Save ArrayList to SharedPreferences

Question

I have an ArrayList with custom objects. Each custom object contains a variety of strings and numbers. I need the array to stick around even if the user leaves the activity and then wants to come back at a later time, however I don't need the array available after the application has been closed completely. I save a lot of other objects this way by using the SharedPreferences but I can't figure out how to save my entire array this way. Is this possible? Maybe SharedPreferences isn't the way to go about this? Is there a simpler method?

Answer 1

This method is used to store/save array list:-

 public static void saveSharedPreferencesLogList(Context context, List<String> collageList) {
            SharedPreferences mPrefs = context.getSharedPreferences("PhotoCollage", context.MODE_PRIVATE);
            SharedPreferences.Editor prefsEditor = mPrefs.edit();
            Gson gson = new Gson();
            String json = gson.toJson(collageList);
            prefsEditor.putString("myJson", json);
            prefsEditor.commit();
        }

This method is used to retrieve array list:-

public static List<String> loadSharedPreferencesLogList(Context context) {
        List<String> savedCollage = new ArrayList<String>();
        SharedPreferences mPrefs = context.getSharedPreferences("PhotoCollage", context.MODE_PRIVATE);
        Gson gson = new Gson();
        String json = mPrefs.getString("myJson", "");
        if (json.isEmpty()) {
            savedCollage = new ArrayList<String>();
        } else {
            Type type = new TypeToken<List<String>>() {
            }.getType();
            savedCollage = gson.fromJson(json, type);
        }

        return savedCollage;
    }

Answer 2

With Kotlin, for simple arrays and lists, you can do something like:

class MyPrefs(context: Context) {
    val prefs = context.getSharedPreferences("x.y.z.PREFS_FILENAME", 0)
    var listOfFloats: List<Float>
        get() = prefs.getString("listOfFloats", "").split(",").map { it.toFloat() }
        set(value) = prefs.edit().putString("listOfFloats", value.joinToString(",")).apply()
}

and then access the preference easily:

MyPrefs(context).listOfFloats = ....
val list = MyPrefs(context).listOfFloats

Answer 3

You can save String and custom array list using Gson library.

=>First you need to create function to save array list to SharedPreferences.

public void saveListInLocal(ArrayList<String> list, String key) {

        SharedPreferences prefs = getSharedPreferences("AppName", Context.MODE_PRIVATE);
        SharedPreferences.Editor editor = prefs.edit();
        Gson gson = new Gson();
        String json = gson.toJson(list);
        editor.putString(key, json);
        editor.apply();     // This line is IMPORTANT !!!

    }

=> You need to create function to get array list from SharedPreferences.

public ArrayList<String> getListFromLocal(String key)
{
    SharedPreferences prefs = getSharedPreferences("AppName", Context.MODE_PRIVATE);
    Gson gson = new Gson();
    String json = prefs.getString(key, null);
    Type type = new TypeToken<ArrayList<String>>() {}.getType();
    return gson.fromJson(json, type);

}

=> How to call save and retrieve array list function.

ArrayList<String> listSave=new ArrayList<>();
listSave.add("test1"));
listSave.add("test2"));
saveListInLocal(listSave,"key");
Log.e("saveArrayList:","Save ArrayList success");
ArrayList<String> listGet=new ArrayList<>();
listGet=getListFromLocal("key");
Log.e("getArrayList:","Get ArrayList size"+listGet.size());

=> Don't forgot to add gson library in you app level build.gradle.

implementation 'com.google.code.gson:gson:2.8.2'

Answer 4

    public  void saveUserName(Context con,String username)
    {
        try
        {
            usernameSharedPreferences= PreferenceManager.getDefaultSharedPreferences(con);
            usernameEditor = usernameSharedPreferences.edit();
            usernameEditor.putInt(PREFS_KEY_SIZE,(USERNAME.size()+1)); 
            int size=USERNAME.size();//USERNAME is arrayList
            usernameEditor.putString(PREFS_KEY_USERNAME+size,username);
            usernameEditor.commit();
        }
        catch(Exception e)
        {
            e.printStackTrace();
        }

