3java Advanced Oop

1 © 2001-2003 Marty Hall, Larry Brown http://www.corewebprogramming.com
core
programming
Advanced
Object-Oriented
Programming in Java

Advanced Object Oriented Programming2 www.corewebprogramming.com
Agenda
• Overloading
• Designing “real” classes
• Inheritance
• Advanced topics
– Abstract classes
– Interfaces
– Understanding polymorphism
– Setting a CLASSPATH and using packages
– Visibility modifiers
– Creating on-line documentation using JavaDoc

Example 4: Overloading
class Ship4 {
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);
}
...

Overloading (Continued)
...
public void move() {
move(1);
}
public void move(int steps) {
double angle = degreesToRadians(direction);
x = x + (double)steps * speed * Math.cos(angle);
y = y + (double)steps * speed * Math.sin(angle);
}
public void printLocation() {
System.out.println(name + " is at ("
+ x + "," + y + ").");
}
}

Overloading: Testing and
Results
public class Test4 {
public static void main(String[] args) {
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();
}
}
• Compiling and Running:
javac Test4.java
java Test4
• Output:
Ship1 is at (1,0).
Ship2 is at (-4.24264,4.24264).

Overloading: Major Points
• Idea
– Allows you to define more than one function or
constructor with the same name
• Overloaded functions or constructors must differ in
the number or types of their arguments (or both), so
that Java can always tell which one you mean
• Simple examples:
– Here are two square methods that differ only in the
type of the argument; they would both be permitted inside
the same class definition.
// square(4) is 16
public int square(int x) { return(x*x); }
// square("four") is "four four"
public String square(String s) {
return(s + " " + s);
}

Example 5: OOP Design and
Usage
/** 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. */
public void setX(double x) {
this.x = x;
}

Example 5: Major Points
• Encapsulation
– Lets you change internal representation and data
structures without users of your class changing their code
– Lets you put constraints on values without users of your
class changing their code
– Lets you perform arbitrary side effects without users of
your class changing their code
• Comments and JavaDoc
– See later slides (or book) for details

Example 6: Inheritance
public 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 void printLocation() {
System.out.print(getColor().toUpperCase() + " ");
super.printLocation();
}
...
}

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();
s3.printLocation();
}
}

Inheritance Example: Result
javac SpeedboatTest.java
– The above calls javac on Speedboat.java and
Ship.java automatically
java SpeedboatTest
• Output
RED Speedboat1 is at (20,0).
BLUE Speedboat2 is at (-1.41421,1.41421).
Ship1 is at (-1.41421,1.41421).

Example 6: Major Points
• Format for defining subclasses
• Using inherited methods
• Using super(…) for inherited constructors
– Only when the zero-arg constructor is not OK
• Using super.someMethod(…) for inherited
methods
– Only when there is a name conflict

Inheritance
• Syntax for defining subclasses
public class NewClass extends OldClass {
...
}
• Nomenclature:
– The existing class is called the superclass, base class or parent class
– The new class is called the subclass, derived class or child class
• Effect of inheritance
– Subclasses automatically have all public fields and methods of the
parent class
– You don’t need any special syntax to access the inherited fields and
methods; you use the exact same syntax as with locally defined
fields or methods.
– You can also add in fields or methods not available in the superclass
• Java doesn’t support multiple inheritance

Inherited constructors and
super(...)
• When you instantiate an object of a subclass, the
system will automatically call the superclass
constructor first
– By default, the zero-argument superclass constructor is called
unless a different constructor is specified
– Access the constructor in the superclass through
super(args)
– If super(…) is used in a subclass constructor, then super(…)
must be the first statement in the constructor
• Constructor life-cycle
– Each constructor has three phases:
1. Invoke the constructor of the superclass
2. Initialize all instance variables based on their initialization
statements
3. Execute the body of the constructor

Overridden methods and
super.method(...)
• When a class defines a method using the same name,
return type, and arguments as a method in the
superclass, then the class overrides the method in the
superclass
– Only non-static methods can be overridden
• If there is a locally defined method and an inherited
method that have the same name and take the same
arguments, you can use the following to refer to the
inherited method
super.methodName(...)
– Successive use of super (super.super.methodName) will not
access overridden methods higher up in the hierarchy; super can
only be used to invoke overridden methods from within the class
that does the overriding

Advanced OOP Topics
• Abstract classes
• Interfaces
• Polymorphism details
• CLASSPATH
• Packages
• Visibility other than public or private
• JavaDoc details

