ClientBuilder.java

import org.checkerframework.checker.nullness.qual.NonNull;

class ClientBuilder<T extends @NonNull ClientBuilder<T>> {

    static @NonNull ClientBuilder<?> newBuilder() {
        return new BuilderImpl();
    }

    // Dummy class to get the recursive Builder typing right.
    static class BuilderImpl extends ClientBuilder<BuilderImpl> {}

    T setThing() {
        return (T) this;
    }
}