public class RememberMe {

    private SharedPreferences           sharedPreferences;
    private SharedPreferences.Editor    sharedEditor;
    public  static  int                 USERNAME_AND_PASSWORD = 0,
            USERNAME = 1;

    public RememberMe(Context context) {
        sharedPreferences  = PreferenceManager.getDefaultSharedPreferences(context);
        sharedEditor       = sharedPreferences.edit();
    }

    /**
     * @param userIdentity Example; username, email.
     * @param userPassword
     * @param isLastUser If this value is true 'getLastUser' function will get this user.
     */
    public void addUser(String userIdentity, String userPassword, boolean isLastUser){
        // Save user.
        sharedEditor.putString(userIdentity, userPassword);

        // Make last user.
        if (isLastUser){
            sharedEditor.putString("remember_me_last_user", userIdentity);
        }

        // Commit
        sharedEditor.apply();
    }

    @Nullable
    public String getUserPassword(String userIdentity){
        return sharedPreferences.getString(userIdentity, null);
    }

    public RememberMeModel getLastUser(int resultType){
        if (resultType == USERNAME_AND_PASSWORD){
            return new RememberMeModel(
                    sharedPreferences.getString("remember_me_last_user", null),
                    getUserPassword(sharedPreferences.getString("remember_me_last_user", null))
            );
        }

        return new RememberMeModel(
                sharedPreferences.getString("remember_me_last_user", null),
                null
        );
    }
}

android:allowBackup="true"
android:fullBackupContent="true"

android {
    compileSdkVersion 27
    defaultConfig {
        applicationId "com.oka.okaoka"
        minSdkVersion 19
        targetSdkVersion 27
        versionCode 8
        versionName "v1.0.8"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
    compileOptions {
        targetCompatibility 1.8
        sourceCompatibility 1.8
    }
}