This document discusses best practices for handling null values in Java. It recommends: failing fast with NullPointerExceptions instead of silently returning null; never returning null values from methods; marking optional parameters and return values with Java's Optional class; using Optional's map and flatMap methods to avoid deep null checks; and using Guava transforms if on Java 6/7 prior to Java 8 streams. The document provides code examples demonstrating these techniques for optional user objects returned from databases and assigned to groups.

1. Fighting NULL with memes
Jarek Ratajski 2016

4. LESSON NR 1

5. In fact...

6. NPE shows where the problem is
and protects your application
from making further damage

7. Unless you do like below...
public String getUserName(User user) {
if (user != null) {
return user.getName();
} else {
return null;
}
}

8. Never do it

9. Better
Validate.notNull(user);
return user.getName();

10. and the best...
return user.getName();

11. LESSON NR 2

12. what if???
public User getUserFromDB(long key) {
if ( userFoundInDB ) {
return userFromDB;
} else {
???
}
}

13. Solution 1
public User getUserFromDB(long key) throws
UserNotFoundException {
if ( userFoundInDB ) {
return userFromDB;
} else {
throw new
UserNotFoundException();
}
}

14. and if I do not want exception?
@CheckForNull
public User getUserFromDB(long key) {
if ( userFoundInDB ) {
return userFromDB;
} else {
return null;
}
}

15. @Nullable, @CheckForNull – do not
work for real

16. Better – NullObject Pattern
abstract class AbstractUser {
public void addToGroup(String name);
}
class NullUser extends AbstractUser {
@Override
public void addToGroup(String name) {//do nothing
}
}

17. usersService.getUserFromDb(1001).addToGro
up("admins");

18. Cool! But des not work for
getName()...

19. LESSON NR 3

20. Optional<User>
● Is like List<User> with max 1 element
● Has isDefined() method
● Has get()
● Has orElse() and more...
usersService.getUserFromDB(1001).getUserN
ame().orElse("----undefined---");

21. ...
public Optional<User> getUserFromDB(long
key) {
if ( userFoundInDB ) {
return Optional.of(userFromDB);
} else {
return Optional.empty();
}
}

22. You haven't seen best yet...
But we have to make a short break for yet another lesson

23. LESSON 4
class User {
public User(Optional<Group> parent){
}
}

24. API Design
● Do not pass null as method or constructor
param
● Design API so that null is not needed
● Mark optional params with Optional<>

25. Passing null in place of Optional<> argument is a serious crime or idiocy

26. and back to the best...

27. User user = userService.getUserFromDB(10001);
if ( user != null) {
final Group group = user.getGroup();
if ( group != null ) {
final User owner = group.getGroupOwner();
if ( owner != null ) {
owner.sendMessage("hello there!!!");
}
}
}
Lets consider...

28. if you however define
class Group {
Optional<User> getGroupOwner() {...
}
}
class User {
Optional<Group> getGroup() {...
}
void sendMessage(String msg) {...
}
}
class UserService {
Optional<User> getUserFromDB(long key) {
..}
}

29. Then you can...
Optional<User> user = userService.getUserFromDB(10001);
if ( user.isPresent()) {
Optional<Group> group = user.get().getGroup();
if ( group.isPresent() ) {
Optional<User> owner = group.get().getGroupOwner();
if ( owner.isPresent() ) {
owner.get().sendMessage("hello there!!!");
}
}
}

30. userService.getUserFromDB(10001)
.flatMap(user -> user.getGroup() )
.flatMap(group -> group.getGroupOwner() )
.ifPresent(user-> user.sendMessage("Hallo there!"));
BUT! - LESSON 5

31. By the way

32. But .. .problem

33. Guava
userService.getUserFromDB(10001)
.transform(getUserGroupFunction )
.transform(flatten)
.transform(getGroupOwnerFunction)
.transform(flatten)
.transform(sendMessageFunction("Hallo there!"));
Function<User,Group> getUserGroupFunction = new
Function<User,Group>() {
public Group apply( User u ) {…. }
}

34. and it is easier to migrate to Java 8
later

35. Summary
● Fail on null with NPE – mostly best you can do
● Never return null
● Do not pass null – ( unless some badly
designed API asks for it)
● Mark optional params and returns with
Optional<> container
● Learn map and flatMap power of Java 8
● If Java 6,7 -> use sometimes Guava