1.调用Environment.getExternalStorageState()判断手机上是否插入了SD卡，并且应用程序具有读写SD卡的权限。
Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)
返回值为true代表该应用程序具有读写SD卡的权限。
2.调用Environment的getExternalStorageDirectory()方法来获取外部存储器，也就是SD卡的目录。
3.使用FileInputStream FileOutputStream FileReader FIleWriter 读写SD卡里的文件。
class Utils { public void MyWrite(String data) throws Exception { File sDfile = Environment.getExternalStorageDirectory(); File f = new File(sDfile, "demo.txt"); FileOutputStream fos = new FileOutputStream(f); fos.write(data.getBytes()); fos.flush(); fos.close(); } public String MyRead() throws Exception { File sdFile = Environment.getExternalStorageDirectory(); File f = new File(sdFile, "demo.txt"); FileInputStream fis = new FileInputStream(f); StringBuffer sb = new StringBuffer(); int len = 0; while ((len = fis.read()) != -1){ sb.append((char)len); } return sb.toString(); }}public class MainActivity extends Activity { private EditText write; private Button saveButton, readButton; private TextView show; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); show = (TextView) findViewById(R.id.show); write = (EditText) findViewById(R.id.write); saveButton = (Button) findViewById(R.id.save); readButton = (Button) findViewById(R.id.read); final Utils s = new Utils(); try { show.setText(s.MyRead()); } catch (Exception e1) { // TODO Auto-generated catch block e1.printStackTrace(); } saveButton.setOnClickListener(new OnClickListener() { @Override public void onClick(View v) { // TODO Auto-generated method stub try { s.MyWrite(write.getText().toString()); } catch (Exception e) { // TODO Auto-generated catch block e.printStackTrace(); } } }); readButton.setOnClickListener(new OnClickListener() { @Override public void onClick(View v) { // TODO Auto-generated method stub try { show.setText(s.MyRead()); } catch (Exception e) { // TODO Auto-generated catch block e.printStackTrace(); } } }); }}为了读写SD卡上的数据，需要添加权限：
创建于删除的权限：
<uses-permission android:name="android.permission.MOUNT_UNMOUNT_FILESYSTEMS"/>
向SD卡中写入的权限：
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />