java _io_面向对象风格递归获得文件夹大小

public  class test {

private  int sum; //文件夹大小
private String path;  //路径
private File f;

public static void main(String[]args) throws IOException
{   
    test t=new test("D:/d");
    System.out.println(t.getSum());
}

public test(String path)
{
    this.path=path;
    this.f=new File(path);
    N(this.f);
}
public int getSum()
{
    return this.sum;
}
//递归打印获得文件夹大小
private int N(File f)
{
    if(null!=f&&f.exists())
    {       
        if(f.isFile()) //是文件才能获得大小
        {
            sum+=f.length();
        }
        else if(f.isDirectory())
        {

            File[] ss=f.listFiles();
            for(File r:ss)
            {
                 N(r);
            }
        }
    }
    return sum; 
}

}