在模拟器中使用SDCard，你需要先创建一张SDCard卡（当然不是真的SDCard，只是镜像文件）。创建SDCard可以在Eclipse创建模拟器时随同创建，也可以使用DOS命令进行创建，如下：在Dos窗口中进入android SDK安装路径的tools目录，输入以下命令创建一张容量为2G的SDCard，文件后缀可以随便取，建议使用.img：
mksdcard 2048M D:\AndroidTool\sdcard.img

注意：在程序中访问SDCard，你需要申请访问SDCard的权限。
在AndroidManifest.xml中加入访问SDCard的权限如下:

<uses-permission android:name="android.permission.MOUNT_UNMOUNT_FILESYSTEMS"/>

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>

要往SDCard存放文件，程序必须先判断手机是否装有SDCard，并且可以进行读写。
注意：访问SDCard必须在AndroidManifest.xml中加入访问SDCard的权限
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();
}
Environment.getExternalStorageState()方法用于获取SDCard的状态，如果手机装有SDCard，并且可以进行读写，

那么方法返回的状态等于Environment.MEDIA_MOUNTED。

Environment.getExternalStorageDirectory()方法用于获取SDCard的目录，当然要获取SDCard的目录，你也可以这样写：
File sdCardDir = new File("/mnt/sdcard"); //获取SDCard目录
File saveFile = new File(sdCardDir, "ljq.txt");
//上面两句代码可以合成一句： File saveFile = new File("/mnt/sdcard/ljq.txt");
FileOutputStream outStream = new FileOutputStream(saveFile);
outStream.write("abc".getBytes());
outStream.close();

案例

main.xml布局文件

<?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>

strings.xml

<?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>

FileService工具类

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);
    }
}

SdcardActivity类

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;
            }
        }

    };
}

清单文件

<?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>

运行结果