Transcript ch04_notes

Java
Methods
TM
Maria Litvin
Gary Litvin
An Introduction
to Object-Oriented Programming
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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
4-2
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


java compiler automatically looks for classes
(.class files first and then .java files) in the
current folder.
If you made changes to more than one .java
file you must select BuildAll or the java
compiler will use the last .class file which
does not have your changes saved.

A missing file may be reported as a syntax
error when compiling another file.

IDE (JCreator in our case) 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

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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");
4-9
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.
4-10
public class SomeClass
Attributes / variables that define the
{



}
Fields
object’s state. Can hold numbers,
characters, strings, other objects.
Usually private.
Constructors
Code for constructing a
new object and initializing
its fields. Usually public.
Methods
Actions that an object
can take. Can be
public or private.
private: visible only inside this class
public: visible in other classes
4-11
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
4-13
Fields (cont’d)

May have primitive data types:
int, char, double, etc.
private int cubeX, cubeY; // cube coordinates
...
private char randomLetter; // cube letter
4-14
Fields (cont’d)

May be objects of different types:
private FallingCube cube;
private Timer t;
private static final String letters;
4-15
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");
4-17
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);
in other words, if a class is static you can only
refer to it by using the name of the class.
4-19
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
4-20
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)


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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(ActionEvent 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