# JAVA教學---Defining Classes I

###### tags: `JAVA教學`

## 目錄
[TOC]

## define
Java的class是什麼?
>Every program is a class
>All helping software consists of classes
>All programmer-defined types are classes

:::warning
:bulb: **原則**: define your own classes and their attribute and methods, and to create your own objects from classes
:::

## 名詞解釋

public class Car {
	public int xPosition = 0;
    
	public void moveForward(){
		xPosition++;
	}
}

int 等敘述稱為這個 class(Car) 的 attribute、field、instance、variable。
function 稱為這個 class(Car) 的 method、function、behabior、operator。
但是兩者皆是這個 class(Car) 的 member。
>A primitive type value is a **single piece of data**

>A class type value or object can have **multiple
pieces of data**, as well as actions called methods

>For a given object, each piece of data can hold a **different** value

## 使用方式
宣告

ClassName classVar;

給值，可在括號內給予data做initialization

classVar = new ClassName();

宣告同時給值

ClassName classVar = new ClassName();

## Local Variables
A variable declared within a method definition is called a local variable
>All variables declared in a method are local variables.
>All method parameters are local variables.
>

If two methods each have a local variable of the same name, they are still two entirely different variables

## Variable Argument (Varargs)
接收多個相同型態變數的方法，若有不同型態則不能用

public static void main(String args[]) {
    display("Tom");// zero argument
    display("my", "name", "is", "Tom");// four arguments
}
static void display(String... values) {
    System.out.println(values[0]); 
}

## Call-by-value (Primitive Type)
The value of each argument (not the variable name) is plugged into the corresponding method parameter
>known as the call-by-value mechanism

## this. 用法
如果想要透過Class裡的**method**修改Class裡的**instance**時，==而且method的parameter和instance名稱相同時==，需要在前面加上**This.**，才能存取該**instance**，否則會變成local variable。
:::warning
即便 method 的 parameter 和 Class 裡的 instance 名稱不同，也最好加上**this.**，用以表明是用來修改 Class 裡的 instance。
:::

public class Car {
	public int xPosition = 0;
	
	public void moveToGiven(int xPosition){
		this.xPosition = xPosition;  //用來修改Class裡的xPosition
	}
}

## Encapsulation 封裝
Encapsulation means that the data and methods of a class are combined into a single unit (i.e., a class object),which hides the implementation details
> Knowing the details is unnecessary because interaction with the object occurs via a well-defined and simple interface

>In Java, hiding details is done by marking them **private**

:::danger
把 object 的 attribute 設為 private，包起來成為一個set，不讓外界輕易來改，而是透過寫 get 和 set 的 public method 來決定是否要讓外界class來 read 或 write 這些 object 的 attribute ，這一系列操作稱為 Encapsulation，是OOP裡最基本且重要的原則，也可減少 bug 發生的機率。
:::

## public and private Modifiers
* public : there are **no restrictions** on where an instance variable or method can be used.
* private : an instance variable or method **can only be accessed** by name **inside** of the class
:::warning
1. It is considered good programming practice to make all instance variables **private**.
2. Most methods are **public**, and thus provide controlled access to the object.
3. Usually, methods are **private** only if used as helping methods for other methods in the class.
:::

demo如下

public class Car {
	private int position = 0;
	
	public Car(int position){
		this.position = position;
	}
	public void moveForward(){
		position++;
	}
	public void moveToGiven(int position){
		this.position = position;
	}
	public int getPosition(){
		return position;
	}
}

public class demo {
	public static void main(String[] args) {
		Car bmw = new Car(2);
		System.out.println("Car position = " + bmw.getPosition());
		bmw.moveForward();
		System.out.println("Car position = " + bmw.getPosition());
		bmw.moveForward();
		System.out.println("Car position = " + bmw.getPosition());
		bmw.moveToGiven(6);
		System.out.println("Car position = " + bmw.getPosition());
	}
}

## Overloading
定義 : 發生於class中的兩個method有**相同name**時
解決方法 : 讓兩個method有不同的parameter list，及參數不同

public class Duck {	
	public void quack(){
		System.out.println("Quack!!");
	}
	public void quack(String sound){
		System.out.println(sound);
	}
}

:::warning
Overloading可用於user輸入多種parameters的狀況
:::

## Constructors 建構子
定義 : a special kind of **method** that is designed to **initialize** the **instance variables** for an object.
* Constructors 的名稱需和 class **相同**。
* A constructor has no type returned, not even void.
* Constructors are typically overloade.

定義 Constructors

public ClassName(anyParameters){code}

使用 Constructors

ClassName objectName = new ClassName(anyParameters);

:::warning
若不想用 Constructor 做任何初始化，則不用重新定義Constructor，在 new Object 時不用帶入任何參數，Java 會自動 new 一段 memory location 並給予 default 值。
:::

## StringTokenizer Class
用途 : 重新定義字串的separator

import java.util.StringTokenizer;
public class StringTokenizerTest {
    public static void main(String[] args) {
        String input = "Hello,World,Java"; 
        StringTokenizer st = new StringTokenizer(input, ","); 
        while(st.hasMoreTokens()) {
            String token = st.nextToken(); 
            System.out.println(token); 
            System.out.println("Reamin Tokens = " + st.countTokens());
        } 
    }
}