LQAS-JAVA-OOP-More

7/28/2019 LQAS-JAVA-OOP-More

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2010 Marty Hall

Ob ect-OrientedProgramming in Java:

Originals of Slides and Source Code for Examples:

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2010 Marty Hall

For live Java EE training, please see training courses

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Taught by the author ofCore Servlets and JSP, More


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Agenda

Overloading

Best practices for real classesEncapsulation and accessor methods

ava oc

Inheritance

Abstract classes

CLASSPATH

PackagesVisibility modifiers

JavaDoc options4

2010 Marty Hall

Overloading




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Overview

IdeaClasses can have more than one method with the same

name, or more than one constructor.

other by having different number or types of arguments

Syntaxpublic class MyClass {

public double getRandomNum() { }; // Range 1-10

}

MotivationMethods: lets you have similar names for similar ops

Constructors: let you build instances in different ways6

Ship Example: Overloadingpublic class Ship4 { (In Ship4.java)public double x=0.0, y=0.0, speed=1.0, direction=0.0;public String name;

public Ship4(double x, double y,double speed, double direction,String name) {

this.x = x;this.y = y;

this.speed = speed;this.direction = direction;this.name = name;

}

public Ship4(String name) {this.name = name;

}

private double degreesToRadians(double degrees) {return(degrees * Math.PI / 180.0);

}

...7


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Overloading (Continued)

...

public voidmove() {

move(1);}

public voidmove(int steps) {

double an le = de reesToRadians(direction);

x = x + steps * speed * Math.cos(angle);

y = y + steps * speed * Math.sin(angle);

}

public void printLocation() {

System.out.println(name + " is at ("

+ x + "," + y + ").");

}

}8

Overloading: Testing and

public class Test4 { (In Test4.java)

ublic static void main Strin ar s

Ship4 s1 = new Ship4("Ship1");

Ship4 s2 = new Ship4(0.0, 0.0, 2.0, 135.0, "Ship2");

s1.move();

s2.move(3);

s1.printLocation();

s2.printLocation();

}

- ,DOS> javac Test4.java

DOS> java Test4

Ship1 is at (1.0,0.0).

Ship2 is at (-4.24264...,4.24264...).

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Overloading: Major Points

Idea ows you o e ne more an one unc on or

constructor with the same name Overloaded functions or constructors must differ in the

,can always tell which one you mean

Simple examples:

type of the argument; they would both be permitted insidethe same class definition.

// square(4) is 16public int square(int x) { return(x*x); }

square our s our ourpublic String square(String s) {

return(s + " " + s);}10

2010 Marty Hall

Best Practices




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Overview

Ideas Instance variables should always be private

And hooked to outside world with getBlah and/or setBlah -,

Syntax/** Short summary. More detail. Can use HTML. */

public class MyClass {

private String firstName;

ublic Strin etFirstName return firstName

public void setFirstName(String s) { firstName = s; }

} o va on

Limits ripple effect. Makes code more maintainable.12

Ship Example: OOP Design and

/** Ship example to demonstrate OOP in Java. */

public class Ship {

private double x=0.0, y=0.0, speed=1.0, direction=0.0;

private String name;

/** Get current X location. */

public double getX() {

return(x);

}

/** Set current X location. */

this.x = x;

}13


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OOP Design: Testing and

public class ShipTest { (In ShipTest.java)

ublic static void main Strin ar s

Ship s1 = new Ship("Ship1");

Ship s2 = new Ship(0.0, 0.0, 2.0, 135.0, "Ship2");s1.move();

s2.move(3);

s1.printLocation();

s2.printLocation();

}

- ,javac ShipTest.java

java ShipTest

javadoc *.java

Output:Ship1 is at (1.0,0.0).

Ship2 is at (-4.24264...,4.24264...).14

OOP Design: Testing and Results

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Major Points

EncapsulationLets you change internal representation and data

structures without users of your class changing their code

class changing their code

Lets you perform arbitrary side effects without users ofyour class changing their code

Comments and JavaDoc ommen s mar e w ... w e par o e

online documentation

Call "javadoc *.java" to build online documentation.

