mock_fake.java

class FakeBlogRepository implements BlogRepository {

    private Map<String, List<Post>> data = new HashMap<>();
    private long lastPostId = 0L;

    public long save(long userId, Post post) {
        lastPostId++;

        post.setId(lastPostId);
        List<Post> posts = data.get(userId);
        if (posts == null) {
            posts = new ArrayList<>();
        }
        posts.add(post);

        return lastPostId;
    }

    public findByUserId(long userId) {
        List<Post> posts = data.get(userId);
        if (posts == null) {
            throw new IllegalStateException("Not found");
        } 
        return posts;
    }
}