markdown
#說明
這個是一種彈性的建構方法，鈄過這樣的方式就可以方便呼叫，不需要再個別處理變數以及宣告。
#操作流程
##Code
```
package overload;
class Point {
public int x,y;
public Point(int x, int y) {
this.x=x;
this.y=y;
}
}
class Rectangle {
Point upperleft;
Point lowerright;
//full create method
public Rectangle (Point upperleft,Point lowerright) {
this.upperleft=upperleft;
this.lowerright=lowerright;
}
//don't need argv
public Rectangle() {
this(new Point(0,0),new Point(5,-5));
}
//
public Rectangle(int x1,int y1, int x2 ,int y2) {
this(new Point(x1,y1 ),new Point(x2,y2));
}
//square
public Rectangle(Point upperleft, int length) {
this(upperleft, new Point(upperleft.x+length, upperleft.y -length));
}
//caculate
public int area() {
return (lowerright.x-upperleft.x)*(upperleft.y - lowerright.y);
}
}
public class constructor {
public static void main(String[] argv) {
Rectangle a =new Rectangle (0,0,5,-5);
Rectangle b =new Rectangle(new Point (3,3),4);
System.out.println("a's area is :" +a.area());
System.out.println("b's area is :" +b.area());
}
}
```
##DEMO
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