Filtering avoiding unexpected null using lambda

问题:

I have list for each element I would like to do this(using Java 8):

disabledUsersOnLDAP.stream().forEach(user ->usersRepository.findEnabledByUsername(user.getUserName()).setEnabled(false));

How ever usersRepository.findEnabledByUsername might return null's. Of course I can do this instead:

disabledUsersOnLDAP.stream().forEach(user -> {
            UserEntity userEntity = usersRepository.findEnabledByUsername(user.getUserName());
            {
                if (userEntity != null) {
                    userEntity.setEnabled(false);
                }
            }
        });

But I wonder if I could do the null check inline(in the first option)?

回答1:

You could do a mapping + filtering before running the forEach part, which also makes it a bit more readable:

disabledUsersOnLDAP.stream()
            .map(user -> usersRepository.findEnabledByUsername(user.getUserName()))
            .filter(userEntity -> userEntity != null)
            .forEach(userEntity -> userEntity.setEnabled(false));

Or as an alternative:

disabledUsersOnLDAP.stream()
            .map(User::getUsername)
            .map(usersRepository::findEnabledByUsername)
            .filter(Objects::nonNull)
            .forEach(userEntity -> userEntity.setEnabled(false));

回答2:

Some alternatives to the @assylias answser.

Use a method reference to Objects==nonNull for the null check:

disabledUsersOnLDAP.stream()
    .map(User::getUsername)
    .map(usersRepository::findEnabledByUsername)
    .filter(Objects::nonNull)
    .forEach(userEntity -> userEntity.setEnabled(false));

---

if you can update UserEntity with a disable method

class UserEntity {
    public void disable() {
        setEnabled(false);
    }
}

you can again use a method reference (UserEntity::disable) :

disabledUsersOnLDAP.stream()
    .map(User::getUsername)
    .map(usersRepository::findEnabledByUsername)
    .filter(Objects::nonNull)
    .forEach(UserEntity::disable);

---

Some resources:

• Method References on the java tutorial
• JSR 335: Lambda Expressions for the JavaTM Programming Language