Title: "[[Creating-Java-Classes]]"
Author:
  - AllenYGY
status: DONE
tags:
  - NOTE
  - Java
  - Lec3
  - Program
created: 2023-11-05T00:59
updated: 2024-05-31T01:01

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.

Class

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

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. ^[1]

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 ^[2] to an object that is stored elsewhere.

Initialize

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

MemoryAnalysis

Instance Variables

Define:
[<modifiers>] type <fields_name> [= 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:

person1.name // "." is the period operator
person2.age

instance variables store the data of the object.

Each object has its own copy of the variables.

Every object has a state that is determined by the values stored in the instance variables of the object.

DefaultValue

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

Methods

Define:
[<modifiers>] <return_type><name>([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(){
        this("Bob",0);
        System.out.println("Inside Constructor without parameter");
    }
    public Person(String name, intage){
        this.name = 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) 
    {
        this.name = name;
        this.age= age;
    }
    public void method1(Person pp) 
    {
        System.out.println("Inside Method1");
        System.out.println("Age:"+pp.age);
    }
    public void method2() 
    {
        this.method1(this);
        System.out.println("Inside Method2");
    }
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Person p = new Person("Bob",23);
        p.method2();
    }
}

MemoryAnalysis2

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

AccessModifiers

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;

(there is more than one constructor in the same class)↩︎
(basically an address of a memory location)↩︎