Ppt chapter10

Chapter 10
Classes Continued
Fundamentals of Java

2
Objectives
 Know when it is appropriate to include class
(static) variables and methods in a class.
 Understand the role of Java interfaces in a
software system and define an interface for
a set of implementing classes.
 Understand the use of inheritance by
extending a class.

3
Objectives (cont.)
 Understand the use of polymorphism and
know how to override methods in a
superclass.
 Place the common features (variables and
methods) of a set of classes in an abstract
class.

4
Objectives (cont.)
 Understand the implications of reference
types for equality, copying, and mixed-mode
operations.
 Know how to define and use methods that
have preconditions, postconditions, and
throw exceptions.

5
Vocabulary
 Abstract class
 Abstract method
 Aggregation
 Class (static) method
 Class (static) variable
 Concrete class
 Dependency

6
Vocabulary (cont.)
 Final method
 Inheritance
 Interface
 Overriding
 Postcondition
 Precondition

7
Class (static) Variables and
Methods
 static variables and methods belong to a
class.
– Not an instance of the class
 Class variable: Storage allocated at
program startup
– Independent of number of instances created
 Class method: Activated when message
sent to the class rather than to an object

8
Methods (cont.)
 Class variables and methods are declared
with the keyword static.
 Example:
– private static int studentCount = 0;
 Shared by all instances of the class with this
declaration
 Use a static variable in any situation in which
all instances share a common data value.

9
Methods (cont.)
 Use static methods to provide public access
to static variables.
 Class constants: Combine keyword final
with keyword static
– Example:
 public static final int MIN_SCORE = 0;

10
Methods (cont.)
Static variables, methods, and constants example

11
Methods (cont.)
Static variables, methods, and constants example (cont.)

12
Methods (cont.)
 Using class variables example:
 Using class constants example:

13
Methods (cont.)
 Two rules for using static variables:
– Class methods can reference only static
variables.
 Never instance variables
– Instance methods can reference static and
instance variables.
 The main method for an executable java
class is static.
– JVM sends main message to start a program.

14
Turtle Graphics
 Open-source Java package for drawing
– Used in this text to illustrate features of object-
oriented programming
– Pen used for drawing on a window
 StandardPen is a specific type of Pen.

15
Turtle Graphics (cont.)
Table 10-1: Pen messages

16
Turtle Graphics (cont.)
Example 10.1: Drawing a square using Turtle Graphics

17
Java Interfaces—The Client
Perspective
 Interface: A list of a class’s public methods
– Provides information to use a class without
revealing its implementation
– When related classes have same interface, they
can be used interchangeably in a program.
 In Turtle Graphics, Pen is an interface.
– StandardPen, WigglePen, and RainbowPen
are examples of classes that conform to the
Pen interface.

18
Perspective (cont.)
 A Java interface specifies the method
signatures for an interface.

19
Perspective (cont.)
 An interface is not a class.
– But can be used as a data type
 Having multiple classes conform to the same
interface allows for polymorphic behavior:

20
Perspective (cont.)
 A class that conforms to an interface is said
to implement the interface.
 When declaring a variable or parameter, use
the interface type when possible.
– Methods using interface types are more
general.
– They are easier to maintain.

21
Java Interfaces—The
Implementation Perspective
 A class
implements an
interface using
the implements
keyword.

22
Java Interfaces – The
Implementation Perspective (cont.)
 A class that implements an interface must
implement every method in the interface.
 A variable declared with the interface type
can reference objects of any class that
implements the interface.

23
The Circle class

24
The Circle class (cont.)

25
Example 10.3: Try out some shapes

26
Example 10.3: Try out some shapes (cont.)

27
Figure 10.3: Output from the TestShapes program

28
 Important interface concepts:
– Interface contains only methods, never
variables.
– Interface methods are usually public.
– If more than one class implements an interface,
its methods are polymorphic.

29
 Important interface concepts (cont.):
– A class can implement methods in addition to
those listed in the interface.
– A class can implement more than one interface.
– Interfaces can be in an inheritance hierarchy.

30
Code Reuse Through Inheritance
 All classes are part of a large class hierarchy.
– Object class is the root.
– Each class inherits variables and methods of
the classes above it in the hierarchy.
 New classes can add new variables, add new
methods, or alter existing inherited methods.
– Class immediately above a class is a
superclass.
– Any classes that inherit are subclasses.

31
(cont.)
 A class can only have one superclass, but
may have many subclasses.
 The descendents of a class consist of its
subclasses, their subclasses, etc.
Figure 10-4: Part of a class hierarchy

32
(cont.)
 A class can inherit the characteristics of another
class using the extends keyword.
The Wheel class extends the Circle class.

33
(cont.)
The Wheel class extends the Circle class (cont.).

34
(cont.)
Example 10.4: Draw a wheel and a circle.

35
(cont.)
Figure 10-5: A circle and a wheel with the
same radius but different positions

36
(cont.)
 Wheel implements Shape because it
extends Circle, which implements Shape.
 xPos, yPos, and radius inherited by
Wheel from Shape
– Must modify these instance variables in Shape
to be protected rather than private
 Protected access modifier means variables
are accessible only in current class and its
descendents.

37
(cont.)
 Methods may also be protected.
 A constructor may call superclass constructor
using super();.
 To call one of superclass methods:
 Overriding: A subclass can modify a
superclass method by re-implementing it.

38
Inheritance and Abstract Classes
 Abstract class: Provides functionality
(methods), but can never be instantiated
– Can only be extended
– Declared with keyword abstract
– May contain standard methods and abstract
methods

39
(cont.)
 Abstract method: A method with no body
– Defined using keyword abstract
– A class with an abstract method will also be
abstract.
– A subclass of an abstract class must implement
every abstract method in that class.
 Final methods: Methods that cannot be
overridden in a subclass

40
(cont.)
 Partial abstract class/method example:

41
 A Java interface has a name and consists of
a list of method headers.
 One or more classes can implement the
same interface.
 If a variable is declared as an interface type,
it can be associated with an object from any
class that implements the interface.
Interfaces, Inheritance, and
Relationships among Classes

42
Relationships among Classes (cont.)
 If a class implements an interface, then all of
its subclasses do so implicitly.
 A subclass inherits all of the characteristics of
its superclass.
– Subclass can add new variables and methods or
modify inherited methods.
 Characteristics common to several classes
can be collected in a common abstract
superclass that is never instantiated.

43
 An abstract class can contain headers for
abstract methods that are implemented in the
subclasses.
 A class’s constructors and methods can use
constructors and methods in the superclass.
 Inheritance reduces repetition and promotes
the reuse of code.

44
 Interfaces and inheritance promote the use
of polymorphism.
 When a message is sent to an object, Java
looks for a matching method.
– Search starts in the object’s class and, if
necessary, continues up the class hierarchy

45
 Four ways in which methods in a subclass
can be related to methods in a superclass:
– Implementation of an abstract method
– Extension
– Overriding
– Finality
 It is possible to work only with abstract
classes, forgoing interfaces.

46
Figure 10-7: Three types of relationships among classes

47
Acceptable Classes for
Parameters and Return Values
 If an object of class BBB is expected, it is
always acceptable to substitute an object of a
subclass.
– But never of a superclass
 A subclass of BBB inherits all of BBB’s methods.
 No guarantees about the methods in the
superclass

48
Error Handling with Classes
 Before implementing error handling code,
must determine error conditions for a class
 Preconditions: Describe what must be true
before a particular method is called
– Values for parameters/instance variables
 Postconditions: Describe what must be true
after a particular method has been executed
– Return values and altered instance variables

49
Error Handling with Classes
(cont.)
 Example:

50
Exceptions
 JVM can throw exceptions when illegal
operations are attempted.
 Commonly used exception classes:

51
Exceptions (cont.)
 Can throw exceptions in methods:
– To enforce pre-conditions or any other condition
 Syntax:
 Example:

52
Exceptions (cont.)
 Exceptions to enforce pre-conditions:

53
Exceptions (cont.)
 Clients who call such methods need to catch
the exceptions so the program does not halt.
– Embed method call in a try-catch statement

54
Exceptions (cont.)
 A method may throw more than one type of
exception.
– Client can handle each type explicitly.
 Example:

55
Reference Types, Equality,
and Object Identity
 Aliasing: Two reference variables point to
same object
 Comparing objects for equality:
– Comparing two reference variables using ==
indicates whether the variables point to the
same object.
– To compare values of two distinct objects for
equality, use the equals method.

56
and Object Identity (cont.)
 equals method: Defined in Object class
– Uses == operator by default
– A class must override equals to allow for
comparison of object’s contents.
 Example:

57
and Object Identity (cont.)
 Copying objects:
– Aliasing is not copying.
– Use a copy constructor:
– Implement Clonable:

58
Summary
 Class (static) variables provide storage for
data that all instances of a class can access
but do not have to own separately.
 Class (static) methods are written primarily
for class variables.
 An interface specifies a set of methods that
implementing classes must include.
– Gives clients enough information to use a class

59
Summary (cont.)
 Polymorphism and inheritance reduce the
amount of code that must be written by
servers and learned by clients.
 Classes that extend other classes inherit
their data and methods.
 Methods in different classes that have the
same name are polymorphic.

60
Summary (cont.)
 Abstract classes are not instantiated.
– Help organize related subclasses
– Contain their common data and methods
 Error handling can be distributed among
methods and classes by using preconditions,
postconditions, and exceptions.

61
Summary (cont.)
 Because of the possibility of aliasing, the
programmer should provide:
– equals method for comparing for equality
– clone method for creating a copy of an object

Ppt chapter10

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