Download Ch04

Java
Methods
TM
Maria Litvin
Gary Litvin
An Introduction
to Object-Oriented Programming
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4
Java Classes, Objects, and
Events: A Preview
Copyright © 2003 by Maria Litvin, Gary Litvin, and Skylight Publishing. All rights reserved.
Objectives:

Get an introduction to classes, objects, fields,
constructors, and methods; get a general
idea of how a small program is put together

Explore how library classes are used in Java
programs

Get a feel for how methods call each other;
learn about private and public methods

Learn a little about event-driven applications
and the event-handling mechanism in Java
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Objects in the Ramblecs Applet
Ramblecs, the
applet itself
FallingCube
cube
LetterPanel
whiteboard
JButton
go
4-3
Classes and Source Files

A class defines a class of objects.
Convention:
a class name
starts with a
capital letter
Class name:
File name:
SomeClass
SomeClass.java
Ramblecs
Ramblecs.java
FallingCube
FallingCube.java
Same upper / lower case letters
4-4
Programmers write classes

And extensively use library classes
– either directly:
JButton go = new JButton("Click here");
– or through inheritance:
public class LetterPanel extends JPanel
4-5
Classes in the Ramblecs Applet
Ramblecs (applet)
LetterPanel whiteboard
FallingCube cube
JButton go
From the library
package javax.swing
Written by us
4-6
Files and Folders


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javac automatically looks for classes (.java or
.class files) in the current folder, or, if classpath
is set, in folders listed in the classpath string.
A missing file may be reported as a syntax
error when compiling another file.
If you set classpath, include the current folder.
It is denoted by a dot. For example:
.;C:\javamethods\EasyIO

IDE helps take care of the file locations.
4-7
Libraries

Java programs are usually not written from
scratch.

There are hundreds of library classes for all
occasions.

Library classes are organized into packages.
For example:
java.util — miscellaneous utility classes
java.awt — windowing and graphics toolkit
javax.swing — newer GUI package Swing
4-8
import


Full library class names include the package
name. For example:
java.awt.Color
javax.swing.JButton
import statements at the top of your program
let you refer to library classes by their short
names:
Fully-qualified
name
import javax.swing.JButton;
...
JButton go = new JButton("Click here");
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import (cont’d)

You can import names for all the classes in
a package by using a wildcard .*:
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

Imports all classes
from awt, awt.event,
and swing packages
java.lang is imported automatically into all
classes; defines System, Math, Object, String,
and other commonly used classes.
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public class SomeClass
Attributes / variables that define the
{



}
Fields
object’s state. Can hold numbers,
characters, strings, other objects.
Usually private.
Constructors
Methods
Code for constructing a
new object and initializing
its fields. Usually public.
Actions that an object
can take. Can be
public or private.
private: visible only inside this class
public: visible in other classes
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public class FallingCube
{
private final int cubeSize;
private int cubeX, cubeY;
// Cube coordinates
...
private char randomLetter; // Cube letter
public FallingCube(int size)
{
cubeSize = size;
...
Constructor
}
}
public void start()
{
cubeX = 0;
cubeY = -cubeSize;
...
}
...
Fields
The name of a
constructor is
always the same
as the name of
the class.
Methods
4-12
Fields
You name it!
private (or public) [static] [final]
datatype name;
Usually
private
May be present:
means the field is
shared by all objects
in the class
May be present:
means the field
is a constant
int, double, etc., or an
object: String, JButton,
FallingCube, Timer
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Fields (cont’d)

May have primitive data types:
int, char, double, etc.
private int cubeX, cubeY; // cube coordinates
...
private char randomLetter; // cube letter
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Fields (cont’d)

May be objects of different types:
private FallingCube cube;
private Timer t;
private static final String letters;
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Constructors

Constructors are like methods for creating
objects of a class.

Most constructors initialize the object’s fields.

Constructors may take parameters.

A class may have several constructors that
differ in the number or types of their
parameters.

All of a class’s constructors have the same
name as the class.
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Constructors (cont’d)
go = new JButton("Go");
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Constructors (cont’d)

Call them using the new operator:
cube = new FallingCube(CUBESIZE);
...
Calls FallingCube’s constructor with
CUBESIZE as the parameter
t = new Timer(delay, this)
Calls Timer’s constructor with delay and
this (i.e. this object) as the parameters
(see Java docs for javax.swing.Timer)
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Methods


Call them for a particular object:
cube.start();
whiteboard.dropCube();
randomLetter = letters.charAt(i);
But call static (“class”) methods for the whole
class, not a specific object:
y = Math.sqrt (x);
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Methods (cont’d)

Constructors and methods can call other
public and private methods of the same class.

Constructors and methods can call only
public methods of another class.
Class X
public method
Class Y
public method
private method
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Methods (cont’d)

You can call methods with specific arguments:
g.drawRect (75, 25, 150, 50);
g.drawString ("Welcome", 120, 50);

The number and types of arguments must
match the method’s parameters:
public void drawRect ( int x, int y,
int width, int height ) {...}
public void drawString ( String msg, int x, int y ) {...}
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Events

Can originate in the real world (mouse
clicked, keyboard key pressed, cable gets
connected, etc.)

Can come from the operating system
(window resized or closed, e-mail message
received, etc.)

Can originate in your program (a timer fires,
a panel needs to be repainted, etc.)
4-22
Events (cont’d)



An object that
generates events
may have one or
several listeners
attached to it.
A listener is an
object.
A listener’s
method is called
for each event.
Click!
ActionListener object
public void actionPerformed
(ActionEvent e)
{
whiteboard.dropCube();
}
4-23
Events (cont’d)
public class Ramblecs extends JApplet
implements ActionListener
{
Add a listener to
...
private JButton go;
the button. In
public void init()
{
go = new JButton("Go");
go.addActionListener(this);
...
}
this case, the
listener object is
the applet itself.
public void actionPerformed(AcionEvent e)
{
whiteboard.dropCube();
Describes the details of
}
}
this event. Not used here.
4-24
Ramblecs Events
Ramblecs class
applet object
Applet
starts
 “Go”
clicked
init method
creates the whiteboard
panel and the “Go”
button
actionPerformed method
calls whiteboard’s
dropCube
4-25
Ramblecs Events (cont’d)
LetterPanel class
whiteboard object
dropCube method
starts the timer and the cube
Timer
fires
actionPerformed method
moves the cube down;
generates a repaint request
Repaint
paintComponent method
request
restores the background; calls
cube’s draw method
4-26
Ramblecs Events (cont’d)
FallingCube class
cube object
start method
picks a random letter; resets
cube’s position to the top
moveDown method
checks whether cube reached the
bottom; moves the cube down
draw method
draws the cube
4-27
Review:
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How many classes did we write for
Ramblecs?
Name a few library classes that we used.
What are import statements used for?
What is a field? A constructor? A method?
Which operator is used to construct an
object?
What is the difference between private and
public methods?
Why are fields usually private?
4-28
Review (cont’d):
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Define an event-driven application.
Why are GUI applications event-driven?
Is an event listener a class, an object, or a
method?
How many action listeners are used in
Ramblecs?
What does the following statement do?
w.addWindowListener (new ExitButtonListener ( ) );
4-29