up vote 4 down vote favorite

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]
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4 Answers

up vote 7 down vote accepted
+50

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);
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I changed the array elements to strings, I'm sure it must be a lot easier now. – Liso22 Jan 13 at 23:51
It doesn't matter. With Gson, you can convert any java object to a string and then convert it back. So even if you have an arraylist of custom objects, there's no problem. – Dhruv Gairola Jan 13 at 23:54
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up vote 0 down vote

Build your own converter, for example:

public static String toLisoString(ArrayList<String> lisoList)
{
    if(lisoList == null) return null;

    StringBuilder sb = new StringBuilder();

    for(String s : lisoList)
    {
        if(s == null) sb.append("-1_");
        else
        {
            sb.append(s.length() + "_" + s);
        }
    }

    return sb.toString();
}

/* Take care of any runtime exception here */
public static ArrayList<String> toLisoArrayList(String lisoString) throws CorruptedLisoStringException
{
    if(lisoString == null) return null;

    ArrayList<String> lisoList = new ArrayList<String>();

    while(lisoString.length() > 0)
    {
        try
        {
            int next_ = lisoString.indexOf("_");
            int size = Integer.parseInt(lisoString.substring(0, next_));
            lisoString = lisoString.substring(next_ + 1);
            String token;
            if(size >= 0) token = lisoString.substring(0, size);
            else token = null;
            lisoString = lisoString.substring(size);
            lisoList.add(token);
         }
         catch(Exception e)
         {
             throw new CorruptedLisoStringException(e);
         }
    }

    return lisoList;
}

I didn't test it, but I hope it will work!

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up vote 0 down vote

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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Did you see my answer? – Eng.Fouad Jan 16 at 3:32
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up vote 1 down vote

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.

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