开发家庭记账本小程序必然会用到数据的存储，所以今天主要学习Android开发的数据存储。

##Android开发的数据存储

• 文件存储

1. 应用程序只可以把数据存储在自己私有的文件夹里， data/data/<包名>/文件名...
   1. 上下文：应用程序运行的环境
   2. api获取目标目录：
      • this.getFileDir（）===>data/data/<包名>/files     //保存重要的配置信息

• this.getCacheDir（） ===>data/data/<包名>/cache   //缓存目录

  • package com.fmd.qqlander;import java.io.BufferedReader;
    import java.io.File;
    import java.io.FileInputStream;
    import java.io.FileNotFoundException;
    import java.io.FileOutputStream;
    import java.io.IOException;
    import java.io.InputStreamReader;import android.app.Activity;
    import android.os.Bundle;
    import android.text.TextUtils;
    import android.util.Log;
    import android.view.Menu;
    import android.view.MenuItem;
    import android.view.View;
    import android.widget.CheckBox;
    import android.widget.EditText;
    import android.widget.Toast;public class MainActivity extends Activity {private static String MainActivity;private static final String TAG = MainActivity;private EditText et_qq;private EditText et_password;private CheckBox cb_remember;@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);et_qq = findViewById(R.id.et_qq);et_password = findViewById(R.id.et_password);cb_remember = findViewById(R.id.cb_remember);//获取原来在文件中保存的QQ号码和密码，回到界面上。//File file = new File("data/data/com.fmd.qqlander/info.txt");//getFilesDir() === /data/data/<当前应用程序的包名>/filesFile file = new File(this.getFilesDir(),"info.txt");if(file.exists()&&file.length()>0) {try {FileInputStream fis = new FileInputStream(file);BufferedReader br = new BufferedReader(new InputStreamReader(fis));String info = br.readLine();String qq = info.split("##")[0];String pwd = info.split("##")[1];et_qq.setText(qq);et_password.setText(pwd);} catch (IOException e) {// TODO Auto-generated catch block
                    e.printStackTrace();}}}public void login(View view) {String qq = et_qq.getText().toString().trim();String pwd = et_password.getText().toString().trim();if(TextUtils.isEmpty(qq)||TextUtils.isEmpty(pwd)){Toast.makeText(this, "用户名或密码不能为空", 0).show();return;}if(cb_remember.isChecked()) {//CheckBox被勾选时记住密码Log.i(TAG,"记住密码");}else {//不需要记住密码Log.i(TAG,"不需要记住密码");}try {//File file = new File("data/data/com.fmd.qqlander/info.txt");File file = new File(this.getFilesDir(),"info.txt");FileOutputStream fos = new FileOutputStream(file);fos.write((qq+"##"+pwd).getBytes());fos.close();Toast.makeText(this, "数据保存成功", 0).show();} catch (FileNotFoundException e) {// TODO Auto-generated catch block
                e.printStackTrace();} catch (IOException e) {// TODO Auto-generated catch block
                e.printStackTrace();Toast.makeText(this, "数据保存异常", 0).show();}}@Overridepublic boolean onCreateOptionsMenu(Menu menu) {// Inflate the menu; this adds items to the action bar if it is present.
            getMenuInflater().inflate(R.menu.main, menu);return true;}@Overridepublic boolean onOptionsItemSelected(MenuItem item) {// Handle action bar item clicks here. The action bar will// automatically handle clicks on the Home/Up button, so long// as you specify a parent activity in AndroidManifest.xml.int id = item.getItemId();if (id == R.id.action_settings) {return true;}return super.onOptionsItemSelected(item);}
    }

• sd卡存储

1. 应用程序可以把数据存储在外存储卡
   • 需要声明权限
   • Environment.getExternalStorageDirectory() //获取外部存储卡的目录
   • 需要注意检查sd卡是否可用，检查sd卡的剩余空间

File file = new File(Environment.getExternalStorageDirectory(),"info.txt");if(file.exists()&&file.length()>0) {try {FileInputStream fis = new FileInputStream(file);BufferedReader br = new BufferedReader(new InputStreamReader(fis));String info = br.readLine();String qq = info.split("##")[0];String pwd = info.split("##")[1];et_qq.setText(qq);et_password.setText(pwd);} catch (IOException e) {// TODO Auto-generated catch block
                e.printStackTrace();}

• 文件权限

1. 应用程序在data/data/包名/目录下创建的文件都是私有的，其他应用程序是不可以访问的。
2. 默认生成的文件都是私有的

• 如何生成公开权限的文件

//生成公开权限的文件public void getpublicFile(View view) {try {FileOutputStream fos = openFileOutput("public.txt",Context.MODE_WORLD_READABLE+Context.MODE_WORLD_READABLE);fos.write("private".getBytes());fos.close();} catch (IOException e) {// TODO Auto-generated catch block
            e.printStackTrace();}}

• 更改文件权限的方法

1. chmod:change mode       Linux下更改文件访问的权限

• 参数 sharedperference

1. 声明 sharedperference  //sp = this.getSharedPreferences("config", 0);
2. 获取到一个参数
3. 获取编辑器  //Editor editor = sp.edit();
4. editor.putString(key,value);  //  editor.putInt();  //  editor.putDouble();
5. editor.commit();保存数据完毕，必须记得调用commit的方法
6. 获取数据 sp.getString(key,dafvalue);  //  sp.getInt();  //  sp.getDouble();

public class MainActivity extends Activity {private static String MainActivity;private static final String TAG = MainActivity;private EditText et_qq;private EditText et_password;private CheckBox cb_remember;//1.声明spprivate SharedPreferences sp;@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);et_qq = findViewById(R.id.et_qq);et_password = findViewById(R.id.et_password);cb_remember = findViewById(R.id.cb_remember);//2.获取到一个参数sp = this.getSharedPreferences("config", 0);String qq = sp.getString("qq", "");    String pwd = sp.getString("pwd", "");et_qq.setText(qq);et_password.setText(pwd);}public void login(View view) {String qq = et_qq.getText().toString().trim();String pwd = et_password.getText().toString().trim();if(TextUtils.isEmpty(qq)||TextUtils.isEmpty(pwd)){Toast.makeText(this, "用户名或密码不能为空", 0).show();return;}if(cb_remember.isChecked()) {//CheckBox被勾选时记住密码Log.i(TAG,"记住密码");//3.得到sp文件的编辑器Editor editor = sp.edit();editor.putString("qq",qq);editor.putString("pwd",pwd);//4.保存数据完毕，必须记得调用commit的方法
            editor.commit();}else {//不需要记住密码Log.i(TAG,"不需要记住密码");}}@Overridepublic boolean onCreateOptionsMenu(Menu menu) {// Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);return true;}@Overridepublic boolean onOptionsItemSelected(MenuItem item) {// Handle action bar item clicks here. The action bar will// automatically handle clicks on the Home/Up button, so long// as you specify a parent activity in AndroidManifest.xml.int id = item.getItemId();if (id == R.id.action_settings) {return true;}return super.onOptionsItemSelected(item);}