Mapper.java

static <T, R> Publisher<R> map(Publisher<T> upstream, Function<T, R> mapper) {
    return downstream -> upstream.subscribe(new Mapper<>(downstream, mapper));
}

class Mapper<T, R> implements Subscriber<T> {

    final Subscriber<? super R> downstream;  
  
    final Function<T, R> mapper;
  
    Mapper(Subscriber<? super R> downstream, Function<T, R> mapper) {
        this.downstream = downstream;
        this.mapper = mapper;
    }
  
    @Override
    public void onSubscribe(Subscription s) {
         downstream.onSubscribe(s);
    }     
  
    @Override
    public void onNext(T item) {
         downstream.onNext(mapper.apply(item));
    }
  
    @Override
    public void onError(Throwable throwable) {
         downstream.onError(throwable);
    }
  
    @Override
    public void onComplete() {
         downstream.onComplete();
    }
}