See later slides for details

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More Details on Getters and

Eclipse will automatically buildgetters/setters from instance variablesR-click anywhere in code

However, if you later click on instance variable and do

Refactor Rename, Eclipse will not automaticallyrename the accessor methods

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There need not be both getters and setters It is common to have fields that can be set at instantiation,

but never changed again (immutable field). It is evenuite common to have classes containin onl immutable

fields (immutable classes)

private final String name;

pu c p name = ;

public String getName() { return(name); }

// No setName method18


Getter/setter names need not correspond toinstance variable namesCommon to do so if there is a simple correspondence, but

In fact, there doesnt even have to be a corresponding

instance variable

public class Customer {

public String getFirstName() { getFromDatabase(); }

public void setFirstName() { storeInDatabase(); }

.

}

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2010 Marty Hall

Inheritance



Overview

Ideas ou can ma e a c ass t at n er ts c aracter st cs o

another class The original class is called parent class, super class, or

. ,subclass, or extended class.

The child class has access to all non-private methods of.

No special syntax need to call inherited methods

Syntaxpublic String ChildClass extends ParentClass { }

Motivation . .,

the same code twice). Design class hierarchies so thatshared behavior is in inherited to all classes that need it.

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Simple Example

Personpublic class Person {

public String getFirstName() { }public String getLastName() { }

}

public class Employee extends Person {

public double getSalary() { }

public String getEmployeeInfo() {

return(getFirstName() + " " + getLastName() +earns ge a ary ;

}

}22

Ship Example: Inheritancepublic class Speedboat extends Ship {private String color = "red";

public Speedboat(String name) {super(name);setSpeed(20);

}

public Speedboat(double x, double y,

double speed, double direction,String name, String color) {

super(x, y, speed, direction, name);setColor(color);

}

public voidprintLocation() {System.out.print(getColor().toUpperCase() + " ");super.printLocation();

}

...

}23


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Inheritance Example: Testing

public class SpeedboatTest {

Speedboat s1 = new Speedboat("Speedboat1");

Speedboat s2 = new Speedboat(0.0, 0.0, 2.0, 135.0,"Speedboat2", "blue");

Ship s3 = new Ship(0.0, 0.0, 2.0, 135.0, "Ship1");

s1.move();

s2.move();

s3.move();

s1.printLocation();

s2.printLocation();

s .pr n oca on ;

}

}

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Inheritance Example: Result

Compiling and running in EclipseSave SpeedBoatTest.java

R-click, Run As Java Application

DOS> javac SpeedboatTest.java

The above calls javac on Speedboat.java and Ship.javaautomatically

DOS> java SpeedboatTest

u puRED Speedboat1 is at (20,0).

BLUE S eedboat2 is at (-1.41421,1.41421).

Ship1 is at (-1.41421,1.41421).

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Ship Inheritance Example:

Format for defining subclasses

Using inherited methods

Using super() for inherited constructorsOnly when the zero-arg constructor is not OK

Using super.someMethod() for inherited

Only when there is a name conflict

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Inheritance Syntax for defining subclasses

...

}

Nomenclature: The old class is called the superclass,base class orparent class The new class is called the subclass, derived class orchild class

Effect of inheritance Subclasses automatically have all public fields and methods of theparent class

You dont need any special syntax to access the inherited fields and

fields or methods. You can also add in fields or methods not available in the superclass

A class can only have one directparent. But grandparent and great-

grandparent (etc.) are legal and common.27


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Inherited constructors and...

When you instantiate an object of a subclass, thesys em w au oma ca y ca e superc ass

constructor first By default, the zero-argument superclass constructor is called If you want to specify that a different parent constructor is called,

invoke the parent class constructor with super(args)

Ifsuper() is used in a subclass constructor, then super()must be the first statement in the constructor

Constructor life-cycle

1. Invoke the constructor of the superclass

The zero-argument constructor is called automatically. No special syntaxis needed unless you want a different parent constructor.

