This document discusses abstract classes and interfaces in Java. It provides examples of abstract classes that cannot be instantiated but can be extended by subclasses. Interfaces are defined as collections of method signatures without implementations that classes can implement to define common behaviors. The document compares abstract classes and interfaces, and provides examples of defining and implementing interfaces. It also discusses abstract and final methods, and the use of abstract classes and interfaces in Java applications.

1. Object Oriented Design
and Programming II
Chapter 10
Abstract classes and
Interfaces
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2. Software Engineering Principle
 Minimize change

3. From concrete classes
 To abstract classes
 To interface
 Examples:
 BJ_1_initial_SalCal

BJ_2_class_SalCal
 BJ_3_abstract_SalCal
 BJ_4_abstract_method_SalCal

BJ_5_interface_SalCal

4. abstract and final Methods
 An abstract method in a superclass has
no implementation, and it is to be
overridden by a method in its subclass.
 A final method in a superclass cannot be
overridden at its subclass.
 Why do I need to bother with abstract
methods and final methods? Can I live
happily in Java without them?
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5. Abstract Class
 A class that cannot instantiate objects.
Message
Text Voice Fax
Message Message Message
Public abstract class Message {
}
Do I and should I use non-abstract methods
in abstract classes?
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6. Abstract or not-abstract
Contains Contains non-
abstract abstract
methods methods
Class abstract May contain OK
abstract methods. E.g. class
abstract methods BJ_abstract_Figure
must be in abstract
classes
GeometricObject in
BJ_GeomObj.
Situation 3
Class non- Not allowed Fine. You are
abstract
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6

7. Example BJ_abstract_Figure
 Problems of Bj_FindArea
 BJ_abstract_Figure
 An abstract class used as supertype
 An object cannot be created from an abstract
class

An abstract class can be extended by a
subclass
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8. Example BJ_abstract_Figure2
 An abstract class used as supertype
 An object cannot be created from an
abstract class
 An array of the abstract type is used to
contain objects of the concrete subclasses
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9. Example BJ_GeomObj_Circle9
 Circle9 extends an abstract class
GeometricObject
 Note the 4 situations in the project:

Circle class has concrete method getArea()

No abstract method getArea(); concrete
getArea() in Circle

abstract method getArea(); concrete
getArea() in Circle
 Instantiate object of abstract class
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10. Empty vs abstract methods
 Method with empty body

protected abstract double getArea();
 Abstract method
 protected abstract double getArea();
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11. From abstract class
To Interface

12. What is Interface
 An interface is a named collection of method
definitions and constants ONLY.
 An interface defines a protocol of behavior that
can be implemented by any class anywhere in
the class hierarchy.
 An interface defines a set of methods but does
not implement them.
 A class that implements the interface agrees to
implement all the methods defined in the
interface, thereby agreeing to certain behaviors.
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13. Interface and Abstract Classes
 An interface cannot implement any
methods, whereas an abstract class can.
 A class can implement many interfaces
but can have only one superclass.
 An interface is not part of the class
hierarchy. Unrelated classes can
implement the same interface.
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14. Multiple Inheritance
Class A Class B Class C
Class ABC
Class ABC inherits all variables and methods from
Class A, Class B, and Class C.
Java does NOT support multiple inheritances.
However, you can use interface to implement the
functionality of multiple inheritance.
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15. Defining Interfaces
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16. Interface Declaration
public interface StockWatcher{ }
public interface Sortable{ }
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17. Interface Body
 The interface body contains method
declarations for ALL the methods included
in the interface.
 A method declaration within an interface is
followed by a semicolon (;) because an
interface does not provide
implementations for the methods declared
within it.
 All methods declared in an interface are
implicitly public and abstract.
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18. Implement an Interface
 An interface defines a protocol of
behavior.
 A class that implements an interface
adheres to the protocol defined by that
interface.
 To declare a class that implements an
interface, include an implements clause in
the class declaration.
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19. Implement Interface (Example)
public class StockApplet extends Applet implements StockWatcher {
...
public void valueChanged(String tickerSymbol, double newValue) {
if (tickerSymbol.equals(sunTicker)) {
// record newValue for sunTicker...
} else if (tickerSymbol.equals(oracleTicker)) {
// record newValue for oracleTicker
} else if (tickerSymbol.equals(ciscoTicker)) {
// record newValue for ciscoTicker
}
}
}
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20. Code Review
 BJ_Interface

Objects inheriting properties of superclass
and implementing properties of interface
 A class may have only one superclass but
may implement multiple interfaces

Using an array of supertype
 polymorphism
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21. Sorting
 It is easy to write a sorting method for numbers
of a specific type.

bubblesort, shellsort, quicksort, heapsort.
 It is not easy to write a method to sort numbers
of any primitive type: short, int, long, float, and
double.
 See Example 9.2 GenericSort.java
 It is a challenge to write a method to sort
objects
 How do you do the above in C?
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22. Example: Comparable interface
 Java.lang.Comparable
 See BJ_Max
 See BJ_GenericSort
 Still relying on Java 1.5 unboxing feature of
wrapper objects

23. Source Code Review and Demo
 Exercise 1: Download, run, and study
BJ_Sort/SortTest.java
 Each class needs to implement the Sortable
interface with a compare() method
 Exercise 2: Add other sorting methods
Bubble, Insertion, Selection
 Exercise 3: C:ProgramFilesJava
jdk1.5.0_12demoappletsSortDemo
 To compare speed

24. Application Programming
Interface (API)
 How do I learn to use the packages in the
Java platform?
 Answer: API
 http://java.sun.com/j2se/1.5.0/docs/api/
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25. API Study
example: StringTokenizer
1. Location of the API( package)
2. Class definition
3. Constructor(s) – usually more than one
4. Methods (you can see public only)
5. Variables (any public ones)
6. Interfaces – supply your own methods
7. Exceptions
8. Examples
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26. Clone()
 newObject = someObject;
 Only assigns the reference of someObject to
newObject. No copy is made
 newObject = someObject.clone();

Copies someObject to a new memory location
 See BJ_House

27. hashCode()
 hashCode() returns the object’s hash code
 Hash code is an integer to store the object
in a hash set.
 If you override the equal() method, you
should also override the hashCode
 See BJ_House