//往SD卡寫入檔案的方法
    public void savaFileToSD(String filename, String filecontent) throws Exception {
        //如果手機已插入sd卡,且app具有讀寫sd卡的許可權
        if (Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {
            filename = Environment.getExternalStorageDirectory().getCanonicalPath() + "/" + filename;
            
 
//這裡就不要用openFileOutput了,那個是往手機記憶體中寫資料的
            FileOutputStream output = new FileOutputStream(filename);
            output.write(filecontent.getBytes());
            //將String字串以位元組流的形式寫入到輸出流中
            output.close();
            //關閉輸出流
        } else Toast.makeText(context, "SD卡不存在或者不可讀寫", Toast.LENGTH_SHORT).show();
    }

    
 
//讀取SD卡中檔案的方法
    //定義讀取檔案的方法:
    public String readFromSD(String filename) throws IOException {
        StringBuilder sb = new StringBuilder("");
        if (Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {
            filename = Environment.getExternalStorageDirectory().getCanonicalPath() + "/" + filename;
            
 
//開啟檔案輸入流
            FileInputStream input = new FileInputStream(filename);
            byte[] temp = new byte[1024];

            int len = 0;
            //讀取檔案內容:
            while ((len = input.read(temp)) > 0) {
                sb.append(new String(temp, 0, len));
            }
            //關閉輸入流
            input.close();
        }
        return sb.toString();
    }

/*摘自菜鳥教程,侵刪*/

//許可權

<!-- 在SDCard中建立與刪除檔案許可權 -->
<uses-permission android:name="android.permission.MOUNT_UNMOUNT_FILESYSTEMS"/>
<!-- 往SDCard寫入資料許可權 -->
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>