Workshop for Programming And
Systems Management Teachers
Chapter 9
Source File Anatomy
Georgia Institute of Technology
Learning Goals
• Understand at a conceptual and practical level
the anatomy of a Java source file
–
–
–
–
Optional Comments
Optional Package Declaration
Import Statements as needed
Class Declarations
• Attributes
• Constructors
• Methods
– Applet Anatomy
Georgia Institute of Technology
Structure of a Java Source File
Comments (like copyright)
Package statement
Import statements
Class definition
{
Attributes
Constructors
Methods
Main method (if application)
}
Georgia Institute of Technology
Comments
• There are three types of comments in Java
– A multi-line comment
/* here is a multi-line comment
it ends here */
– A single line comment
// comments out the rest of this line
– A javadoc comment
/** Method to return the name of the person */
@author name - adds an author entry
@version text - adds a version section
@param name description - adds a parameter description
@return description - describes the return value of a method
Georgia Institute of Technology
Comment Exercise
• Open SimplePicture.java and read some of the
comments
– Which are Javadoc comments?
– Which are single line comments?
– Which are multi-line comments?
• Click on the Javadoc button to generate
documentation from the Javadoc comments
– Compare the html documentation to the original
Javadoc comments
Georgia Institute of Technology
Packages
• Packages allow you to organize your classes
into a hierarchy. The package hierarchy is also
used for the directory structure.
– edu.gatech.ice.Class should be in edu/gatech/ice/
• Package names for packages intended for
release outside the organization are like the
reverse of the internet domain names.
– com.sun.java.swing
– edu.gatech
java.lang
• Every class belongs to a package. If none is
specified then the unnamed package is used.
Georgia Institute of Technology
Common Packages
• java.lang - Basic classes, including strings and
objects
• java.util - Utility classes including generic data
structures
• java.awt - User interface and graphics
• javax.swing - Newer user interface classes
• java.applet - Support for applets
• java.net - Networking support (URLs, TCP, UDP)
• java.io - Input and output to files and streams
Georgia Institute of Technology
Package Statement
• The package statement has to be the first line of
code in a source file, if it is specified. It can
follow a comment.
– package packageName;
for example
/** Copyright notice */
package edu.gatech.ice;
Georgia Institute of Technology
Import Statements
• If you wish to use classes from a different
package than the declared package you can
import the class using
• import java.awt.Color;
or
• import java.awt.*;
// import just Color
// all classes in java.awt
– doesn’t include the file like in C so there is no penalty to using
the wildcard
– doesn’t extend to children of that level. For example importing
java.awt.* doesn’t import java.awt.event.*;
• Or you can use the full name
– java.awt.Color
Georgia Institute of Technology
How do you know what to import?
• If you use a class that is
part of Java and you get
a compiler error
– Error: Undefined class
• Use the all classes list of
the API to find the class
and check the
documentation for the
package it is in
Georgia Institute of Technology
Import Exercise
• Change SimplePicture.java to specify all the
classes that are being imported
– Rather than using
• import java.awt.*;
• Import java.io.*;
• Comment out the import statements with a
wildcard ‘*”
– select lines-> Edit-> Comment Line(s)
• Compile and see what classes it says are undefined
• Find the classes in the API and add a fully
qualified import statement for each
– import java.awt.Color;
Georgia Institute of Technology
Class Definition
[Class modifiers] class ClassName [extends
ClassName] [implements Interface1,…]
{
attributes, constructors, methods
}
• Visibility Class Modifiers
– no modifier means the class is visible to all classes in
the same package
– public - visible to any class. A file can have only one
public class.
– private - visible only to classes in the same file
Georgia Institute of Technology
Class Definition Examples
public class Person implements Comparable
{
attributes, methods, and inner classes
}
private class Helper extends Object
{
attributes, methods, and inner classes
}
Georgia Institute of Technology
Optional Extends Clause
• The “extends Class” clause tells you which class
is the parent class
– The one the current class inherits from
– The parent, super, or base class
• If there is no “extends Class” clause
– The class inherits from Object
• In the package java.lang
Georgia Institute of Technology
Inheritance Exercise
• What is the parent class of each of these
classes?
–
–
–
–
Picture.java
SimplePicture.java
Sound.java
Pixel.java
• Where is the show method defined that we have
been using to show Picture objects?
• Where is the play method defined that we used
to play Sound objects?
Georgia Institute of Technology
Optional Implements Clause
• The “implements interface1, interface2” lets the
class specify the interfaces that the class
promises to implement
– Any methods defined in the interfaces must be
defined in the class
• Or the code will not compile
• An interface is like a communications contract
– How two classes will communicate
– Without worrying what about what classes they are
• An interface name can be used as a type
– For variable declarations
Georgia Institute of Technology
Comparable Interface
• What if you want to compare two objects?
