// 取数据
@Override
protected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);etUserName = findViewById(R.id.et_username);etPassword = findViewById(R.id.et_password);SharedPreferences login_info = getSharedPreferences("login_info", MODE_PRIVATE);// 第一个参数为Key，第二个是默认值etUserName.setText(login_info.getString("user_name", ""));etPassword.setText(login_info.getString("password", ""));
}
​
​
// 存储据
public void userLogin(View view) {String userName = etUserName.getText().toString();String password = etPassword.getText().toString();if(TextUtils.isEmpty(userName) || TextUtils.isEmpty(password)){Toast.makeText(MainActivity.this, "输入不完整", Toast.LENGTH_SHORT).show();}
​// 存储输入的信息// 1、拿到SharedPreference对象SharedPreferences loginInfoSP = getSharedPreferences("login_info", MODE_PRIVATE);// 2、获取Editor对象SharedPreferences.Editor editor = loginInfoSP.edit();// 3、通过Editor存储数据editor.putString("user_name", userName);editor.putString("password", password);// 3、调用提交方法boolean commit = editor.commit();Log.i(TAG, "userLogin: commitRet = " + commit);
​// 校验登录结果if(!("admin".equals(userName) && "123456".equals(password))){Toast.makeText(MainActivity.this, "用户名或密码错误", Toast.LENGTH_SHORT).show();}else {Toast.makeText(MainActivity.this, "登录成功", Toast.LENGTH_SHORT).show();// TODO ...}
}

public void saveToFileDir() {File filesDir = MainActivity.this.getFilesDir();File myFileTxt = new File(filesDir, "myFile.txt");try (BufferedWriter writer = new BufferedWriter(new FileWriter(myFileTxt))){writer.write("这是内部存储文件目录的文件内容！");} catch (IOException e) {e.printStackTrace();}
}
​
public void saveToCacheDir() {File cacheDir = MainActivity.this.getCacheDir();File myCacheTxt = new File(cacheDir, "myCache.txt");try (BufferedWriter writer = new BufferedWriter(new FileWriter(myCacheTxt))){writer.write("这是内部存储缓存文件内容！");} catch (IOException e) {e.printStackTrace();}
}

<?xml version="1.0" encoding="utf-8"?>
<LinearLayoutxmlns:android="http://schemas.android.com/apk/res/android"xmlns:tools="http://schemas.android.com/tools"android:layout_width="match_parent"android:layout_height="match_parent"android:padding="20dp"android:orientation="vertical"tools:context=".MainActivity">
​<EditTextandroid:id="@+id/et_content"android:layout_width="match_parent"android:layout_height="wrap_content"android:minLines="10" />
​<Buttonandroid:id="@+id/btn_save"android:onClick="save"android:layout_width="wrap_content"android:layout_height="wrap_content"android:text="保存" />
​<Buttonandroid:onClick="read"android:id="@+id/btn_read"android:layout_width="wrap_content"android:layout_height="wrap_content"android:text="读取" />
​<TextViewandroid:id="@+id/tv_show"tools:text="Content Text！"android:layout_width="match_parent"android:layout_height="wrap_content"/>
</LinearLayout>

public class MainActivity extends AppCompatActivity {private static final String TAG = "MainActivity";private EditText etContent;private TextView tvShow;
​// 申请权限请求码private static final int REQUEST_EXTERNAL_STORAGE = 1001;
​// 检查权限，这种写法主要是针对比较新的Android6.0及以后的版本public static void verifyStoragePermissions(Activity activity) {int writePermission = ActivityCompat.checkSelfPermission(activity, Manifest.permission.WRITE_EXTERNAL_STORAGE);int readPermission = ActivityCompat.checkSelfPermission(activity, Manifest.permission.READ_EXTERNAL_STORAGE);
​if (writePermission != PackageManager.PERMISSION_GRANTED|| readPermission != PackageManager.PERMISSION_GRANTED) {// 如果没有权限需要动态地去申请权限ActivityCompat.requestPermissions(activity,// 权限数组new String[]{Manifest.permission.READ_EXTERNAL_STORAGE, Manifest.permission.WRITE_EXTERNAL_STORAGE},// 权限请求码REQUEST_EXTERNAL_STORAGE);}}// 如果在申请权限的过程中需要做一些对应的处理，则在此方法中处理@Overridepublic void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {super.onRequestPermissionsResult(requestCode, permissions, grantResults);if(REQUEST_EXTERNAL_STORAGE == requestCode){// TODO ...}}
​@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);// 获得外部存储的目录File externalStorageDirectory = Environment.getExternalStorageDirectory();Log.i(TAG, "onCreate: externalStorageDirectory = " + externalStorageDirectory.getAbsolutePath());etContent = findViewById(R.id.et_content);tvShow = findViewById(R.id.tv_show);verifyStoragePermissions(this);}
​public void save(View view) {String content = etContent.getText().toString();if(TextUtils.isEmpty(content)) {Toast.makeText(MainActivity.this, "输入为空", Toast.LENGTH_SHORT).show();return;}
​// 判断外部存储的状态if(Environment.MEDIA_MOUNTED.equals(Environment.getExternalStorageState())){// 外部存储已挂载String absolutePath = Environment.getExternalStorageDirectory().getAbsolutePath();String descPath = absolutePath + "/input_content.txt";File descFile = new File(descPath);Log.i(TAG, "save: descFile = " + descFile.getAbsolutePath());try {BufferedWriter writer = new BufferedWriter(new FileWriter(descFile, true));writer.write(content);writer.close();Toast.makeText(MainActivity.this, "写入成功", Toast.LENGTH_SHORT).show();} catch (IOException e) {e.printStackTrace();}}else{Toast.makeText(MainActivity.this, "存储设备未挂载", Toast.LENGTH_SHORT).show();}}
​public void read(View view) {String absolutePath = Environment.getExternalStorageDirectory().getAbsolutePath();String descPath = absolutePath + "/input_content.txt";try {BufferedReader bufferedReader = new BufferedReader(new FileReader(new File(descPath)));String line;StringBuilder builder = new StringBuilder();while((line = bufferedReader.readLine()) != null){builder.append(line).append("n");}tvShow.setText(builder.toString());} catch (IOException e) {e.printStackTrace();}}
}

<!-- 在SDCard中创建与删除文件的权限 -->
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>

// 存储一个字符串到私有外部存储空间 Downloads
public void saveToPrivate(View view) {File externalPrivateDir = MainActivity.this.getExternalFilesDir(Environment.DIRECTORY_DOWNLOADS);File myDownloadTxt = new File(externalPrivateDir, "myDownload.txt");try {BufferedWriter writer = new BufferedWriter(new FileWriter(myDownloadTxt));writer.write("这是下载目录的文件内容！");writer.close();} catch (IOException e) {e.printStackTrace();}
​
}
​
// 存储一个字符串到私有外部存储空间 cache
public void saveToCache(View view) {File externalPrivateDir = MainActivity.this.getExternalCacheDir();File myCacheTxt = new File(externalPrivateDir, "myCache.txt");try {BufferedWriter writer = new BufferedWriter(new FileWriter(myCacheTxt));writer.write("这是缓存文件内容！");writer.close();} catch (IOException e) {e.printStackTrace();}
}