused the event?

There can be multiple event-producing components.

Use getSource to figure out which one caused the event

E.g. following applet can either lighten or darken the rectangle:


Action event example

public class ActionApplet2 extends Applet implements ActionListener { Button darken = new Button(“Darken”), lighten = new Button(“Lighten”); int red = 255, green = 255, blue = 255;

public void init () { add(darken); darken.addActionListener(this); add(lighten); lighten.addActionListener(this); }


Action event example (cont.)

public void paint (Graphics g) { g.setColor(new Color(red,green,blue)); g.fillRect(10,40,100,50); }

public void actionPerformed (ActionEvent e) { int incr = (e.getSource() == lighten) ? 10 : -10; red = red+incr; green = green+incr; blue = blue+incr; repaint() ; }}


Catching different types of events

An applet can catch events of any or all types. It just declares that it wants to catch events of those types, registers all components, and defines all required operations.

E.g. following applet responds to button click by darkening box, and to mouse click by lightening it:


Action & mouse examplepublic class MouseButtonApplet extends Applet implements ActionListener, MouseListener { Button darken = new Button(“Darken”); int red = 255, green = 255, blue = 255;

public void init () { add(darken); darken.addActionListener(this); addMouseListener(this); }

public void paint (Graphics g) { g.setColor(new Color(red,green,blue)); g.fillRect(10,10,100,50); }


Action & mouse example (cont.)

public void actionPerformed (ActionEvent e) { red = red-10; green = green-10; blue = blue-10; repaint() ; }

public void mouseClicked (MouseEvent e) { red = red+10; green = green+10; blue = blue+10; repaint(); } public void mousePressed (MouseEvent e) {} public void mouseReleased (MouseEvent e) {} public void mouseEntered (MouseEvent e) {} public void mouseExited (MouseEvent e) {}}

7/3/00SEM107- © Kamin & ReddyClass 11 - Events - 1 Class 11 - Events r A couple of odds & ends m...

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