How to Convert JSON to Java Object with List Fields Using Moshi

What is the process for converting JSON to a Java object that includes a List<Object> field when using Moshi?

Moshi is a modern JSON library for Android and Java, designed to efficiently parse and serialize JSON data. When working with JSON that includes a field that's a List<Object>, it's important to understand how to structure your Java classes and how to use annotations to facilitate this conversion.

import com.squareup.moshi.Json;
import com.squareup.moshi.Moshi;
import com.squareup.moshi.Types;
import java.util.List;

// Define your model class
public class User {
    @Json(name = "name")
    public String name;
    
    @Json(name = "friends")
    public List<Friend> friends;
}

public class Friend {
    @Json(name = "friend_name")
    public String friendName;
}

// Usage example
Moshi moshi = new Moshi.Builder().build();
Type userListType = Types.newParameterizedType(List.class, User.class);
JsonAdapter<List<User>> jsonAdapter = moshi.adapter(userListType);

String json = "[{ \"name\": \"Alice\", \"friends\": [ { \"friend_name\": \"Bob\" } ] }]";
List<User> users = jsonAdapter.fromJson(json);  // Converts JSON into List<User>

Causes

• The JSON structure does not match the Java object structure, leading to parsing issues.
• Missing Moshi annotations that provide serialization/deserialization guidance.
• Incompatible types between the List in your Java class and the JSON elements.

Solutions

• Define your Java object with a List field using generics, making sure it matches the JSON structure.
• Use Moshi's `@Json` annotation to specify the name of the JSON field if it differs from your Java variable name.
• Implement custom adapters if you need more complex deserialization logic for the objects contained in the list.

Mistake: Assuming all necessary relationships within the Java classes are accounted for in the JSON structure.

Solution: Always validate your JSON structure against your defined models to prevent parsing exceptions.

Mistake: Not handling potential null values in JSON which may result in NullPointerExceptions.

Solution: Use `@Json` with default values or check for null when parsing.