Creating Java Classes


Class and object

  • A class is a programmer-defined type.
  • A value of a class type is called an object or an instance of the class: an object exhibits the properties and behaviors defined by the object’s class.

  • Objects have data and behavior.
    • Data in an object is called instance fields.
    • Behaviors that operate on the data are called methods.


  • A class definition specifies the data items and methods that all of it subjects will have:
    • Data items and methods are sometimes called members of the object.
    • Data items are usually called instance fields/instance variables/attributes/properties.

UML and class diagram

  • UML : pictures of an Object-Oriented system
    • Programming languages are not abstract enough for OO design.
    • Open standard to model the OO system.
  • The most popular diagrammatic notation used for Object-Oriented Development.
  • Consists of:
    • Class diagrams;
    • Sequence diagrams;
    • Use case diagrams;
    • Activity diagrams;

Diagram of one class

  • Class name in the top of box
  • Attributes (optional)
    • Include all instance fields of the object
    • visibility name : type
  • Operations / methods (optional)
    • visibility name (parameters) : return_type
    • May omit trivial methods (get/set)
  • Visibility:
    • + public
    • # protected
    • - private
    • ~ package (default)
  • Underline static instance variables and static methods ClassDiagram

Constructor 构造器

In java we use constructors to construct new instances.

  • Constructors always have the same name as the class name.
  • A constructoris a special kind of method that is designed to initialize the instance variables for a new object:

public ClassName(Parameters){ code }

  • A constructor must have the same name as the class.
  • A constructor has no type returned, not even void.
  • Constructors are often overloaded. ^[(there is more than one constructor in the same class)]

The new Operator

The new operator is used in combination with constructor to create an object from a particular class.

> ClassName classVar= new ClassName();
  • The object variable refers to the newly constructed object.

In Java, the value of any object variable is a reference ^[(basically an address of a memory location)] to an object that is stored elsewhere.


  • Initialize the object variable with a newly constructed object 创建一个新的对象
  • Set the variable to refer to an existing object 对存在的对象进行引用
Person person1 = new Person();
Person person2 = person1;
Person person3 = null;       //refers to no object


Instance Variables

  • Define: [<modifiers>] type [= defaultValue] **定义在类里,方法外的变量**
  • In order to refer to a particular instance variable, preface it with its object name as follows: // "." is the period operator
  1. instance variables store the data of the object.
  2. Each object has its own copy of the variables.
  3. Every object has a state that is determined by the values stored in the instance variables of the object.


Local variable

A variable declared within a method definition is called a local variable. 定义在方法里的变量

  • If two methods each have a local variable of the same name, they are still two entirely different variables.
  • A local variable is only valid for calculation insideits method definition.
  • Local variables aredestroyed automatically when the method call ends


  • Define: [<modifiers>] \([parameter_list]) {}

this Keyword

It(this) works as a reference to the current Object, whose method or constructor is being invoked.

this Keyword with a field

  • this must be used if a parameter or a local variable with the same name is used in a method.

this Keyword with Constructor

  • this keyword can be used inside the constructor to call another overloaded constructor in the same class.
  • By this way, we can call the constructor explicitly.
  • Note: thiskeyword can only be the first statement in Constructor
public class Person{
    private intage;
    private String name;
    public Person(){
        System.out.println("Inside Constructor without parameter");
    public Person(String name, intage){ = name;
        this.age= age;
        System.out.println("Inside Constructor with two parameters");

this Keyword as a method parameter

public class Person {
    private intage;
    private String name;
    public Person(String name, intage) 
    { = name;
        this.age= age;
    public void method1(Person pp) 
        System.out.println("Inside Method1");
    public void method2() 
        System.out.println("Inside Method2");
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Person p = new Person("Bob",23);


Overloading 重载

  • Java Method Overloading
    • Overloading occurs if several methods have the same name but different parameters.
    • Overloaded methods:
    • appear in the same class;
    • have the same name; but
    • have different parameter lists; and
    • can have different return types

Java access modifiers


  • Only public and default can be used in the class declaration 只有public和default可以用于类名的申明
  • A public class can be accessed from any place.
  • A no modifier class can only be accessed from within the same package.
  • protected instance variables or methods can be accessed by:
    • the code of the class itself;
    • subclasses;
    • other code in the same package.
  • private instance variables or methods can be accessed by:
    • the code of the class itself;