public class FileActivity extends Activity {
 @Override
 public void onCreate(Bundle savedInstanceState) {  
   FileOutputStream outStream = this.openFileOutput("ljq.txt", Context.MODE_PRIVATE);
   outStream.write("我叫林计钦".getBytes());
   outStream.close(); 
 }
} 

package com.ljq.service;
import java.io.ByteArrayOutputStream;
import java.io.InputStream;
import java.io.OutputStream;
public class FileService {
 /**
  * 保存数据
  * 
  * @param outputStream
  * @param content
  * @throws Exception
  */
 public static void save(OutputStream outputStream, String content) throws Exception {
  outputStream.write(content.getBytes());
  outputStream.close();
 }
 /**
  * 读取数据
  * 
  * @param inputStream
  * @return
  * @throws Exception
  */
 public static String read(InputStream inputStream) throws Exception {
  // 往内存写数据
  ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream(); 
  // 缓冲区
  byte[] buffer = new byte[1024]; 
  int len = -1;
  while ((len = inputStream.read(buffer)) != -1) {
   byteArrayOutputStream.write(buffer, 0, len);
  }
  // 存储数据
  byte[] data = byteArrayOutputStream.toByteArray(); 
  byteArrayOutputStream.close();
  inputStream.close();
  return new String(data);
 }
}

package com.ljq.service;
import java.io.InputStream;
import java.io.OutputStream;
import android.content.Context;
import android.test.AndroidTestCase;
import android.util.Log;
/**
 * android测试
 * 
 * @author jiqinlin
 *
 */
public class FileServiceTest extends AndroidTestCase {
 private final String TAG = "FileServiceTest"; 
 public void testSave() throws Exception{
  OutputStream outputStream = this.getContext().openFileOutput("ljq.txt", Context.MODE_PRIVATE);
  FileService.save(outputStream, "abc");
 }
 public void testRead() throws Exception{
  InputStream inputStream= this.getContext().openFileInput("ljq.txt");
  String content = FileService.read(inputStream);
  Log.i(TAG, content);
 }
}

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
  package="com.ljq.activity"
  android:versionCode="1"
  android:versionName="1.0">
 <application android:icon="@drawable/icon" android:label="@string/app_name">
  <uses-library android:name="android.test.runner" />
  <activity android:name=".FileActivity"
     android:label="@string/app_name">
   <intent-filter>
    <action android:name="android.intent.action.MAIN" />
    <category android:name="android.intent.category.LAUNCHER" />
   </intent-filter>
  </activity>
 </application>
 <uses-sdk android:minSdkVersion="7" />
 <instrumentation android:name="android.test.InstrumentationTestRunner"
   android:targetPackage="com.ljq.activity" android:label="Tests for My App" />
</manifest>