java问题。。。刚学我自己写了一个有问题

public class Rectangel{

double length,width;
Rectangel(double a,double b )
{
length=a;
width=b; // 方法体
}
double getPerimeter( )// perimeter方法定义
{
double perimeter;
perimeter=2*(length+width);
return perimeter;
}
double getArea()// area方法定义
{
double area;
area= length*width;
return area;
}
public static void main(String []args)
{

double perimeter,area;

Rectangelr;
r=new Rectangel(3,4);
// 调用perimeter方法，并显示周长
perimeter=r.getPerimeter();
System.out.println("矩形的周长:"+perimeter);
// 调用area方法，并显示面积
area=r.getArea();
System.out.println("矩形的面积:"+area);
}
}

给你来个优化版的:

public class Rectangel
{

private double length, width;

Rectangel(double length, double width)
{
this.length = length;
this.width = width;
}

public double getPerimeter()// perimeter方法定义
{
return 2 * (length + width);
}

public double getArea()// area方法定义
{
return length * width;
}

public static void main(String[] args)
{
Rectangel r = new Rectangel(3, 4);
System.out.println("矩形的周长:" + r.getPerimeter());
System.out.println("矩形的面积:" + r.getArea());
}
}