– To find out if one is less than another (< 0)
– Or equal to the other (= 0)
– Or greater than the other (> 0)
• How would you compare people?
– playing cards?
– shoppers?
• Each class needs to decide what this means
– But we need to know what method to call
int compareTo(Object o)
Georgia Institute of Technology
Interface Exercise
• What interface does SimplePicture.java
implement?
• Find that interface
– Defined in a file with the interface Name.java
• What methods are declared in the interface?
• Find the same methods in SimplePicture.java
Georgia Institute of Technology
Attributes
• Attributes are the data that each object knows
about itself.
– All object methods in the class have access to the
object attributes
• Can use just the attribute name
• Can also use this.attribute name
• Can also use getAttribute() if you follow Java naming
conventions
– Class and object methods have access to the class
attributes
• Declared with “static” keyword
Georgia Institute of Technology
Attribute Exercise
• Open SimplePicture.java
• What are the attributes (fields)?
– Which are object fields?
– Which are class fields?
• Change the load method to use
– setFileName(fileName) instead of
• this.fileName = fileName;
– Compile
– Test by creating a picture and using
System.out.println(picture);
Georgia Institute of Technology
Declaring Attributes
• Attributes are declared as
[visibility] type name [= value];
Visibility
none specified - classes in the same package
public - all classes
protected - class, subclasses, and other classes in the same
package
private - only the class itself
Type - the type can be one of the following
primitive type
object type: class, interface, or array
Georgia Institute of Technology
Visibility Exercise
• Open SimplePicture.java
– Which fields have public visibility?
• Why are these public?
– Which fields have private visibility?
• Why are these private?
Georgia Institute of Technology
Constructors
• Constructors are called when the new object is
created. They initialize the new object.
Person currPerson = new Person(“Fred Flintstone”);
• Constructors don’t have a return type and have
the same name as the class.
public Person () // no argument constructor
• There can be many constructors but their
parameter signatures must be different.
public Person() {}
public Person(String theName) {
name = theName; }
}
Georgia Institute of Technology
Advanced Constructor Information
• The compiler will add a no-argument (no
parameters) constructor
– All fields will have their default values
• Objects default to null
• Numbers default to zero
• Booleans default to false
• If you add any constructors
– The compiler will no longer add any for you
– You will need to write the no-argument constructor
• One constructor can invoke another
– Use this(parameter List) as first line
Georgia Institute of Technology
Inheritance and Constructors
• If classes have private attributes
– Meaning they can’t be directly accessed outside the
class
– How can you initialize inherited attributes?
• Use super(parameters) to call the parent constructor
• Must be the first line in a constructor
• If not present the compiler will add a super() as the first line
in a constructor
• Means the parent class must have a no-argument
constructor if you aren’t calling super(parameters) in your
constructors
Georgia Institute of Technology
Constructor Exercise
• Open SimplePicture.java
– How many constructors does it have?
– Why do the constructors call super()?
• Open Picture.java
– How many constructors does it have?
• What is different about the constructors?
– How would the complier decide which one to use?
Georgia Institute of Technology
Methods
• Methods are the things that object of the class
know how to do.
• Methods are declared as
[visibility] returnType methodName (parameter list)
{
…
[return x;]
// if returnType is not void
}
• Example
public String getName() { return name;}
Georgia Institute of Technology
Method Visibility and Return Type
• Visibility
– none specified - classes in the same package
– public - all classes
– protected - class, subclasses, and other classes in
the same package
– private - only the class itself
• Return Type - the return type can be one of the
following
– primitive type: int, float, double ...
– object type: class, interface, or array
– void if no value is returned
Georgia Institute of Technology
Method Parameter List
• Can have no parameters
– public String getName()
• Parameters are listed as type name
– public void setName(String name)
• Separate parameters with commas
– public void multiplyTwo(int x, int y)
• You can have several methods with the same
name but different parameter signatures.
