就好比下面的程序，里面自己定义的方法和变量 有的是public的 有的是private的  这些都在什么情况下用呢，为什么要这么用呢
package jianchayuchuangjianmulu;
import *;

public class CrFileDemo {
  public static void main(String[] args) {
    if(args.length<1)
      myUsage();
    if(args[0].equals("-r"))
       {
       if(args.length!=3)
         myUsage();
       File oldFile=new File(args[1]),rname=new File(args[2]);
       oldFile.renameTo(rname);
       System.out.println(oldFile+"重命名为"+rname);
       fileInfo(oldFile);
       fileInfo(rname);
       return;
    }
    int count=0;
    boolean del=false;
    if(args[0].equals("-d"))
    {
      count++;
      del=true;
    }
    for(;count<args.length;count++)
    {
      File f=new File(args[count]);
      if(f.exists())
      {
        System.out.println(f+"已经存在");
        if(del)
        {
          System.out.println("正在删除..."+f);
          f.delete();
          System.out.println(f+"已经删除");
        }else
        {
          if(!del)
          {
            f.mkdirs();
            System.out.println(f+"已经创建");
          }
        }
        fileInfo(f);
      }
    }
  }
  private final static String myUsage="用法: \n"+"新建: CrFileDemo \n"+
      "新建一个目录: \n"+
      "删除:CrFileDemo -d path1 \n"+"删除一个目录/文件\n"+
      "重命名: CrFileDemo -r path1 path2\n"+"path1 重命名为path2";
  private static void myUsage()
  {
    System.err.println(myUsage);
    System.exit(1);
  }
  private static void fileInfo(File f)
  {
    System.out.println("Absolute path"+f.getAbsolutePath()+"\n Can read:"+f.canRead()
                       +"\n Can write:"+f.canWrite()
                       +"\n getName:"+f.getName()
                       +"\n getParent:"+f.getParent()
                       +"\n getPath:"+f.getPath()
                       +"\n length:"+f.length()
                       +"\n lastModified:"+f.lastModified());
    if(f.isFile())
      System.out.println("its a file");
    else if(f.isDirectory())
      System.out.println("its a directory");
  }
  
}