import java.io.File;
import java.io.FileOutputStream;

public class Writetxt {
 public void DOWriteTxt(String file, String txt) {
  try {
    //寫入文件尾
   FileOutputStream os = new FileOutputStream(new File(file), true);
   //覆蓋
   //FileOutputStream os = new FileOutputStream(new File(file), false);
   //os.write((txt + "\n").getBytes());
   //這里不需要換行，換行另處理
   os.write((txt).getBytes());
  } catch (Exception e) {
   e.printStackTrace();
  }
 }

 public static void main(String[] args) {
  new Writetxt().DOWriteTxt("D:\\問好.txt", "你好！");
 }
}