Java8 .getMethod() vs ::getMethod

Question

I am new in Java8 ,and I created this piece of code that is working fine

 userService.getClient().findUsersByMarkets(marketIds)
                .stream()
                .filter(us -> !alreadyNotifiedUserIds.contains(us.getId()))
                .forEach(usersToBeNotified::add);

but under my understandings this piece of code should also work fine, but is not the case and I am wondering why

     userService.getClient().findUsersByMarkets(marketIds)
        .stream()
        .filter(us -> !alreadyNotifiedUserIds.contains(User::getId))
        .forEach(usersToBeNotified::add);

Answer 1

User::getId is a reference to a function so it's not equivalent to contains(us.getId()).

see --> https://www.codementor.io/eh3rrera/using-java-8-method-reference-du10866vx to familiarise your self with method references.