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https://docs.oracle.com/javase/tutorial/java/javaOO/constructors.html

# Constructor

**constructor is NOT method**

一個class會有constructor, 在建立object時被呼叫。

- 必須與class同名
- 沒有return-type
- 可以寫多個constructor, 但參數不能一樣

如果class沒有任何constructor,
implict constructor就會存在。無參數,無body

class Point {
    public int x;
    public int y;
}

it's actually...

class Point {
    public int x;
    public int y;
    
    // implict constructor
    public Point() { 
    }
}

因此我們來加一些constructor:

class Point {
    public int x;
    public int y;
    
    public Point() {
        x = -1;
        y = -1;
    }
    
    public Point(int startX, int startY) {
        x = startX;
        y = startY;
    }
}
//---------- in Main.java ----------
public class Main {
    public static void main(String[] args) {
        Point p = new Point();
        System.out.println(p.x + " " + p.y); // -1 -1
        
        Point p = new Point(100, 200);
        System.out.println(p.x + " " + p.y); // 100 200
    }
}

<!--
多個constructor:

//---------- in Ship.java ----------
public class Ship {
    
    private String name;
    
    public Ship(String name) { //a-custom-constructor
        this.name = name;
    }
    
    public Ship() { //another-custom-constructor
        this.name = "default name";
    }
    
    //getter & setter
    public String getName() {
        return this.name;
    }
    
    public void setName(String name) {
        this.name = name;
    }
}
//---------- in Main.java ----------
public class Main {
    public static void main(String[] args) {
        Ship s = new Ship("Britannic");
        System.out.println(s.getName()); //"Britannic"
        Ship s2 = new Ship();
        System.out.println(s2.getName()); //"default name"
    }
}

用constructor呼叫其他的constructor可以用this(...);

    public Ship(String name) { //a-custom-constructor
        this.name = name;
    }
    
    public Ship() {
        this("default name");
    }

-->