Chapter 10 Classes Continued. Fundamentals of Java

Transcription

1 Chapter 10 Classes Continued

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. 2

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. 3

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. 4

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

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

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 7

8 Class (static) Variables and 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. 8

9 Class (static) Variables and 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; 9

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

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

12 Class (static) Variables and Methods (cont.) Using class variables example: Using class constants example: 12

13 Class (static) Variables and Methods (cont.) 13 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 objectoriented programming Pen used for drawing on a window StandardPen is a specific type of Pen. 14

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

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

17 Java Interfaces The Client Perspective 17 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 Java Interfaces The Client Perspective (cont.) A Java interface specifies the method signatures for an interface. 18

19 Java Interfaces The Client 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: 19

20 Java Interfaces The Client 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. 20

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

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. 22

23 Java Interfaces The Implementation Perspective (cont.) 23 The Circle class

24 Java Interfaces The Implementation Perspective (cont.) 24 The Circle class (cont.)

25 Java Interfaces The Implementation Perspective (cont.) Example 10.3: Try out some shapes 25

26 Java Interfaces The Implementation Perspective (cont.) 26 Example 10.3: Try out some shapes (cont.)

27 Java Interfaces The Implementation Perspective (cont.) Figure 10.3: Output from the TestShapes program 27

28 Java Interfaces The Implementation Perspective (cont.) 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. 28

29 Java Interfaces The Implementation Perspective (cont.) 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. 29

30 Code Reuse Through Inheritance All classes are part of a large class hierarchy. Object class is the root. 30 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 Code Reuse Through Inheritance (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. 31 Figure 10-4: Part of a class hierarchy

32 Code Reuse Through Inheritance (cont.) A class can inherit the characteristics of another class using the extends keyword. 32 The Wheel class extends the Circle class.

33 Code Reuse Through Inheritance (cont.) 33 The Wheel class extends the Circle class (cont.).

34 Code Reuse Through Inheritance (cont.) Example 10.4: Draw a wheel and a circle. 34

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

36 Code Reuse Through Inheritance (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. 36

37 Code Reuse Through Inheritance (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. 37

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 38

39 Inheritance and Abstract Classes (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 39

40 Inheritance and Abstract Classes (cont.) Partial abstract class/method example: 40

41 Interfaces, Inheritance, and Relationships among Classes 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. 41

42 Interfaces, Inheritance, and Relationships among Classes (cont.) 42 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 Interfaces, Inheritance, and Relationships among Classes (cont.) 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. 43

44 Interfaces, Inheritance, and Relationships among Classes (cont.) 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 44

45 Interfaces, Inheritance, and Relationships among Classes (cont.) 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. 45

46 Interfaces, Inheritance, and Relationships among Classes (cont.) 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 47

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 48

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

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

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

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

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 53

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

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. 55

56 Reference Types, Equality, 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: 56

57 Reference Types, Equality, and Object Identity (cont.) Copying objects: Aliasing is not copying. Use a copy constructor: Implement Clonable: 57

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 58

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. 59

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. 60

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 61