数据库存储方案

Android系统中主要提供了三种方式用于简单地实现数据持久化功能，即文件存储、SharedPreference存储及数据库存储。
（一）文件存储
文件存储不对存储的内容进行任何的格式化处理，所有数据都是原封不动地保存在文件中，所以比较适用于存储一些简单的文本数据或二进制数据。
如何将数据存储到文件中：

String fileName = et1.getText().toString();
String fileContent = et2.getText().toString();
    try {
            FileOutputStream fileOutputStream = openFileOutput(fileName,Context.MODE_PRIVATE);
            //UTF-8是基于Linux的编码，gb2312是基于window的编码
            fileOutputStream.write(fileContent.getBytes("UTF-8"));
            fileOutputStream.close();
            Toast.makeText(this,"保存成功",Toast.LENGTH_SHORT).show();
        } catch (Exception e) {
            e.printStackTrace();
        }
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如何从文件中读取数据：

String fileName = et1.getText().toString();
    try {
        FileInputStream fileInputStream = openFileInput(fileName);
        BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(fileInputStream));
        String msg = "";
        StringBuffer buffer = new StringBuffer();
        while((msg = bufferedReader.readLine()) != null){
            buffer.append(msg);
        }
        tv.setText(buffer);
        bufferedReader.close();
    } catch (Exception e) {
        e.printStackTrace();
    }
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存储到SD卡中（极少用）：

public void showSD(View view){
        //判断有没有SD卡
        if (Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {
            String fileName = et1.getText().toString();
            String fileContent = et2.getText().toString();
            String path = Environment.getExternalStorageDirectory().getAbsolutePath();
            try {
                FileOutputStream fileOutputStream = new FileOutputStream(path +"/"+fileName);
                fileOutputStream.write(fileContent.getBytes("UTF-8"));
                fileOutputStream.close();
                Toast.makeText(this,"保存成功",Toast.LENGTH_SHORT).show();
            } catch (Exception e) {
                Log.i("MainActivity",e.toString());
                e.printStackTrace();
            }
        }else {
            Toast.makeText(this,"SD卡不可用",Toast.LENGTH_SHORT).show();
        }
    }
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从SD卡里面读取数据

public void readSD(View view){
        if (Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {
            String fileName = et1.getText().toString();
            String path = Environment.getExternalStorageDirectory().getAbsolutePath();

            try {
                FileInputStream fileInputStream = new FileInputStream(path +"/"+fileName);
                BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(fileInputStream));
                String msg = "";
                StringBuffer buffer = new StringBuffer();
                while((msg = bufferedReader.readLine()) != null){
                    buffer.append(msg);
                }
                tv.setText(buffer);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }else {
            Toast.makeText(this,"SD卡不可用",Toast.LENGTH_SHORT).show();
        }
    }
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（二）、SharedPreferences是使用键值对的方式来存储，支持多种不同的数据类型存储。
使用SharedPreferences存储数据，需先获取到SharedPreferences对象，以下有三种方法：
1、Context类中的getSharedPreences()方法
2、Activity类中的getPreferences()方法
3、PreferenceManager类中的getDefaultSharedPreferences()方法
获得SharedPreferences对象之后，向SharedPreferences文件中存储数据，主要分以下三步实现：
1、调用SharedPreferences对象的edit()方法获取一个SharedPreferences.Editor对象；
2、向SharedPreferences.Editor对象添加数据；
3、调用commit()方法将添加的数据提交，从而完成数据存储操作

public class MainActivity extends Activity {
    private EditText et;
    private SharedPreferences sharedPreferences;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        et = (EditText) findViewById(R.id.et);

        sharedPreferences = getSharedPreferences("info", MODE_PRIVATE);
    }

    public void show(View view) {
        Editor edit = sharedPreferences.edit();
        String pwd =et.getText().toString();
        edit.putString("pwd",pwd);      
        edit.commit();
    }

    public void read(View view){
        String pwd = sharedPreferences.getString("pwd",null);
        et.setText(pwd);
    }
}
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（三）、SQLite数据库存储
前两种只适用于保存简单的数据和键值对，当需要保存大量复杂的关系数据的时候，需用到SQLite。SQLite是一款轻量级的关系型数据库，运算快，占用资源少，适合在移动设备上适用。
1、适用SQL语句操作数据库：

private SQLiteDatabase db;

db = SQLiteDatabase.openOrCreateDatabase(getFilesDir() + "/info.db", null);

public void save(View view) {
    String userName = userName_et.getText().toString();
    String pwd = pwd_et.getText().toString();
    //若数据库中存在表，则执行insert(userName, pwd)；反之，抛出异常，执行create()和insert(userName, pwd)
    try {
        insert(userName, pwd);
    } catch (Exception e) {
        create();
        insert(userName, pwd);
    }
}
/**
* 建表操作
*/
private void create() {
    String createsql = "create table user(_id integer primary key autoincrement,username,password)";
    db.execSQL(createsql);
}

