android如何操作sd卡

//申明SharedPreferences对象

SharedPreferences mSharedPreferences;
SharedPreferences.Editor mEditor;

mSharedPreferences = getSharedPreferences("user", Context.MODE_PRIVATE);
mEditor = mSharedPreferences.edit();

//在主方法里写一个点击事件保存数据

Button left = (Button) findViewById(R.id.left);

left.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
        WriteToSd();
        mEditor.putString("user",editText1.getText().toString());
        mEditor.putString("passWorld",editText2.getText().toString());
        mEditor.commit();
    }
});

user = mSharedPreferences.getString("user",null);
passWorld = mSharedPreferences.getString("passWorld",null);
editText1.setText(user);
editText2.setText(passWorld);

//读取文件
private void ReadFromSd(){
    String s = Environment.getExternalStorageState();
    File root = Environment.getExternalStorageDirectory();
    File TargetDir = new File(root,super.getPackageName());
    ObjectInputStream ois = null;
    if (TargetDir.exists()){
        try {
            ois = new ObjectInputStream(new FileInputStream(new File(TargetDir+"/liu.txt")));
            People people = (People) ois.readObject();
            Toast.makeText(getApplication(),people.getUser().toString()+people.getPwd().toString(),Toast.LENGTH_SHORT).show();
        } catch (IOException e) {
            e.printStackTrace();
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }finally {
            try {
                ois.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }else {
        Toast.makeText(getApplication(),"false",Toast.LENGTH_SHORT).show();
    }
}
写入文件
private void WriteToSd(){
    String s = Environment.getExternalStorageState();
    ObjectOutputStream oos = null;
    if (s.equals(Environment.MEDIA_MOUNTED)){//判断是否有权限,记得在清单文件里声明权限,6.1版本之后的手机可能还是不行
        File root = Environment.getExternalStorageDirectory();
        File tag = new File(root ,super.getPackageName());
        if (!tag.exists()){  //判断文件是否存在

            tag.mkdir();
        }
        try {
            oos = new ObjectOutputStream(new FileOutputStream(new File(tag,"liu,txt")));
            People people = new People("liu","yifan");
            oos.writeObject(people);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
          package="com.jredu.liuyifan.fragmentapplication">//写在这里面
//两个权限
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
    <uses-permission android:name="android.permission.MOUNT_UNMOUNT_FILESYSTEMS"/>