2. Initialize all instance variables based on their initializationstatements

3. Execute the body of the constructor28

Overridden methods and. ...

When a class defines a method using the, ,

method in the superclass, then the classoverrides the method in the superclass

-

If there is a locally defined method and ann er e me o a ave e same nameand take the same arguments, you can use thefollowing to refer to the inherited method

super.methodName(...)

Successive use ofsu er su er.su er.methodName isnot legal.

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2010 Marty Hall

Advanced Topics



Advanced OOP Topics

Abstract classes

Interfaces

CLASSPATH

Packages

Visibility other than public or private JavaDoc details

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Abstract Classes

IdeaA class that you cannot directly instantiate (i.e., on which

you cannot use new). But you can subclass it andinstantiate the subclasses.

Syntaxpublic abstract class SomeClass {

public abstract SomeType method1(); // No body

public SomeType method2() { } // Not abstract

Motivation Lets you make collections of mixed types

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Abstract Classes: Problem

You haveCircle and Rectangle classes, each with getArea methods

Goal et sum o areas o an array o rc es an ectang es

Object[] shapes =

{ new Circle(), new Rectangle() };

double sum = 0;

for(Object shape: shapes) {

sum = sum + shape.getArea();

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Abstract Classes: Solution

Shapepublic abstract class Shape {

public abstract double getArea();

public void printInfo() {

System.out.println(getClass().getSimpleName() +

" has area " + getArea());

}

}

Circle and similar for Rectan lepublic class Circle extends Shape {

public double getArea() { }

}

34

Interfaces

IdeaA model for a class. More or less an abstract class but

without any concrete methods.

public interface SomeInterface {

public SomeType method1(); // No bodypublic SomeType method2(); // No body

}

public class SomeClass implements SomeInterface {

// Real definitions of method1 and method 2

}

Like abstract classes, guarantees classes have certain

methods. But classes can implement multiple interfaces.35


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Interfaces: Problem

Sum of areasYou again want to get sum of areas of mixture of Circles

and Rectangles. ,

concrete printInfo method

Why interface instead of abstract class?Classes can directly extend only one class (abstract or

otherwise)

public class Foo extends Bar implements Baz, Boo {

36

Interfaces: Solution

Shapepublic interface Shape {

public double getArea();

}

Circlepublic class Circle implements Shape {

ublic double etArea }

Rectanglepu c c ass ec ang e mp emen s ape

public double getArea() { }

}

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CLASSPATH

IdeaThe CLASSPATH environment variable defines a list of

directories in which to look for classes =

Best practice is to not set this when first learning Java!

Setting the CLASSPATHset CLASSPATH = .;C:\java;D:\cwp\echoserver.jar

setenv CLASSPATH .:~/java:/home/cwp/classes/

.

Supplying a CLASSPATHjavac classpath .;D:\cwp WebClient.java

java classpath .;D:\cwpWebClient

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Packages

Idea Organize classes in groups.

Syntax

Make folder called somePackage (in Eclipse, R-click on srcand do New Package)

To use code from another package put import somePackage.* in file below your package

Motivation You only have to worry about name conflicts within your

pac age. So, team members can work on different parts of project without

worrying about what class names the other teams use.39


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Visibility Modifiers

publicThis modifier indicates that the variable or method can be

accessed anywhere an instance of the class is accessible ,

that any other class can use the class definition

The name of a public class must match the filename, thusa file can have only one public class

private

methods within the same class

Declare allinstance variables private

Declare methods private if they are not part of classcontract and are just internal implementation helpers

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Visibility Modifiers (Continued)

protectedProtected variables or methods can only be accessed by

methods within the class, within classes in the sameacka e and within subclasses

[default]

Default visibility indicates that the variable or method canbe accessed by methods within the class, and withinclasses in the same package

omitted . Rarely used!

private: very common. Use this as first choice.

pu c: common or me o s an cons ruc ors. n c o ce

protected: usually for instance vars only. Moderately rare.

default: very rare. Dont omit modifier without good reason.41


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Visibility Summary

Modifiers

Accessible from same class? yes yes yes yes

Accessible to classes (nonsubclass) yes yes yes no

Accessible to subclass from the yes yes yes no

same package?

