Java convert ArrayList to string and back to ArrayList?

Question

I wanted to save an ArrayList to SharedPreferences so I need to turn it into a string and back, this is what I am doing:

// Save to shared preferences
SharedPreferences sharedPref = this.getPreferences(Activity.MODE_PRIVATE);
SharedPreferences.Editor editor = this.getPreferences(Activity.MODE_PRIVATE).edit();
editor.putString("myAppsArr", myAppsArr.toString());
editor.commit();

I can retrieve it with String arrayString = sharedPref.getString("yourKey", null); but I don't know how to convert arrayString back into an ArrayList. How can it be done?

---

My array looks something like:

[item1,item2,item3]

Solution 1

You have 2 choices :

1. Manually parse the string and recreate the arraylist. This would be pretty tedious.

2. Use a JSON library like Google's Gson library to store and retrieve objects as JSON strings. This is a lightweight library, well regarded and popular. It would be an ideal solution in your case with minimal work required. e.g.,

   // How to store JSON string
   Gson gson = new Gson();
   // This can be any object. Does not have to be an arraylist.
   String json = gson.toJson(myAppsArr);
   
   // How to retrieve your Java object back from the string
   Gson gson = new Gson();
   DataObject obj = gson.fromJson(arrayString, ArrayList.class);
   

Solution 2

Try this

ArrayList<String> array = Arrays.asList(arrayString.split(","))

This will work if comma is used as separator and none of the items have it.

Solution 3

The page http://mjiayou.com/2015/07/22/exception-gson-internal-cannot-be-cast-to/ contains the following:

Type     type  = new TypeToken<List<T>>(){}.getType();
List<T>  list  = gson.fromJson(jsonString, type)

perhaps it will be helpful.

Solution 4

      //arraylist convert into String using Gson 
      Gson gson = new Gson();
      String data = gson.toJson(myArrayList);
      Log.e(TAG, "json:" + gson);

      //String to ArrayList

      Gson gson = new Gson();
      arrayList=gson.fromJson(data, new TypeToken<List<Friends>>()
      {}.getType());

Solution 5

I ended up using:

ArrayList<String> appList = new ArrayList<String>(Arrays.asList(appsString.split("\\s*,\\s*")));

This doesn't work for all array types though. This option differs from:

ArrayList<String> array = Arrays.asList(arrayString.split(","));

on that the second option creates an inmutable array.

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