java

package com.test;

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileReader;
import java.io.FileWriter;

public class test {
  public static void main(String[] args){
    String s = new String();
    String s1 = new String();
    try {
     File f = new File("E://123.txt");
     if(f.exists()){
      System.out.print("文件存在");
     }else{
      System.out.print("文件不存在");
      f.createNewFile();// 不存在则创建
     }
     BufferedReader input = new BufferedReader(new FileReader(f));
    
     while((s = input.readLine())!=null){
      s1 += s+"/n";
     }
     System.out.println(s1);
     input.close();
     s1 += "添加的内容!";
    
    
     BufferedWriter output = new BufferedWriter(new FileWriter(f));
     output.write(s1);
     output.close();
    } catch (Exception e) {
     e.printStackTrace();
    }
  }
}