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

# this
this指的是目前的物件(參考)

**不能放在static method裡面**

## 使用this
為什麼我們在這個class裡面了, 還要用目前的物件參考?

class Point {
    public int x;
    public int y;
    
    public Point(int startX, int startY) {
        x = startX; // 明明這裡可以直接存取到x ?
        y = startY; // 明明這裡可以直接存取到y ?
    }
}

public class Main {
    public static void main(String[] args) {
        Point p = new Point(100, 200);
    }
}

有了this, 你可以這樣寫:

class Point {
    public int x;
    public int y;
    
    public Point(int x, int y) {
        /*
          this就是指現在這個物件參考
          也就是 剛剛new出來的物件參考
        */
        this.x = x; 
        this.y = y; 
    }
    
    public void print() {
        System.out.println(this.x + " " + this.y);
    }
}

public class Main {
    public static void main(String[] args) {
        Point p = new Point(100, 200);
        p.print();
    }
}

因為上述寫法中, 建構子中的參數x, y把field x, y隱藏住了,
如果在建構子中寫x, 指的是參數x, 而不是field x,
所以要以this.x 表明是field的x.

## 在Constructor使用this
你可以在呼叫建構子時, 呼叫另一個建構子。
這叫做"explicit constructor invocation"

class Point {
    public int x;
    public int y;
 
    public Point() {
        /*
        this.x = -1;
        this.y = -1;
        */
        this(-1, -1);
    }
    
    public Point(int x, int x) {
        this.x = x;
        this.y = y;
    }
}

public class Main {
    public static void main(String[] args) {
        Point p1 = new Point();
        System.out.println(p1.x + " " + p1.y); // -1 -1
        
        Point p2 = new Point(100, 200);
        System.out.println(p2.x + " " + p2.y); // 100 200
    }
}

## getter, setter

class Point {
    private int x;
    public int y;
 
    public Point() {
        this(-1, -1);
    }
    
    public Point(int x, int x) {
        this.x = x;
        this.y = y;
    }
    
    public void getX() {
        return this.x;
    }
    
    public int setX(int x) {
        this.x = x;
    } 
    
    public void getY() {
        return this.x;
    }
    
    public void setY(int y) {
        this.y = y;
    }
}