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Java Source Anatomy

Barb EricsonGeorgia Institute of Technology

July 2008

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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

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Structure of a Java Source FileOptional Package statementOptional Import statementsComments (like author(s))Class definition{

Attributes - fieldsConstructors - initializersMethods - behaviorsOptional Main method

}

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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• Every class belongs to a package. If none is

specified then the unnamed package is used.

java.lang

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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

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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;

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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; // import just Coloror• import java.awt.*; // 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 in your code– java.awt.Color

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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

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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;

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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

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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 Tools and select Preview Javadoc for Current Document– Compare the html documentation to the original

Javadoc comments

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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

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Class Definition Examples

public class Person implements Comparable{

attributes, methods, and inner classes}

private class Helper extends Object{

attributes, methods, and inner classes}

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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

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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 Picture objects know how to do?

• Where is the play method defined that Sound objects know how to do?

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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

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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)

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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

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Attributes/Fields

• 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

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Attribute Exercise

• Open SimplePicture.java• What are the attributes (fields)?

– Which are object fields?– Which are class (static) 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);

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Declaring Attributes

• Attributes are declared as[visibility] type name [= value];

Visibilitynone specified - classes in the same packagepublic - all classesprotected - class, subclasses, and other classes in the same packageprivate - only the class itself

Type - the type can be one of the following primitive type object type: class, interface, or array

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Visibility Exercise

• Open SimplePicture.java– Which fields have public visibility?

• Why are these public?

– Which fields have private visibility?• Why are these private?

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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; }}

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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

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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

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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?

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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}

• Examplepublic String getName() { return name;}

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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

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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)

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Main Method

• Each class can have a main methodpublic static void main(String[] argv) {}

• You tell Java which class to start withjava 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

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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

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Summary

• 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.*;

• There are three kinds of comments in Java– // comment or /* comment */ or /** comment */

• The class definition follows the import statement.• Inside a class definition you declare attributes,

constructors, and methods.