Main.java

import android.util.SparseArray;
import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;

import java.lang.reflect.Type;

public class Main {

    public static class MyCustomClass {
        public int a;
        public String b;
    }

    public static void main(String[] args) {
        Type sparseArrayType = new TypeToken<SparseArray<MyCustomClass>>() {}.getType();
        Gson gson = new GsonBuilder()
            .registerTypeAdapter(sparseArrayType, new SparseArrayTypeAdapter<MyCustomClass>(MyCustomClass.class))
            .create();
    }

}

SparseArrayTypeAdapter.java

import android.util.SparseArray;
import com.google.gson.Gson;
import com.google.gson.JsonElement;
import com.google.gson.TypeAdapter;
import com.google.gson.reflect.TypeToken;
import com.google.gson.stream.JsonReader;
import com.google.gson.stream.JsonToken;
import com.google.gson.stream.JsonWriter;

import java.io.IOException;
import java.lang.reflect.Type;

public class SparseArrayTypeAdapter<T> extends TypeAdapter<SparseArray<T>> {

    private final Gson gson = new Gson();
    private final Class<T> classOfT;
    private final Type typeOfSparseArrayOfT = new TypeToken<SparseArray<T>>() {}.getType();
    private final Type typeOfSparseArrayOfObject = new TypeToken<SparseArray<Object>>() {}.getType();

    public SparseArrayTypeAdapter(Class<T> classOfT) {
        this.classOfT = classOfT;
    }

    @Override
    public void write(JsonWriter jsonWriter, SparseArray<T> tSparseArray) throws IOException {
        if (tSparseArray == null) {
            jsonWriter.nullValue();
            return;
        }
        gson.toJson(gson.toJsonTree(tSparseArray, typeOfSparseArrayOfT), jsonWriter);
    }

    @Override
    public SparseArray<T> read(JsonReader jsonReader) throws IOException {
        if (jsonReader.peek() == JsonToken.NULL) {
            jsonReader.nextNull();
            return null;
        }
        SparseArray<Object> temp = gson.fromJson(jsonReader, typeOfSparseArrayOfObject);
        SparseArray<T> result = new SparseArray<T>(temp.size());
        int key;
        JsonElement tElement;
        for (int i = 0; i < temp.size(); i++) {
            key = temp.keyAt(i);
            tElement = gson.toJsonTree(temp.get(key));
            result.put(key, gson.fromJson(tElement, classOfT));
        }
        return result;
    }
    
}