Senior Java Interview: Bug Fix in a Generic Repository

Understanding wildcard variance in Java generics — and why you can never have both safe reads and safe writes at once

Scenario

Your team has built a generic repository used by several Spring Boot services for caching reference data before it’s persisted.

A teammate recently tried to reuse it for a new feature and hit a compilation error.

You’re reviewing their pull request.

The new feature does this:

Repository<User> repository = new Repository<>();

List<AdminUser> admins = List.of(
        new AdminUser("Alice"),
        new AdminUser("Bob")
);

repository.saveAll(admins);   // Compilation error

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Why this code doesn't compile?

A common misconception is:

"AdminUser extends User, so List<AdminUser> should extend List<User>."

That's not how Java generics work. Generics are invariant.