How to save an instance of a custom class in onSaveInstanceState?

Custom objects can be saved inside a Bundle when they implement the interface Parcelable. Then they can be saved via:

    @Override
    public void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        outState.putParcelable("key", myObject);
    }

Basically the following methods must be implemented in the class file:

 public class MyParcelable implements Parcelable {
     private int mData;

     public int describeContents() {
         return 0;
     }

     /** save object in parcel */
     public void writeToParcel(Parcel out, int flags) {
         out.writeInt(mData);
     }

     public static final Parcelable.Creator<MyParcelable> CREATOR
             = new Parcelable.Creator<MyParcelable>() {
         public MyParcelable createFromParcel(Parcel in) {
             return new MyParcelable(in);
         }

         public MyParcelable[] newArray(int size) {
             return new MyParcelable[size];
         }
     };

     /** recreate object from parcel */
     private MyParcelable(Parcel in) {
         mData = in.readInt();
     }
 }

Check this answer.

Basically you have to save it inside a Bundle.

I know "that this case is cold", but because i found this thread first, when I was searching for exactly the same thing (and found an answer by now):

Imagine Bundle as an XML file. If you create a new <BUNDLE name="InstanceName" type="ClassName"> you can freely add elements and attributes in a fresh and empty namespace.

When onSaveInstance(Bundle outState) of your MainActivity is called (you can also force this in onPause), you can create a new: Bundle b = new Bundle();

Then call your (probably not inherited and not overriden) custom Method onSaveInstance(Bundle b) in your own class with your newly created Bundle b. Then (in onSaveInstance(Bundle outState)) of your MainActivity, call outState.putBundle("StringClassAndInstanceName", b);

When you find this string in onCreate, you can either use a switch/case to recreate this object or (better) have a factory function in your custom class to work with Bundle and "StringClassAndInstanceName".

Kotlin Solution: For custom class save in onSaveInstanceState you can be converted your class to JSON string and restore it with Gson convertion. The following example is for Fragment and Activity:

For Activity:

For put class in saveInstanceState:

override fun onSaveInstanceState(outState: Bundle) {
        super.onSaveInstanceState(outState)
        val gson = Gson()
        val json = gson.toJson(your_custom_class)
        outState.putString("CUSTOM_CLASS", json)
    }

Restore class:

 override fun onRestoreInstanceState(savedInstanceState: Bundle) {
    super.onRestoreInstanceState(savedInstanceState)
    val json = savedInstanceState?.getString("CUSTOM_CLASS")
    if (!json!!.isEmpty()) {
        val gson = Gson()
        testBundle = gson.fromJson(json, Session::class.java)
    }
 }

You can restore it on Activity onCreate also.

For fragment:

For put class in saveInstanceState:

 override fun onSaveInstanceState(outState: Bundle) {
        super.onSaveInstanceState(outState)
        val gson = Gson()
        val json = gson.toJson(customClass)
        outState.putString("CUSTOM_CLASS", json)
    }

Restore class:

 override fun onActivityCreated(savedInstanceState: Bundle?) {
        super.onActivityCreated(savedInstanceState)
        if (savedInstanceState != null) {
            val json = savedInstanceState.getString("CUSTOM_CLASS")
            if (!json!!.isEmpty()) {
                val gson = Gson()
                val customClass: CustomClass = gson.fromJson(json, CustomClass::class.java)
            }
        }
    }