Saving data across app launches and terminations  is easy. SharedPreferences is good for storing small amounts of data, such as a username (String) and password (String). For example, if you are building an Android app that saves the user’s email address and password, SharedPreferences can be used to store the email and password. Under normal conditions, setting a variables to “daniel@example.com” and “password1234” will function when the app is open, but not if the user navigates to another app then back to your app.

SharedPreferences is good for storing info like an email and password. It’s fast, reliable and performant. With only 2 lines of code, we can save and retrieve data throughout the app in milliseconds.

But what is it not good for? Large amounts of structure data. For example, SharedPreferences is not good for storing product data in your Ecommerce app or podcasting app. For structured data like an Excel spreadsheet, Room is a viable option. For storing audio and video files, device storage should be utilized. It’s probably best not to store JSON, although it certainly can be used to store JSON, which can be complimented with the GSON library by Google.

With that, let’s begin by saving a value that can be retrieved throughout the app. First we call getSharedPreferences, passing it a file name and read/write mode. For the file name, I’ve chosen production, but you can call it articles, weather, username, password, first_name, or something else.

package com.example.sharedpreferences

import android.content.Context
import android.os.Bundle
import android.support.v7.app.AppCompatActivity
import kotlinx.android.synthetic.main.activity_main.*

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        setSupportActionBar(toolbar)

        val sharedPreferences = getSharedPreferences("production", Context.MODE_PRIVATE)
        sharedPreferences.edit().putString("email_address", "danieljmalone@gmail.com").apply()
    }
}

Then we call edit() on the SharedPreferences variable, passing in a key and value. The key is the field name, and the value is the data to be saved. In this case, an email address.

If you run the code, it successfully saves the email address to local storage. Now let’s see how we can retrieve the email address.

package com.example.sharedpreferences

import android.content.Context
import android.os.Bundle
import android.support.v7.app.AppCompatActivity
import android.util.Log.d
import kotlinx.android.synthetic.main.activity_main.*

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        setSupportActionBar(toolbar)

        val sharedPreferences = getSharedPreferences("production", Context.MODE_PRIVATE)
        sharedPreferences.edit().putString("email_address", "danieljmalone@gmail.com").apply()

        val savedEmailAddress = sharedPreferences.getString("email_address", "")

        d("daniel", "savedEmailAddress is: $savedEmailAddress")
    }
}

With only one line of code, we can get the saved email_address. On line 21 above, we output savedEmailAddress to the Logcat console for testing.

But there are some tricks for saving a lot of data to SharedPreferences. Using the .apply lambda we can save multiple values with less code all things to Kotlin. When saving a user’s email, phone, address, age and city we can simply write the following.

        val sharedPreferences = getSharedPreferences("production", Context.MODE_PRIVATE)
        sharedPreferences.edit().apply {
            putString("emailAddress", "danieljmalone@gmail.com")
            putString("phoneNumber", "512-483-4863")
            putInt("age", 28)
            putString("address", "129 West 4th St. Apt. 1543")
            putString("city", "Austin")
        }.apply()

That’s it! We’ve successfully saved a String to SharedPreferences.