if(Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)){
  File sdCardDir = Environment.getExternalStorageDirectory();//获取SDCard目录
  File saveFile = new File(sdCardDir, "ljq.txt");
  FileOutputStream outStream = new FileOutputStream(saveFile);
  outStream.write("abc".getBytes());
  outStream.close();
}

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
 android:orientation="vertical"
 android:layout_width="fill_parent"
 android:layout_height="fill_parent"
 >
 <!-- 相对布局 -->
 <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
 android:orientation="vertical"
 android:layout_width="fill_parent"
 android:layout_height="wrap_content"> 
 <TextView android:layout_width="wrap_content"
  android:layout_height="wrap_content" 
  android:text="@string/filename"
  android:id="@+id/filenameLable" />
 <EditText android:layout_width="250px"
  android:layout_height="wrap_content" 
  android:layout_toRightOf="@id/filenameLable"
  android:layout_alignTop="@id/filenameLable"
  android:layout_marginLeft="10px"
  android:text="ljq.txt"
  android:id="@+id/filename" />
 </RelativeLayout>
 <TextView android:layout_width="fill_parent"
 android:layout_height="wrap_content" 
 android:text="@string/content" />
 <EditText android:layout_width="fill_parent"
 android:layout_height="wrap_content"
 android:minLines="3"
 android:id="@+id/content" />
 <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
 android:orientation="vertical"
 android:layout_width="fill_parent"
 android:layout_height="wrap_content"> 
 <Button android:layout_width="wrap_content"
  android:layout_height="wrap_content"
  android:text="@string/button" 
  android:id="@+id/button" />
 <Button android:layout_width="wrap_content"
  android:layout_height="wrap_content"
  android:layout_toRightOf="@id/button"
  android:layout_alignTop="@id/button"
  android:layout_marginLeft="10px"
  android:minLines="3"
  android:text="@string/readButton"
  android:id="@+id/readButton" />
 </RelativeLayout>
 <TextView android:layout_width="wrap_content"
  android:layout_height="wrap_content" 
  android:id="@+id/resultView" />
</LinearLayout>

<?xml version="1.0" encoding="utf-8"?>
<resources>
 <string name="hello">Hello World, FileActivity!</string>
 <string name="app_name">数据保存</string>
 <string name="filename">文件名称</string>
 <string name="content">文件内容</string>
 <string name="button">保存</string>
 <string name="readButton">读取内容</string>
 <string name="error">保存失败</string>
 <string name="success">保存成功</string>
</resources>

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 {
 ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream(); // 往内存写数据
 byte[] buffer = new byte[1024]; // 缓冲区
 int len = -1;
 while ((len = inputStream.read(buffer)) != -1) {
  byteArrayOutputStream.write(buffer, 0, len);
 }
 byte[] data = byteArrayOutputStream.toByteArray(); // 存储数据
 byteArrayOutputStream.close();
 inputStream.close();
 return new String(data);
 }
}

package com.ljq.sdcard;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.InputStream;
import android.app.Activity;
import android.os.Bundle;
import android.os.Environment;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;
import com.ljq.service.FileService;
public class SdcardActivity extends Activity {
 private final String TAG = "FileActivity";
 private EditText filenameText;
 private TextView resultView;
 private EditText contentText;
 @Override
 public void onCreate(Bundle savedInstanceState) {
 super.onCreate(savedInstanceState);
 setContentView(R.layout.main);
 filenameText = (EditText)this.findViewById(R.id.filename);
 contentText = (EditText)this.findViewById(R.id.content);
 resultView = (TextView)this.findViewById(R.id.resultView);
 Button button = (Button)this.findViewById(R.id.button);
 button.setOnClickListener(listener);
 Button readButton = (Button)this.findViewById(R.id.readButton);
 readButton.setOnClickListener(listener);
 }
 private View.OnClickListener listener = new View.OnClickListener() {
 public void onClick(View v) {
  Button button = (Button) v;
  String filename = filenameText.getText().toString();
  // Environment.getExternalStorageDirectory()等价于new File("/sdcard")---->获取sdcard目录
  //获取sdcard目录
  //File file = new File("/sdcard" + filename);
  File file = new File(Environment.getExternalStorageDirectory(), filename);
  switch (button.getId()) {
  case R.id.button:
  int resId = R.string.success; // 默认成功
  String content = contentText.getText().toString();
  //sdcard存在并且可以读写
  if (Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {
   try {
   FileOutputStream fileOutputStream = new FileOutputStream(file);
   FileService.save(fileOutputStream, content);
   } catch (Exception e) {
   Log.e(TAG, e.toString());
   resId = R.string.error;
   }
   Toast.makeText(SdcardActivity.this, resId, Toast.LENGTH_LONG).show();
  }else {
   Toast.makeText(SdcardActivity.this, "sdcard不存在或写保护", Toast.LENGTH_LONG).show();
  }
  break;
  case R.id.readButton:
  try {
   InputStream inputStream= new FileInputStream(file);
   String text = FileService.read(inputStream);
   resultView.setText(text);
   } catch (Exception e) {
   Log.e(TAG, e.toString());
   Toast.makeText(SdcardActivity.this, "读取失败", Toast.LENGTH_LONG).show();
   } 
  break;
  }
 }
 };
}

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
 package="com.ljq.sdcard" android:versionCode="1"
 android:versionName="1.0">
 <application android:icon="@drawable/icon"
 android:label="@string/app_name">
 <activity android:name=".SdcardActivity"
  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" />
 <!-- 在SDCard中创建与删除文件权限 -->
 <uses-permission
 android:name="android.permission.MOUNT_UNMOUNT_FILESYSTEMS" />
 <!-- 往SDCard写入数据权限 -->
 <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
</manifest>