21[分析题]
import java.io.*;
public class  TestFile
{
  public static void main(String args[]) throws Exception
    {
    BufferedReader br=new BufferedReader(new InputStreamReader(System.in);
    BufferedWriter bw = new BufferedWriter(new FileWriter(“input.txt");
    String s;
    while (true)
{
      System.out.print("请输入一个字符串： ");
      System.out.flush();
      s=br.readLine();
      if (s.length()==0) break;
      bw.write(s);
      bw.newLine();
    }
    bw.close();
    }
}