    }
    public void loadUserName(Context con)
    {  
        try
        {
            usernameSharedPreferences= PreferenceManager.getDefaultSharedPreferences(con);
            size=usernameSharedPreferences.getInt(PREFS_KEY_SIZE,size);
            USERNAME.clear();
            for(int i=0;i<size;i++)
            { 
                String username1="";
                username1=usernameSharedPreferences.getString(PREFS_KEY_USERNAME+i,username1);
                USERNAME.add(username1);
            }
            usernameArrayAdapter = new ArrayAdapter<String>(this, android.R.layout.simple_dropdown_item_1line, USERNAME);
            username.setAdapter(usernameArrayAdapter);
            username.setThreshold(0);

        }
        catch(Exception e)
        {
            e.printStackTrace();
        }
    }

Answer 5

All of the above answers are correct. :) I myself used one of these for my situation. However when I read the question I found that the OP is actually talking about a different scenario than the title of this post, if I didn't get it wrong.

"I need the array to stick around even if the user leaves the activity and then wants to come back at a later time"

He actually wants the data to be stored till the app is open, irrespective of user changing screens within the application.

"however I don't need the array available after the application has been closed completely"

But once the application is closed data should not be preserved.Hence I feel using SharedPreferences is not the optimal way for this.

What one can do for this requirement is create a class which extends Application class.

public class MyApp extends Application {

    //Pardon me for using global ;)

    private ArrayList<CustomObject> globalArray;

    public void setGlobalArrayOfCustomObjects(ArrayList<CustomObject> newArray){
        globalArray = newArray; 
    }

    public ArrayList<CustomObject> getGlobalArrayOfCustomObjects(){
        return globalArray;
    }

}

Using the setter and getter the ArrayList can be accessed from anywhere withing the Application. And the best part is once the app is closed, we do not have to worry about the data being stored. :)

Answer 6

After API 11 the SharedPreferences Editor accept Sets. You could convert your List into a HashSet or something similar and store it like that. When your read it back, convert it into an ArrayList, sort it if needed and you're good to go.

//Retrieve the values
Set<String> set = myScores.getStringSet("key", null);

//Set the values
Set<String> set = new HashSet<String>();
set.addAll(listOfExistingScores);
scoreEditor.putStringSet("key", set);
scoreEditor.commit();

---

You can also serialize your ArrayList and then save/read it to/from SharedPreferences. Below is the solution:

EDIT:
Ok, below is the solution to save ArrayList as serialized object to SharedPreferences and then read it from SharedPreferences.

Because API supports only storing and retrieving of strings to/from SharedPreferences (after API 11, its simpler), we have to serialize and de-serialize the ArrayList object which has the list of tasks into string.

In the addTask() method of the TaskManagerApplication class, we have to get the instance of the shared preference and then store the serialized ArrayList using the putString() method:

public void addTask(Task t) {
  if (null == currentTasks) {
    currentTasks = new ArrayList<task>();
  }
  currentTasks.add(t);

  // save the task list to preference
  SharedPreferences prefs = getSharedPreferences(SHARED_PREFS_FILE, Context.MODE_PRIVATE);
  Editor editor = prefs.edit();
  try {
    editor.putString(TASKS, ObjectSerializer.serialize(currentTasks));
  } catch (IOException e) {
    e.printStackTrace();
  }
  editor.commit();
}

Similarly we have to retrieve the list of tasks from the preference in the onCreate() method:

public void onCreate() {
  super.onCreate();
  if (null == currentTasks) {
    currentTasks = new ArrayList<task>();
  }

  // load tasks from preference
  SharedPreferences prefs = getSharedPreferences(SHARED_PREFS_FILE, Context.MODE_PRIVATE);

  try {
    currentTasks = (ArrayList<task>) ObjectSerializer.deserialize(prefs.getString(TASKS, ObjectSerializer.serialize(new ArrayList<task>())));
  } catch (IOException e) {
    e.printStackTrace();
  } catch (ClassNotFoundException e) {
    e.printStackTrace();
  }
}

You can get ObjectSerializer class from Apache Pig project ObjectSerializer.java