How to make a field optional?

Question

I have a class like so

class User {
  private String firstName;
  private String lastName;

  ...
}

I want the lastName property to be optional. How do I accomplish this in Java?

In some languages it's quite elegant and simple wherein we just append a question mark after the field name.

Answer 1

Overview

There is no language concept for this in Java.

In Java, all (non-primitive) types are nullable, hence can be seen optional. So you could just assign it null and call it a day.

To overcome the problems related with that, make intention clearer and strive for fail-fast, what I have seen being used a lot would be to assign null to it internally and return Optional<String> on methods that would return it, such as a getLastName() method.

---

Code example

So one way doing this could be:

class User {
  private String firstName;
  private String lastName;

  User(@NotNull String firstName) {
     this(firstName, null);
  }

  User(@NotNull String firstName, @Nullable String lastName) {
    this.firstName = firstName;
    this.lastName = lastName;
  }

  @NotNull Optional<String> getLastName() {
    return Optional.ofNullable(lastName);
  }

  void setLastName(@Nullable String lastName) {
    this.lastName = lastName;
  }

  ...
}

I used the @NotNull and @Nullable annotations intentionally to make it clearer for you where null could be allowed in such a design.

Note that if you have more optional parameters, a builder-pattern might also become handy.

Ultimately, also think about whether having optional values is a good design in the first place - maybe you can redesign the entire thing on a bigger picture to aovid the situation all together.

---

Optional as field or argument

Note that it is usually not recommended to store the field as Optional<String>, nor to take such arguments, for example a setLastName(Optional<String>).

Optional was designed only to be used by methods wo want to indicate the lack of a return value.

You can read more about that here:

• Is it a good practice to use Optional as an attribute in a class?
• Uses for Optional

Answer 2

Just assign null to it, when you call it. you can also just assign it a empty string and then if needed for your use case have a method that checks if it is empty and from there knows that that particular user only has a first name user("John","")

Answer 3

You could use the Optional monad. Though honestly, I'd just keep the values as-is, and treat the fact that they are nullable elsewhere (depends on the case):

Optional.ofNullable(user.getFirstName()).orElse("unknown");

Optional.ofNullable(user.getFirstName()).orElseThrow(() 
    -> new InvalidArgumentException("First name is required);

Answer 4

Maybe just create a getter that returns Optional:

class User {
  private String firstName;
  private String lastName;

  public String getFirstName() {
    return firstName;
  }

  public Optional<String> getLastName() {
    return Optional.ofNullable(lastName);
  }
}

and then:

User user = new User();
...
String lastName = user.getFirstName();
Optional<String> lastName = user.getLastName();