Question
Why am I receiving a ClassCastException when trying to deserialize JSON to my custom Product class with Gson?
String name = Util.stringToArray(message).get(i).getName();
Answer
When deserializing JSON strings into Java objects using Gson, you may encounter a ClassCastException, specifically when Gson interprets the data as LinkedTreeMap instead of your custom class. This occurs when Gson cannot determine the target type correctly during deserialization.
public static ArrayList<Product> stringToArray(String s) {
Gson g = new Gson();
Type listType = new TypeToken<ArrayList<Product>>(){}.getType();
ArrayList<Product> list = g.fromJson(s, listType);
return list;
}
Causes
- Gson does not know the type of elements in the ArrayList when converting from JSON to Java objects, leading it to use a default type (LinkedTreeMap).
- The TypeToken is not utilized correctly or not respected during the deserialization process.
Solutions
- Use a concrete TypeToken to inform Gson about the specific type being deserialized.
- Change the type parameter in your stringToArray method to ensure type information is preserved during deserialization.
Common Mistakes
Mistake: Not specifying the correct type token when deserializing.
Solution: Ensure that the TypeToken accurately reflects the generic type you expect, such as ArrayList<Product>.
Mistake: Using raw types instead of proper generic declarations.
Solution: Always specify types with generic collections to prevent type erasure issues.
Helpers
- Gson deserialization
- ClassCastException
- LinkedTreeMap
- Gson TypeToken
- Java JSON parsing
- custom objects Gson
- Product class Gson