Saturday, May 23, 2015

Java 8: Using Optional Objects

Consider the following nested object structure for a person with a camera phone:

class Person {
  private Phone phone;

  public Phone getPhone() {
    return phone;
  }
}

class Phone {
  private Camera camera;

  public Camera getCamera() {
    return camera;
  }
}

class Camera {
  private Resolution resolution;

  public Resolution getResolution() {
    return resolution;
  }
}

class Resolution {
  private int width;
  private int height;
}

Now, let's say that you want to get the resolution of a person's camera phone.

You might be tempted to do this:

public Resolution getPhoneCameraResolution(Person person) {
  return person.getPhone().getCamera().getResolution();
}

However, not everyone owns a phone, so person.getPhone() might return null and consequently person.getPhone().getCamera() will result in a NullPointerException at run-time! Similarly, a person might own a phone, but the phone might not have a camera.

One way to avoid NullPointerExceptions is to check for nulls, as shown below:

public Resolution getPhoneCameraResolution(Person person) {
  if (person != null) {
    Phone phone = person.getPhone();
    if (phone != null) {
      Camera camera = phone.getCamera();
      if (camera != null) {
        return camera.getResolution();
      }
    }
  }
  return Resolution.UNKNOWN;
}

This code doesn't scale very well and makes your code harder to read and maintain. Every time a variable could be null, you have to add another nested if statement.

Java 8: Optional class

Java 8 introduced a new class called java.util.Optional<T> to model potentially absent values. It forces you to actively unwrap an optional and deal with the absence of a value. It also leads to better APIs because, just by reading the signature of a method, you can tell whether to expect an optional value.

We can re-write the original object model using the Optional class as shown below:

import java.util.Optional;

class Person {
  private Optional<Phone> phone;

  public Optional<Phone> getPhone() {
    return phone;
  }
}

class Phone {
  private Optional<Camera> camera;

  public Optional<Camera> getCamera() {
    return camera;
  }
}

class Camera {
  // resolution is not optional - all cameras must have it
  private Resolution resolution;

  public Resolution getResolution() {
    return resolution;
  }
}

Creating Optional objects:

  • Empty Optional: You can create an empty optional object use Optional.empty:
    Optional<Phone> optPhone = Optional.empty();
    
  • Optional from a non-null value: To create an optional from a non-null value use Optional.of. Note that if the value is null a NullPointerException is thrown immediately:
    Optional<Phone> optPhone = Optional.of(phone);
    
  • Optional from null: Use Optional.ofNullable to create an optional that may hold a null value. If the value is null, an empty optional is created.
    Optional<Phone> optPhone = Optional.ofNullable(phone);
    

Extracting values from Optional objects:

Optional supports a map method, which can be used to extract information from an object. For example, to get the Resolution of a Camera:

Optional<Camera> optCamera = Optional.ofNullable(camera);
Optional<Resolution> resolution = optCamera.map(Camera::getResolution);

What this means is that if the camera optional contains a value, getResolutions is called, otherwise nothing happens and an empty optional is returned.

Optional also contains a flatMap method in order to "flatten" nested optionals. For example, if you were to call map with a function that returns an Optional, you will get an Optional containing an Optional, as illustrated below:

Optional<Phone> optPhone = Optional.ofNullable(phone);

// calling map returns a two-level optional
Optional<Optional<Camera>> optOptCamera = optPhone.map(Phone::getCamera);

// but flatMap, returns a single-level optional
Optional<Camera> optCamera = optPhone.flatMap(Phone::getCamera);

Chaining Optional objects:

Now let's get back to our original problem of getting the resolution of a person's camera phone and re-write those ugly, nested if-statements using optionals instead:

public Resolution getPhoneCameraResolution(final Optional<Person> person) {
  return
    person.flatMap(Person::getPhone)  // returns Optional<Phone>
          .flatMap(Phone::getCamera) // returns Optional<Camera>
          .map(Camera::getResolution) // returns Optional<Resolution>
          .orElse(Resolution.UNKNOWN); // returns Resolution or UNKNOWN if not found
}

This code returns the resolution of the person's camera phone. If the person is null, doesn't have a phone, or the phone doesn't have a camera, an UNKNOWN resolution is returned.

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