Accessible to classes (nonsubclass) yes no no no

from differentpackage?

Accessible to from es no no no

different package?

Inherited by subclass in the yes yes yes nosame package?

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Inherited by subclass in different yes yes no no

package?

Other Modifiers

final or a var a e: canno e c ange a er ns an a on

Widely used to make immutable classes

For a class: cannot be subclassed

For a method: cannot be overridden in subclasses

synchronized

Only one thread can access the code at any given time

volatileGuarantees other threads see changes to variable

transient

native Indicates that the method is implement using C or C++43


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Comments and JavaDoc

Java supports 3 types of comments // Comment to end of line.

/* Block comment containing multiple lines.*.

/** A JavaDoc comment placed before classdefinition and nonprivate methods.Text may contain (most) HTML tags,hyperlinks, and JavaDoc tags. */

Used to generate on-line documentation

javadoc Foo.java Bar.java (or *.java)

JavaDoc Home Page http://java.sun.com/javase/6/docs/technotes/tools/windows/javadoc.html

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Useful JavaDoc Tags

@author pec es e au or o e ocumenMust use javadoc author ... to generate in output

/** Description of some class ...** @author * Marty Hall*/

Version number of the documentMust use javadoc version ... to generate in output

@param Documents a method argument

Documents the return type of a method

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Useful JavaDoc Command-line

-author Inc u es aut or n ormat on om tte y e au t

-version Includes version number (omitted by default)

-noindex Tells javadoc not to generate a complete index

- Tells javadoc not to generate the tree.html class hierarchy

-link, -linkoffline Tells javadoc where to look to resolve links to other packages

-link http://java.sun.com/j2se/1.5.0/docs/api/-linkoffline http://java.sun.com/j2se/1.5.0/docs/api/

c:\jdk1.5\docs\api

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JavaDoc: Example/** Ship example to demonstrate OOP in Java.** @author* Marty Hall*/

public class Ship {private double x=0.0, y=0.0, speed=1.0, direction=0.0;private String name;

/** Build a ship with specified parameters. */

public Ship(double x, double y, double speed,double direction, String name) {

setX(x);setY(y);setSpeed(speed);setDirection(direction);setName(name);

}...

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JavaDoc: Example

> javadoc -link http://java.sun.com/j2se/1.5.0/docs/api/

-author *. ava

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2010 Marty Hall

Wrap-Up




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Java OOP References

Online as cs sect on o un ava utor a

http://java.sun.com/docs/books/tutorial/java/ Books

Murachs Java SE 6(Murach, Steelman, and Lowe) Excellent Java intro for beginners to Java (but not first-

time ro rammers . Ver ood OOP section.

Thinking in Java (Bruce Eckel) Perhaps not quite as good as Murachs book in general,

but possibly the best OOP coverage of any Java book.

Effective Java, 2ndEdition (Josh Bloch)

In my opinion, the best Java book ever written. Fantasticcoverage of OOP best practices. However, very advanced. Other than the OOP chapter, you wont

understand much unless you have been doing Java fulltime for atleast a year. Even experts will learn a lot from this book.

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Summary

Overloading ou can ave mu p e me o s or cons ruc ors w e same

name. They must differ in argument signatures (number and/ortype of arguments).

Make all instance variables private

Hook them to the outside with getBlah and/or setBlah Use JavaDoc-style comments from the very beginning

Inheritance ublic class Subclass extends Su erclass Non-private methods available with no special syntax Use super() in constructor if you need non-default parent

constructor moderatel common Use super.method() if local method and inherited method

have the same name (rare!)51


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2010 Marty Hall

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