采用openFileOutput获取输出流

 package com.example.login.service;

 import java.io.BufferedReader;
 import java.io.File;
 import java.io.FileInputStream;
 import java.io.FileNotFoundException;
 import java.io.FileOutputStream;
 import java.io.InputStreamReader;
 import java.util.HashMap;
 import java.util.Map;

 import android.annotation.SuppressLint;
 import android.content.Context;

 /*
  * mode 1私有、2可读、3可写、4可读写
  */
 public class LoginService {
     public static boolean savaUserInfo(Context context, String username, String password,int mode)
     {
         try{
             //File file = new File("/data/data/com.example.login/info.txt");
             //File file = new File(context.getFilesDir(),"info.txt");
             //FileOutputStream fos = new FileOutputStream(file);
             //FileOutputStream fos = context.openFileOutput("private.txt", context.MODE_PRIVATE);
             FileOutputStream fos = null;
             switch(mode)
             {
             case 1:
                 fos = context.openFileOutput("private.txt", context.MODE_PRIVATE);
                 break;
             case 2:
                 fos = context.openFileOutput("readable.txt", context.MODE_WORLD_READABLE);
                 break;
             case 3:
                 fos = context.openFileOutput("writeable.txt", context.MODE_WORLD_WRITEABLE);
                 break;
             case 4:
                 fos = context.openFileOutput("public.txt", context.MODE_WORLD_READABLE + context.MODE_WORLD_WRITEABLE);
                 break;
             }
             fos.write((username + "##" + password).getBytes());
             fos.close();
             return true;
         }catch(Exception e){
             e.printStackTrace();
             return false;
         }
     }

 }

2、关于在另外项目测试文件的读写权限

 package com.example.other;

 import java.io.BufferedReader;
 import java.io.File;
 import java.io.FileInputStream;
 import java.io.FileOutputStream;
 import java.io.InputStreamReader;

 import android.os.Bundle;
 import android.app.Activity;
 import android.view.Menu;
 import android.view.View;
 import android.widget.Toast;

 public class MainActivity extends Activity {

     @Override
     protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
         setContentView(R.layout.activity_main);
     }

     public void readPrivate(View view)
     {
         try
         {
             File file = new File("/data/data/com.example.login/files/private.txt");
             FileInputStream fis = new FileInputStream(file);
             BufferedReader br = new BufferedReader(new InputStreamReader(fis));
             String result = br.readLine();
             Toast.makeText(this, result, 0).show();
         }catch(Exception e){
             e.printStackTrace();
             Toast.makeText(this, "读取私有文件失败!", 0).show();
         }
     }

     public void writePrivate(View view)
     {
         try
         {
             File file = new File("/data/data/com.example.login/files/private.txt");
             FileOutputStream fos = new FileOutputStream(file);
             fos.write("aaa".getBytes());
             fos.close();
         }catch(Exception e){
             e.printStackTrace();
             Toast.makeText(this, "写入私有文件失败!", 0).show();
         }
     }

     public void readPublic(View view)
     {
         try
         {
             File file = new File("/data/data/com.example.login/files/public.txt");
             FileInputStream fis = new FileInputStream(file);
             BufferedReader br = new BufferedReader(new InputStreamReader(fis));
             String result = br.readLine();
             Toast.makeText(this, result, 0).show();
         }catch(Exception e){
             e.printStackTrace();
             Toast.makeText(this, "读取私有文件失败!", 0).show();
         }
     }

     public void writePublic(View view)
     {
         try
         {
             File file = new File("/data/data/com.example.login/files/public.txt");
             FileOutputStream fos = new FileOutputStream(file);
             fos.write("aaa##bbb".getBytes());
             fos.close();
         }catch(Exception e){
             e.printStackTrace();
             Toast.makeText(this, "写入私有文件失败!", 0).show();
         }
     }

     @Override
     public boolean onCreateOptionsMenu(Menu menu) {
         // Inflate the menu; this adds items to the action bar if it is present.
         getMenuInflater().inflate(R.menu.main, menu);
         return true;
     }

 }