本章節翻譯自《Beginning-Android-4-Application-Development》，如有翻譯不當的地方，敬請指出。

原書購買地址 http://www.amazon.com/Beginning-Android-4-Application-Development/dp/1118199545/

上一節介紹了如何把文件存儲到內部設備。有的時候，需要把文件存儲到外部存儲設備，比如SD卡。因為SD卡具有更大的存儲空間，同時也可以很容易的和其他用戶分享這些文件。

使用上一節的例子，把用戶輸入的文字保存在SD卡，修改onClick()事件。如下：

    public class FilesActivity extends Activity {
	EditText textBox;
	static final int READ_BLOCK_SIZE = 100;

	/** Called when the activity is first created. */
	@Override
	public void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.main);

		textBox = (EditText) findViewById(R.id.txtText1);
		
        InputStream is = this.getResources().openRawResource(R.raw.textfile);
        BufferedReader br = new BufferedReader(new InputStreamReader(is));
        String str = null;
        try {
            while ((str = br.readLine()) != null) {
                Toast.makeText(getBaseContext(), 
                    str, Toast.LENGTH_SHORT).show();
            }
            is.close();
            br.close();
        } catch (IOException e) {
            e.printStackTrace();
        }

	}

	public void onClickSave(View view) {
		String str = textBox.getText().toString();
		try
		{
            //---SD Card Storage---
            File sdCard = Environment.getExternalStorageDirectory();
            File directory = new File (sdCard.getAbsolutePath() +
                "/MyFiles");
            directory.mkdirs();
            File file = new File(directory, "textfile.txt");
            FileOutputStream fOut = new FileOutputStream(file);

            /*			
			FileOutputStream fOut =
					openFileOutput("textfile.txt",
							MODE_WORLD_READABLE);
			*/
                        
			OutputStreamWriter osw = new
					OutputStreamWriter(fOut);

			//---write the string to the file---
			osw.write(str);
			osw.flush(); 
			osw.close();

			//---display file saved message---
			Toast.makeText(getBaseContext(),
					"File saved successfully!",
					Toast.LENGTH_SHORT).show();

			//---clears the EditText---
			textBox.setText("");
		}
		catch (IOException ioe)
		{
			ioe.printStackTrace();
		}
	}

}
  

上面的代碼中，使用getExternalStorageDirectory()方法去獲取SD卡的路徑。通常，在真機上面返回“/sdcard”，在模擬器上面返回"/mnt/sdcard"。但是，不要嘗試去寫死SD卡的路徑，因為手機廠商有可能去給SD卡指定一個路徑。因此，確保使用getExternalStorageDirectory()方法去獲取SD卡的路徑。

然后，創建一個MyFiles的文件夾。最終，把文件保存在這個文件夾中。

從外部設備中加載文件，修改onClickLoad()方法：

    public void onClickLoad(View view) {
		try
		{
			//---SD Storage---
            File sdCard = Environment.getExternalStorageDirectory();
            File directory = new File (sdCard.getAbsolutePath() + 
                "/MyFiles");
            File file = new File(directory, "textfile.txt");
            FileInputStream fIn = new FileInputStream(file);
            InputStreamReader isr = new InputStreamReader(fIn);

            /*
			FileInputStream fIn = 
					openFileInput("textfile.txt");
			InputStreamReader isr = new 
					InputStreamReader(fIn);
            */
            
			char[] inputBuffer = new char[READ_BLOCK_SIZE];
			String s = "";

			int charRead;
			while ((charRead = isr.read(inputBuffer))>0)
			{
				//---convert the chars to a String---
				String readString =
						String.copyValueOf(inputBuffer, 0,
								charRead);
				s += readString;

				inputBuffer = new char[READ_BLOCK_SIZE];
			}
			//---set the EditText to the text that has been 
			// read---
			textBox.setText(s);

			Toast.makeText(getBaseContext(),
					"File loaded successfully!",
					Toast.LENGTH_SHORT).show();
		}
		catch (IOException ioe) {
			ioe.printStackTrace();
		}

	}
  

請注意，如果想要往SD卡中寫入文件，需要在AndroidManifest.xml中加入相關的權限：

    <?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="net.manoel.Files"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk android:minSdkVersion="10" />
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
        
    <application
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name" >
        <activity
            android:label="@string/app_name"
            android:name=".FilesActivity" >
            <intent-filter >
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>
  

執行上述代碼，查看SD卡：

【Android 開發教程】保存到外部存儲設備（SD卡）