Save HashMap to SharedPreferences: Android Programming Tips

When developing Android applications, it is often necessary to store data locally on the user's device. One common method of doing this is by using SharedPreferences, which is a lightweight key-value storage system provided by the Android framework. In this article, we will discuss how to save a HashMap to SharedPreferences in an Android application.

First, let's define what a HashMap is. A HashMap is a collection that stores data in key-value pairs. In Android, it is often used to store data retrieved from an API or database. To save a HashMap to SharedPreferences, we must first convert it into a format that SharedPreferences can understand.

To do this, we can use the Gson library, which is a popular library used for converting Java objects into JSON and vice versa. To use Gson, we must first add it to our project's dependencies. We can do this by adding the following line to our app module's build.gradle file:

implementation 'com.google.code.gson:gson:2.8.6'

Once we have added the Gson library, we can use it to convert our HashMap into a JSON string. We can do this by creating a Gson object and calling its toJson() method, passing in our HashMap as a parameter. Here is an example:

// Create a HashMap
HashMap<String, String> myMap = new HashMap<>();
myMap.put("key1", "value1");
myMap.put("key2", "value2");

// Convert the HashMap to a JSON string using Gson
Gson gson = new Gson();
String jsonString = gson.toJson(myMap);

Now that we have a JSON string representing our HashMap, we can save it to SharedPreferences using the putString() method. We can retrieve the SharedPreferences object using the getSharedPreferences() method, passing in a unique name for our preferences and a mode (usually MODE_PRIVATE). Here is an example:

// Get the SharedPreferences object
SharedPreferences prefs = getSharedPreferences("myPrefs", MODE_PRIVATE);

// Save the JSON string to SharedPreferences
SharedPreferences.Editor editor = prefs.edit();
editor.putString("myMap", jsonString);
editor.apply();

We have now successfully saved our HashMap to SharedPreferences. To retrieve the HashMap, we can simply retrieve the JSON string from SharedPreferences using the getString() method and then use Gson to convert it back into a HashMap. Here is an example:

// Get the JSON string from SharedPreferences
String jsonString = prefs.getString("myMap", "");

// Convert the JSON string back into a HashMap using Gson
Type type = new TypeToken<HashMap<String, String>>(){}.getType();
HashMap<String, String> myMap = gson.fromJson(jsonString, type);

In conclusion, saving a HashMap to SharedPreferences in an Android application is a simple process that can be accomplished using the Gson library. By converting the HashMap into a JSON string, we can easily store it in SharedPreferences and retrieve it later when needed.