String a = "d:\\bbb.txt";
  File f = new File(a);
  FileOutputStream b = new FileOutputStream(f,true);//不加true会覆盖之前的内容，加上是在后面写。
  String c = "这是要输出到文本的内容";
  byte[] bbb = c.getBytes(); //字符串转类型换为byte[]数组类型
  b.write(bbb);
  b.close();

String a = "d:\\bbb.txt";
  File f = new File(a);
  FileInputStream b = new FileInputStream(f);
  byte[] bbb = new byte[(int)(f.length())];//长度可以直接取文件字节长度
  b.read(bbb);
  b.close();
  String str = new String(bbb);//转换成字符串
  System.out.println(str);

  String a = "d://bbb.txt";
  File f = new File(a);
  FileWriter c = new FileWriter(f,true);//不加true会覆盖之前内容，加上后会在后面写
  c.write("这是要输入的内容");//可以直接用字符串，不用转换(写文本类型推荐使用)
  c.close(); 

String aa ="d:\\bbb.txt";
  File bb =new File(aa);
  FileReader cc = new FileReader(bb);
  char[] dd = new char[100];//长度不好取(不建议用)
  cc.read(dd);
  cc.close();
  String str = new String(dd);//转换为字符串
  System.out.println(str)

String a = "d:\\bbb.txt";
  File b = new File(a);
  Scanner sc = new Scanner(b);
  //Scanner每次只能取一行，所以全部取出要用while循环
  while(sc.hasNext()){ //当还有下个值的时候循环
  System.out.println(sc.nextLine());
  }
  sc.close();