Chapter 7 Objects and Classes

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Chapter 7

Objects and Classes

2010-11-4

Introduction to Java Programming

Chapter 7- 1

JAVAPPT英文版

7.1 IntroductionObjects:To understand objects and classes and use classes to model objects (§7.2). To learn how to declare a class and how to create an object of a class (§7.3). To understand the roles of constructors and use constructors to create objects (§7.3). To use UML graphical notations to describe classes and objects (§7.3). To distinguish between object reference variables and primitive data type variables (§7.4). To use classes in the Java library (§7.5). To declare private data fields with appropriate get and set methods to make class easy to maintain (§7.6-7.8). To develop methods with object arguments (§7.9). To understand the difference between instance and static variables and methods (§7.10). To determine the scope of variables in the context of a class (§7.11).2010-11-4 Introduction to Java Programming Chapter 7 - 2

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Object-oriented programming (OOP) involves programming using objects. An object represents an entity in the real world that can be distinctly identified. For example, a student, a desk, a circle, a button, and even a loan can all be viewed as objects. An object has a unique identity, state, and behaviors. The state of an object consists of a set of data fields (also known as properties) with their current values. The behavior of an object is defined by a set of methods.

2010-11-4

Introduction to Java Programming

Chapter 7 - 3

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7.2 Defining Classes for Objects Classes are constructs that define objects of the same type. A Java class uses variables to define data fields and methods to define behaviors. Additionally, a class provides a special type of methods, known as constructors, which are invoked to construct objects from the class.

2010-11-4

Introduction to Java Programming

Chapter 7 - 4

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Classesclass Circle { /** The radius of this circle */ double radius = 1.0; /** Construct a circle object */ Circle() { } /** Construct a circle object */ Circle(double newRadius) { radius = newRadius; } /** Return the area of this circle */ double findArea() { return radius * radius * 3.14159; } }2010-11-4 Introduction to Java Programming Chapter 7 - 5

Data field

Constructors

Method

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7.3 Constructing Objects Using ConstructorsCircle() { } Constructors are a special kind of methods that are invoked to construct objects. Circle(double newRadius) { radius = newRadius; }

2010-11-4

Introduction to Java Programming

Chapter 7 - 6

JAVAPPT英文版

Constructors, cont.Constructors must have the same name as the class itself. Constructors do not have a return type—not even void. Constructors are invoked using the new operator when an object is created. Constructors play the role of initializing objects.

2010-11-4

Introduction to Java Programming

Chapter 7 - 7

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Creating Objects Using Constructorsnew ClassName();

Example: new Circle(); new Circle(5.0);

2010-11-4

Introduction to Java Programming

Chapter 7 - 8

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Default

ConstructorA class may be declared without constructors. In this case, a no-arg constructor with an empty body is implicitly declared in the class. This constructor, called a default constructor, is provided automatically only if no constructors are explicitly declared in the class.

2010-11-4

Introduction to Java Programming

Chapter 7 - 9

JAVAPPT英文版

Constructing Objects, cont.Circle radius: doubleUML Graphical notation for classes UML Graphical notation for fields UML Graphical notation for methods

findArea(): double new Circle() circle1: Circle radius = 2 ... new Circle() circlen: Circle radius = 5UML Graphical notation for objects

2010-11-4

Introduction to Java Programming

Chapter 7 - 10

JAVAPPT英文版

7.4 Accessing Objects via Reference VariablesReference variables and Reference Types Accessing an Object’s Data and Methods The null value Default value for a data field Differences between variables of primitive types and reference types

2010-11-4

Introduction to Java Programming

Chapter 7 - 11

JAVAPPT英文版

7.4 Accessing Objects via Reference VariablesReference variables and Reference Types

To declare a Object’s Data and Methods Accessing an reference variable, use the syntax: ClassName The null value objectRefVar;

Example: Default value for a data fieldCircle myCircle; Differences between variables of primitive typesand reference types

To create an objectmyCircle = new Circle(2.0);

2010-11-4

Introduction to Java Programming

Chapter 7 - 12

JAVAPPT英文版

7.4 Accessing Objects via Reference VariablesReference variables and Reference Types Accessing an Object’s Data and Methods The null value objectRefVar.data Default value for a data field e.g., myCircle.radius

Referencing the object’s data:

Differences between variables of primitive types and reference types

Invoking the object’s method:

objectRefVar.methodName(arguments)

e.g., myCircle.findArea()2010-11-4 Introduction to Java Programming Chapter 7 - 13

JAVAPPT英文版

7.4 Accessing Objects via Reference VariablesReference variables and Reference Types Accessing an Object’s Data and Methods The null value, Default value for a data fieldIf a variable of a reference type does not reference types Differences between variables of primitive any object, the variable holds and reference types a special literal value, null. The default value of a data field is …… 此处隐藏：5295字，全部文档内容请下载后查看。喜欢就下载吧 ……