Java 语法 索引 ----- 构造函数(constructor)

class MyRectangle{
    int x, y;
    public MyRectangle()             { x = 10; y = 20; }
    public MyRectangle(int a)        { x = a; y = a;   }
    public MyRectangle(int a, int b) { x = a; y = b;   }
}

public MyRectangle()             { this(10,20);  }
public MyRectangle(int a)        { this(a,a);    }
public MyRectangle(int a, int b) { x = a; y = b; }