Abstract Classes
• Idea
– Abstract classes permit declaration of classes that define only part of
an implementation, leaving the subclasses to provide the details
• A class is considered abstract if at least one
method in the class has no implementation
– An abstract method has no implementation (known in C++ as a pure
virtual function)
– Any class with an abstract method must be declared abstract
– If the subclass overrides all the abstract methods in the superclass,
than an object of the subclass can be instantiated
• An abstract class can contain instance variables
and methods that are fully implemented
– Any subclass can override a concrete method inherited from the
superclass and declare the method abstract

Abstract Classes (Continued)
• An abstract class cannot be instantiated,
however references to an abstract class can
be declared
public abstract ThreeDShape {
public abstract void drawShape(Graphics g);
public abstract void resize(double scale);
}
ThreeDShape s1;
ThreeDShape[] arrayOfShapes
= new ThreeDShape[20];
• Classes from which objects can be
instantiated are called concrete classes

Interfaces
• Idea
– Interfaces define a Java type consisting purely of
constants and abstract methods
– An interface does not implement any of the methods, but
imposes a design structure on any class that uses the
interface
– A class that implements an interface must either provide
definitions for all methods or declare itself abstract

Interfaces (Continued)
• Modifiers
– All methods in an interface are implicitly abstract and the
keyword abstract is not required in a method declaration
– Data fields in an interface are implicitly static
final (constants)
– All data fields and methods in an interface are implicitly
public
public interface Interface1 {
DataType CONSTANT1 = value1;
DataType CONSTANT2 = value2;
ReturnType1 method1(ArgType1 arg);
ReturnType2 method2(ArgType2 arg);
}

Interfaces (Continued)
• Extending Interfaces
– Interfaces can extend other interfaces, which brings rise to sub-
interfaces and super-interfaces
– Unlike classes, however, an interface can extend more than one
interface at a time
public interface Displayable extends Drawable, Printable {
// Additonal constants and abstract methods
...
}
• Implementing Multiple Interfaces
– Interfaces provide a form of multiple inheritance because a class can
implement more than one interface at a time
public class Circle extends TwoDShape
implements Drawable, Printable {
...
}

Polymorphism
• “Polymorphic” literally means “of multiple shapes”
and in the context of object-oriented programming,
polymorphic means “having multiple behavior”
• A polymorphic method results in different actions
depending on the object being referenced
– Also known as late binding or run-time binding
• In practice, polymorphism is used in conjunction
with reference arrays to loop through a collection
of objects and to access each object's
polymorphic method

Polymorphism: Example
public class PolymorphismTest {
Ship[] ships = new Ship[3];
ships[0] = new Ship(0.0, 0.0, 2.0, 135.0, "Ship1");
ships[1] = new Speedboat("Speedboat1");
ships[2] = new Speedboat(0.0, 0.0, 2.0, 135.0,
"Speedboat2", "blue");
for(int i=0; i<ships.length ; i++) {
ships[i].move();
ships[i].printLocation();
}
}
}

Polymorphism: Result
javac PolymorphismTest.java
java PolymorphismTest
• Output
RED Speedboat1 is at (20,0).
BLUE Speedboat2 is at (-1.41421,1.41421).
Ship1 is at (-1.41421,1.41421).

CLASSPATH
• The CLASSPATH environment variable
defines a list of directories in which to look
for classes
– Default = current directory and system libraries
– Best practice is to not set this when first learning Java!
• Setting the CLASSPATH
set CLASSPATH = .;C:java;D:cwpechoserver.jar
setenv CLASSPATH .:~/java:/home/cwp/classes/
– The period indicates the current working directory
• Supplying a CLASSPATH
javac –classpath .;D:cwp WebClient.java
java –classpath .;D:cwp WebClient

Creating Packages
• A package lets you group classes in
subdirectories to avoid accidental name conflicts
– To create a package:
1. Create a subdirectory with the same name as the desired
package and place the source files in that directory
2. Add a package statement to each file
package packagename;
3. Files in the main directory that want to use the package should
include
import packagename.*;
• The package statement must be the first
statement in the file
• If a package statement is omitted from a file, then
the code is part of the default package that has
no name

Package Directories
• The package hierarchy reflects the file
system directory structure
– The root of any package must be accessible through a
Java system default directory or through the CLASSPATH
environment variable
Package java.math

Visibility Modifiers
• public
– This modifier indicates that the variable or method can be
accessed anywhere an instance of the class is accessible
– A class may also be designated public, which means
that any other class can use the class definition
– The name of a public class must match the filename, thus
a file can have only one public class
• private
– A private variable or method is only accessible from
methods within the same class
– Declaring a class variable private "hides" the data within
the class, making the data available outside the class only
through method calls

