Why are null fields missing in Gson JSON output?

By default, Gson omits null fields to save bandwidth. To include them, you must create a specific Gson instance using new GsonBuilder().serializeNulls().create().

Is Gson better than Jackson for Android?

Gson is often preferred for Android because it has zero dependencies and a smaller footprint (190KB) compared to Jackson's modular but larger size.

How do I convert a List of objects in Gson?

Because of Java Type Erasure, you cannot use List.class. You must use a TypeToken: gson.fromJson(json, new TypeToken >(){}.getType()).

What is the @SerializedName annotation?

It maps a JSON field name (e.g., 'user_id') to a different Java field name (e.g., 'userId'), allowing you to follow standard Java naming conventions.

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The Simple Logic

Gson shines because it doesn't need to be "Configured". It works out of the box for 90% of use cases.

Gson gson = new Gson();
// One line. Done.
User user = gson.fromJson(jsonString, User.class);

Controlling Null Fields

The Gotcha: Gson does NOT write null values to JSON by default. If user.name is null, the field "name" simply vanishes from the output.

To fix this, you cannot use new Gson(). You must use the Builder:

Gson gson = new GsonBuilder()
.serializeNulls()
.create();

Powerful API Versioning

Gson has a unique feature that Jackson lacks: built-in versioning annotations. This allows you to manage multiple API versions with a single POJO.

public class User {
@Since(1.0) String name;
@Since(2.0) String email;
@Until(1.5) String legacyField;
}

// Usage
Gson v1 = new GsonBuilder().setVersion(1.0).create();

Handling Lists & Generics

Java deletes generic type info at compile time (Type Erasure). To convert a List<User>, you must capture the type definition using a TypeToken.

Type listType = new TypeToken<List<User>>(){}.getType();
List<User> users = gson.fromJson(jsonArray, listType);

Handling Snake_Case

Don't pollute your code with thousands of @SerializedName annotations. Use a global policy to automatically map API defaults (snake_case) to Java (camelCase).

Gson gson = new GsonBuilder()
.setFieldNamingPolicy(FieldNamingPolicy.LOWER_CASE_WITH_UNDERSCORES)
.create();

Android Performance: TypeAdapters

Reflection is slow on Android. For maximum performance, or to parse massive files without crashing memory, use a streaming TypeAdapter.

public class UserAdapter extends TypeAdapter<User> {
@Override
public User read(JsonReader in) throws IOException {
in.beginObject();
while(in.hasNext()) {
// Read tokens manually... extremely fast!
}
in.endObject();
}
}

The "Inner Class" Crash

Problem: Standard inner classes in Java hold a reference to their parent. Gson cannot instantiate them, leading to InstantiationException.

public class Parent { class Child { } } // WRONG

public class Parent { static class Child { } } // RIGHT

Common Questions

How do I convert a List?

Use TypeToken. See the "Handling Lists" section above for the exact code snippet.

What handles Enum conversion?

Gson automatically converts JSON strings to Enums if they match the Enum name exactly. Use @SerializedName on Enum constants if they differ.

Jackson vs. Gson

Feature	Gson	Jackson
Dependencies	Zero (Single Jar)	Multiple Modules
Null Handling	Manual Config	Standard

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