GraphQL APIs with Spring Boot · Lesson

Implementing Interfaces and Union Types

Utilize interfaces and union types to create flexible and polymorphic schema designs.

Implementing Interfaces and Union Types is a free GraphQL APIs with Spring Boot lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the GraphQL APIs with Spring Boot learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Flexible Schemas with Polymorphism

GraphQL allows you to design highly flexible APIs. Sometimes, you need a field that can return different types of objects, or you want to define a shared set of fields that multiple types must implement. This is where polymorphism comes in.

We'll explore two key concepts for achieving this: Interfaces and Union Types.

Understanding GraphQL Interfaces

Imagine you have different types of media, like a Book and a Movie. Both might have a title and a releaseYear. Instead of duplicating these fields, you can define an Interface.

An interface is a contract that specifies a set of fields that any type implementing it must include. It's like a blueprint for common functionality.

Defining an Interface in SDL

In GraphQL's Schema Definition Language (SDL), an interface is defined using the interface keyword. Here, we define a Media interface that both Book and Movie will share.

interface Media {
  id: ID!
  title: String!
  releaseYear: Int
}

Types Implementing an Interface

Now, let's make our Book and Movie types implement the Media interface. They must include all fields defined by Media, and can add their own unique fields. Notice the implements keyword.

type Book implements Media {
  id: ID!
  title: String!
  releaseYear: Int
  author: String
  pages: Int
}

type Movie implements Media {
  id: ID!
  title: String!
  releaseYear: Int
  director: String
  durationMinutes: Int
}

Resolvers for Interfaces

When you query a field that returns an interface type, Spring GraphQL automatically resolves the concrete type based on the data. You usually create Java data classes that implement a common Java interface, and Spring GraphQL handles the rest. Try running this example:

package com.coddykit.graphql;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.graphql.data.method.annotation.QueryMapping;
import org.springframework.stereotype.Controller;

import java.util.List;
import java.util.ArrayList;

@SpringBootApplication
public class GraphqlApp {
    public static void main(String[] args) {
        SpringApplication.run(GraphqlApp.class, args);
    }
}

interface Media {
    String getId();
    String getTitle();
    Integer getReleaseYear();
}

record Book(String id, String title, Integer releaseYear, String author, Integer pages) implements Media {}
record Movie(String id, String title, Integer releaseYear, String director, Integer durationMinutes) implements Media {}

@Controller
class MediaController {

    @QueryMapping
    public List<Media> allMedia() {
        List<Media> mediaList = new ArrayList<>();
        mediaList.add(new Book("b1", "The Hitchhiker's Guide", 1979, "Douglas Adams", 193));
        mediaList.add(new Movie("m1", "Inception", 2010, "Christopher Nolan", 148));
        return mediaList;
    }
}

Exploring GraphQL Union Types

While interfaces ensure types share common fields, Union Types allow a field to return one of several distinct types, without requiring them to share any common fields. Think of it as an 'either-or' situation. For example, a search result might be a Product OR a User.

Schema Definition for Unions

In SDL, union types are defined using the union keyword, followed by the types it can represent, separated by a vertical bar |. Here's a SearchResult union that can be either a Book or a Movie.

union SearchResult = Book | Movie

Resolvers for Union Types

When a field returns a union type, Spring GraphQL needs to know which concrete type it is. For Java, you simply return an instance of one of the types defined in the union. Spring GraphQL automatically adds the __typename field for clients to distinguish. Try running this example:

package com.coddykit.graphql;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.graphql.data.method.annotation.Argument;
import org.springframework.graphql.data.method.annotation.QueryMapping;
import org.springframework.stereotype.Controller;

@SpringBootApplication
public class GraphqlApp {
    public static void main(String[] args) {
        SpringApplication.run(GraphqlApp.class, args);
    }
}

record Book(String id, String title, Integer releaseYear, String author, Integer pages) {}
record Movie(String id, String title, Integer releaseYear, String director, Integer durationMinutes) {}

@Controller
class SearchController {

    @QueryMapping
    public Object search(@Argument String query) {
        if (query.equalsIgnoreCase("inception")) {
            return new Movie("m1", "Inception", 2010, "Christopher Nolan", 148);
        } else if (query.equalsIgnoreCase("hitchhiker")) {
            return new Book("b1", "The Hitchhiker's Guide", 1979, "Douglas Adams", 193);
        }
        return null;
    }
}

Choosing Between Interfaces and Unions

Both interfaces and unions offer polymorphism, but they serve different purposes:

Interfaces: Use when multiple types share a common set of fields and behavior. They enforce a contract.
Union Types: Use when a field can return one of several distinct types that do not necessarily share any common fields. It's about 'either A or B or C'.

Interfaces vs. Unions Quiz

You are designing a GraphQL API. You need a field that can return either a Photo object or a Video object. These two types have completely different fields and do not share any common properties. Which GraphQL schema feature is best suited for this scenario?

Recap: Flexible Schemas

You've learned how GraphQL Interfaces and Union Types enable flexible and polymorphic schema designs. Interfaces define a contract for common fields, while Union Types allow a field to return one of several distinct types. These tools are crucial for building robust and adaptable GraphQL APIs.

Next, you'll explore Input Types to streamline mutation arguments.

Frequently asked questions

Is the “Implementing Interfaces and Union Types” lesson free?

Yes — the full text of “Implementing Interfaces and Union Types” is free to read here on the web, and the GraphQL APIs with Spring Boot course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the GraphQL APIs with Spring Boot course, upgrade to CoddyKit PRO.

What will I learn in “Implementing Interfaces and Union Types”?

Utilize interfaces and union types to create flexible and polymorphic schema designs. You practise GraphQL APIs with Spring Boot with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start GraphQL APIs with Spring Boot?

No prior experience is required. GraphQL APIs with Spring Boot on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Implementing Interfaces and Union Types” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this GraphQL APIs with Spring Boot lesson?

Yes. Every GraphQL APIs with Spring Boot lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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