Visibility Modifiers, cont.
• protected
– Protected variables or methods can only be accessed by
methods within the class, within classes in the same
package, and within subclasses
– Protected variables or methods are inherited by
subclasses of the same or different package
• [default]
– A variable or method has default visibility if a modifier is
omitted
– Default visibility indicates that the variable or method can
be accessed by methods within the class, and within
classes in the same package
– Default variables are inherited only by subclasses in the
same package

Protected Visibility: Example
• Cake, ChocolateCake, and Pie inherit a calories field
• However, if the code in the Cake class had a reference to object of
type Pie, the protected calories field of the Pie object could not
be accessed in the Cake class
– Protected fields of a class are not accessible outside its branch of the class hierarchy
(unless the complete tree hierarchy is in the same package)

Default Visibility: Example
• Even through inheritance, the fat data field cannot cross the
package boundary
– Thus, the fat data field is accessible through any Dessert, Pie, and Cake
object within any code in the Dessert package
– However, the ChocolateCake class does not have a fat data field, nor can the fat
data field of a Dessert, Cake, or Pie object be accessed from code in the
ChocolateCake class

Visibility Summary
Modifiers
Data Fields and Methods public protected default private
Accessible from same class? yes yes yes yes
Accessible to classes (nonsubclass) yes yes yes no
from the same package?
Accessible to subclass from the yes yes yes no
same package?
Accessible to classes (nonsubclass) yes no no no
from different package?
Accessible to subclasses from yes no no no
different package?
Inherited by subclass in the yes yes yes no
same package?
Inherited by subclass in different yes yes no no
package?

Other Modifiers
• final
– For a class, indicates that it cannot be subclassed
– For a method or variable, cannot be changed at runtime or
overridden in subclasses
• synchronized
– Sets a lock on a section of code or method
– Only one thread can access the same synchronized code
at any given time
• transient
– Variables are not stored in serialized objects sent over the
network or stored to disk
• native
– Indicates that the method is implement using C or C++

Comments and JavaDoc
• Java supports 3 types of comments
– // Comment to end of line.
– /* Block comment containing multiple lines.
Nesting of comments in not permitted. */
– /** A JavaDoc comment placed before class
definition and nonprivate methods.
Text may contain (most) HTML tags,
hyperlinks, and JavaDoc tags. */
• JavaDoc
– Used to generate on-line documentation
javadoc Foo.java Bar.java
– JavaDoc 1.4 Home Page
• http://java.sun.com/j2se/1.4/docs/tooldocs/javadoc/

Useful JavaDoc Tags
• @author
– Specifies the author of the document
– Must use javadoc –author ... to generate in output
/** Description of some class ...
*
* @author <A HREF="mailto:brown@lmbrown.com">
* Larry Brown</A>
*/
• @version
– Version number of the document
– Must use javadoc –version ... to generate in output
• @param
– Documents a method argument
• @return
– Documents the return type of a method

Useful JavaDoc Command-line
Arguments
• -author
– Includes author information (omitted by default)
• -version
– Includes version number (omitted by default)
• -noindex
– Tells javadoc not to generate a complete index
• -notree
– 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/1.3/docs/api
-linkoffline http://java.sun.com/j2se/1.3/docs/api
c:jdk1.3docsapi

JavaDoc, Example
/** Ship example to demonstrate OOP in Java.
*
* @author <A HREF="mailto:brown@corewebprogramming.com">
* Larry Brown</A>
* @version 2.0
*/
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);
}
...

JavaDoc, Example
> javadoc -linkoffline http://java.sun.com/j2se/1.3/docs/api
c:jdk1.3docsapi
-author -version -noindex -notree Ship.java

JavaDoc: Result

Summary
• Overloaded methods/constructors, except
for the argument list, have identical
signatures
• Use extends to create a new class that
inherits from a superclass
– Java does not support multiple inheritance
• An inherited method in a subclass can be
overridden to provide custom behavior
– The original method in the parent class is accessible
through super.methodName(...)
• Interfaces contain only abstract methods
and constants
– A class can implement more than one interface

Summary (Continued)
• With polymorphism, binding of a method to
a n object is determined at run-time
• The CLASSPATH defines in which directories
to look for classes
• Packages help avoid namespace collisions
– The package statement must be first statement in the
source file before any other statements
• The four visibility types are: public, private,
protected, and default (no modifier)
– Protected members can only cross package boundaries
through inheritance
– Default members are only inherited by classes in the
same package

42 © 2001-2003 Marty Hall, Larry Brown http://www.corewebprogramming.com
core
programming
Questions?

3java Advanced Oop

More Related Content

What's hot (19)

Viewers also liked (19)

Similar to 3java Advanced Oop (20)

More from Adil Jafri (20)

Recently uploaded (20)

3java Advanced Oop