BufferedWriter.

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

public class IOTest {
public static void main(String[] args){
  File f = new File("E:\\1.txt");
  //读
  String oldStr = "";
  try {
   FileReader fr = new FileReader(f);
   BufferedReader br = new BufferedReader(fr);
   String line = null;
   while((line = br.readLine())!=null){
    oldStr += line;
   }
  } catch (FileNotFoundException e1) {
   e1.printStackTrace();
  } catch (IOException e) {
   e.printStackTrace();
  }
 
  //写
  String newStr = "zxc";
  FileWriter fw;
  try {
   fw = new FileWriter(f);
   BufferedWriter bw = new BufferedWriter(fw);
   bw.write(oldStr+"\r\n"+newStr);//不想换行就把\r\n去掉
   bw.close();
   fw.close();
  } catch (IOException e) {
   e.printStackTrace();
  }
}
}