Can't cast gson to my Object type

Question

I've looked at other questions with the same error but they all seem specific to that user's situation. I was unable to find a solution to my problem.

I'm using gson in my Android app to save a list of my custom object into sharedPreferences. The object is very basic:

public class Idea {
    private String title;
    private String details;

    public Idea (String title, String details) {
        this.title = title;
        this.details = details;
    }

    public String getTitle() { return title; }

    public void setTitle(String newTitle) { title = newTitle; }

    public String getDetails() { return details; }

    public void setDetails(String newDetails) { details = newDetails; }
}

The list is declared like this:

private static List<Idea> ideas = new ArrayList<Idea>();

Serialization seems to work OK but when I try to decode it in the MainActivity like so:

String jsonIdeas = sharedPref.getString(IDEAS_KEY, null);
Gson gson = new Gson();
ideas = gson.fromJson(jsonIdeas, ArrayList.class);

I am getting this error:

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.zarwanhashem.ideatrackr/com.zarwanhashem.ideatrackr.MainActivity}: java.lang.ClassCastException: com.google.gson.internal.LinkedTreeMap cannot be cast to com.zarwanhashem.ideatrackr.Idea
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2325)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)
            at android.app.ActivityThread.access$800(ActivityThread.java:151)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5254)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
     Caused by: java.lang.ClassCastException: com.google.gson.internal.LinkedTreeMap cannot be cast to com.zarwanhashem.ideatrackr.Idea
            at com.zarwanhashem.ideatrackr.MainActivity.onCreate(MainActivity.java:79)
            at android.app.Activity.performCreate(Activity.java:5990)
            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2278)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)
            at android.app.ActivityThread.access$800(ActivityThread.java:151)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5254)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)

I don't think the location I'm getting in the stack trace is accurate because it changes whenever I debug the app. But by break pointing and evaluating in IntelliJ I was able to narrow the error down to decoding code I posted above.

Just in case the serialization is actually not OK, here's the code I'm using:

Gson gson = new Gson();
String json = gson.toJson(ideas);
editor.putString(IDEAS_KEY, json); //putting it into sharedPreferences

The gson was working fine when I was using it to store a list of string lists, but when I carried over the code to use with my custom object I started getting this error.

Answer 1

You need to use a gson TypeToken. TypeTokens are needed when you are trying to deserialize generic types.

For your case, it would be something like --

Type typeOfListOfIdea = new TypeToken<ArrayList<Idea>>(){}.getType();
ideas = gson.fromJson(jsonIdeas, typeOfListOfIdea);

Your other option is to convert it as an array instead of an ArrayList.

Idea ideas[] = gson.fromJson(jsonIdeas, Idea[].class);

Then you can load into an ArrayList (or convert with the Arrays.asList method.