/**
* 插入
* @param userName
* @param pwd
*/
private void insert(String userName,String pwd) {
    Log.i("MainActivity","insert");
    String insertsql = "insert into user(username,password) values(?,?)";
    db.execSQL(insertsql, new String[]{userName,pwd});

    Toast.makeText(this,"插入成功",Toast.LENGTH_SHORT).show();
    userName_et.setText("");
    pwd_et.setText("");
}

/**
* 查询
* @return
*/
private Cursor select(){
    String selectsql = "select * from user";
    Cursor cursor = db.rawQuery(selectsql,null);
    return cursor;
}

/**
* 删除
* @param id
*/
public void delete(String id){
    String deletesql = "delete from user where _id = ?";
    db.execSQL(deletesql, new String[]{id});
}

/**
* 修改
*/
public void update(String name,String pwd){
    String updatasql = "update user set password = ? where username = ?";
    db.execSQL(updatasql, new String[]{pwd,name});
}
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2、借助SQLiteOpenHelper，由于SQLiteOpenHelper是抽象类，所以需创建一个自己的帮助类去继承它。

public class MyOpendhelper extends SQLiteOpenHelper{

    public MyOpendhelper(Context context) {
        //第二个参数是数据库名
        //第三个参数是允许我们在查询数据的时候返回一个自定义的Cursor，一般传入null
        //第四个参数表示当前数据库的版本号
        super(context,"info.db",null,1);
    }

    //用于创建数据库
    @Override
    public void onCreate(SQLiteDatabase db) {
        String createsql = "create table user(_id integer primary key autoincrement,username,password)";
        db.execSQL(createsql);
    }

    //用于对数据库的升级
    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {      
    }
}
public class MainActivity extends Activity {
    private EditText userName_et, pwd_et;
    private SQLiteDatabase db;
    private ListView listView;
    private MyOpendhelper myOpendhelper;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        userName_et = (EditText) findViewById(R.id.username_et);
        pwd_et = (EditText) findViewById(R.id.pwd_et);
        listView = (ListView) findViewById(R.id.lv);

        myOpendhelper = new MyOpendhelper(this);

        //获取一个能读写的数据库
        db  = myOpendhelper.getWritableDatabase();
        //delete();
        update();
    }

    public void save(View view){
        String name = userName_et.getText().toString();
        String pwd = pwd_et.getText().toString();
        insert(name,pwd);
    }

    public void read(View view){
        select();
    }

    private void insert(String userName,String password){
        ContentValues values = new ContentValues();
        values.put("username",userName);
        values.put("password", password);
        db.insert("user", null, values);
        Toast.makeText(this,"插入成功",Toast.LENGTH_SHORT).show();
        userName_et.setText("");
        pwd_et.setText("");
    }

    private void delete(){
        db.delete("user","_id = ?", new String[]{"1"});
    }

    private void update(){
        ContentValues values = new ContentValues();
        values.put("password","0000000");
        db.update("user", values,"username = ?", new String[]{"briup"});
    }

    private void select(){
        Cursor cursor = db.query("user",new String[]{"username","password"},"_id between ? and ?",new String[]{"1","10"},
                null, null, "_id desc", null);

        while(cursor.moveToNext()){
            Log.i("tag",cursor.getString(0));
            Log.i("tag", cursor.getString(1));
        }
    }
}
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SQLiteOpenHelper的两个实例方法：getReadableDatabase()和getWriteableDatabase()都可以创建或打开一个现有的数据库，并返回一个可对数据库进行读写操作的对象，不同的是，当数据库不可写入的时候（如磁盘空间已满）getWriteableDatabase()出现异常，而getReadableDatabase()方法取得实例是先调用getWriteableDatabase()，如数据库的磁盘已满，此时返回打开失败，继而用getWriteableDatabase的实例以只读方式去打开数据库。