– public void multiplyTwo(float x, float y)
– public void multiplyTwo(int x, int y)
Georgia Institute of Technology
Main Method
• Each class can have a main method
public static void main(String[] argv) {}
• You tell Java which class to start with
java ClassName
• Execution starts with the main method
– There are no objects when you begin so the main
method is static (a class method)
• In the main method you create object(s) of the
class
– Then start the simulation
Georgia Institute of Technology
Add Main Method Exercise
• Add a main method to Picture.java to
– Create a picture from a specified file
– Negate the picture
– Show the picture
• Execute by Tools->Run Document’s Main
Method
– Notice that the interactions pane shows
java Picture
Georgia Institute of Technology
Applications Versus Applets
• A Java application will have at least one main
method
– Execution begins at this main
– The Java classes are stored on the machine they run
on
• An applet is a special Java program that runs in
a browser
– The applet tag is used in an html page
– The Java classes are downloaded from a server
– The applet runs in the browser window
Georgia Institute of Technology
Applet Basics
• All applets must inherit from java.awt.Applet or
javax.swing.JApplet
• They don’t have constructors
– Use public void init() instead to set-up the applet
before it is displayed
• Add applet tags to an html document to display
the applet in the html page
Georgia Institute of Technology
How to Embed Applets
<html>
<head>
<title> Message Applet </title>
</head>
<body>
<applet code=“MessageApplet.class" width=200
height=100>
</applet>
</body>
</html>
Georgia Institute of Technology
Applet Tag
<applet code=”x.class" width=200 height=100>
<param name=identifier value=“information”>
<param name=identifier value=“information”>
</applet>
• Attributes of the applet tag
– code - the name of the .class to start the applet
– width - the horizontal size in pixels
– height - the vertical size in pixels
– codebase - where to find .class files
– align - how to align the applet in the page
– name - used for applet to applet communication
– archive - gives the name of the jar file(s) (1.1+)
– hspace - horizontal space around the applet
– vspace - vertical space around the applet
Georgia Institute of Technology
Using Parameters
• Use the param tag to specify the parameters in
the html file.
– <param name=identifier value=“information”>
<param name=name value=“Sue”>
<param name=numItems value=“3”>
• Use getParameter in the applet to read a
parameter as a string.
– public String getParameter(String paramName)
String name = getParameter(“name”);
String numString = getParameter(“numItems”);
if (numString != null)
int numItems = Integer.parseInt(numString);
else
int numItems = 0;
Georgia Institute of Technology
Message Applet Exercise
• Modify
MessageApplet.java to
get the string to display
as a parameter.
• Modify
MessageApplet.html to
pass in the message as a
parameter.
Georgia Institute of Technology
Steps in Exercise
• Edit MessageApplet.java
– Add the following line in the init method to get the
passed parameter
String message = getParameter(“message”);
– Compile MessageApplet.java
javac MessageApplet.java
• Edit MessageApplet.html
– Add the parameter tag between the <applet> and
</applet> tags
<param name=message value=“Hello Class!”>
• Test the applet
– appletviewer Name.html
Georgia Institute of Technology
Applet Methods
• Applets inherit methods from the Applet class
–
–
–
–
init() - initialize and set-up the applet
start() - start execution of applet or restart
stop() - stop execution of applet
destroy() - clean up before termination
Browser calls init to setup
applet or on reload
init()
Stop is called when leaving
the page or quitting.
start()
Start is called to
start execution
(applet visible)
Georgia Institute of Technology
stop()
destroy()
Final cleanup
Run Applet Exercise
• Use appletviewer to test
the applet
– Open a command prompt
window
– Start->Programs>Accessories->Command
Prompt
– Change to the correct
directory
– appletviewer
RunApplet.html
• Stop the applet
• Start the applet
• Restart the applet
Georgia Institute of Technology
Disadvantages to Applets
• You must download the code from the server
– user may have to wait
• Using html, animated gifs, dhtml, and scripting
may be easier for some tasks
• Version problems with browsers
– 3.0 browsers support Java 1.0.2
– 4.0 browsers support Java 1.1
– Need plug-in for 1.2 (only Sun and windows)
• Security restrictions
Georgia Institute of Technology
Advantages to Applets
• Code is downloaded from server
– no distribution
– no installation
– users all on same version of code
• Cross platform GUI environment
• Multimedia support
• The power of an object-oriented programming
language
• Create interactive and dynamic web pages
Georgia Institute of Technology
Applet Resources
• Sun Tutorial for AWT style applets
http://java.sun.com/docs/books/tutorial/applet/index.html
• Sun tutorial for Swing style applets
http://java.sun.com/docs/books/tutorial/uiswing/compone
nts/applet.html
• Applet Home Page
http://java.sun.com/applets/
• Example Applet
– Visual Human
http://www.dhpc.adelaide.edu.au/projects/vishuman2/
Georgia Institute of Technology
Summary
• There are three kinds of comments in Java
– // comment or /* comment */ or /** comment */
• The package statement is the first code in a
class file.
– package edu.gatech.ice;
• Import statements follow the package statement.
– import edu.gatech.edu.*;
• The class definition follows the import statement.
• Inside a class definition you declare attributes,
constructors, and methods.
Georgia Institute of Technology