Every Java developer whether beginner, novice, or seasoned has in his/her lifetime experienced NullPointerException. This is a true fact that no Java developer can deny. We all have wasted or spent many hours trying to fix bugs caused by NullPointerException. According to NullPointerException JavaDoc, NullPointerException is thrown when an application attempts to use null in a case where an object is required.. This means if we invoke a method or try to access a property on null reference then our code will explode and NullPointerException is thrown.
On a lighter note, if you look at the JavaDoc of NullPointerException you will find that author of this exception is unascribed. If the author is unknown unascribed is used(nobody wants to take ownership of NullPointerException ;))
In 2009 at QCon conference Sir Tony Hoare stated that he invented null reference type while designing ALGOL W programming language. null was designed to signify absence of a value. He called null references as a billion-dollar mistake. You can watch the full video of his presentation on Infoq http://www.infoq.com/presentations/Null-References-The-Billion-Dollar-Mistake-Tony-Hoare.
Most of the programming languages like C, C++, C#, Java, Scala, etc. has nullable type as part of their type system which allows the value to be set to a special value Null instead of other possible data type values.
Let's look at the example Task management domain classes shown below. Our domain model is very simple with only two classes -- Task and User. A task can be assigned to a user.
public class Task {
private final String id;
private final String title;
private final TaskType type;
private User assignedTo;
public Task(String id, String title, TaskType type) {
this.id = id;
this.title = title;
this.type = type;
}
public Task(String title, TaskType type) {
this.id = UUID.randomUUID().toString();
this.title = title;
this.type = type;
}
public String getTitle() {
return title;
}
public TaskType getType() {
return type;
}
public User getAssignedTo() {
return assignedTo;
}
public void setAssignedTo(User assignedTo) {
this.assignedTo = assignedTo;
}
}
public class User {
private final String username;
private String fullname;
public User(String username) {
this.username = username;
}
public String getUsername() {
return username;
}
public String getFullname() {
return fullname;
}
public void setFullname(String fullname) {
this.fullname = fullname;
}
}Given the above domain model, if we have to find the User who is assigned a task with id taskId then we would write code as shown below.
public String taskAssignedTo(String taskId) {
return taskRepository.find(taskId).getAssignedTo().getUsername();
}The biggest problem with the code shown above is that absence of the value is not visible in the API i.e. if the task is not assigned to any user then the code will throw NullPointerException when getAssignedTo is called. The taskRepository.find(taskId) and taskRepository.find(taskId).getAssignedTo() could return null. This forces clients of the API to program defensively and check for null checks as shown below.
public String taskAssignedTo(String taskId) throws TaskNotFoundException {
Task task = taskRepository.find(taskId);
if (task != null) {
User assignedTo = task.getAssignedTo();
if (assignedTo != null)
return assignedTo.getUsername();
return "NotAssigned";
}
throw new TaskNotFoundException(String.format("No task exist with id '%s'", taskId));
}The code shown above misses developer intent and bloats client code with if-null checks. The developer somehow wanted to use optional data type but he was forced to write if-null checks. I am sure you would have written this kind of code in your day to day programming.
A common solution to working with null references is to use Null Object pattern. The idea behind this pattern is very simple instead of returning null you should return a null object that implements your interface or class. So, you can create a NullUser as shown below.
public class NullUser extends User {
public NullUser(String username) {
super("NotAssigned");
}
}So now we could return a NullUser when no user is assigned a task. We can change the getAssignedTo method to return NullUser when no user is assigned a task.
public User getAssignedTo() {
return assignedTo == null ? NullUser.getInstance() : assignedTo;
}Now client code can be simplified to not use null check for user as shown below. In this example, it does not make sense to use Null Object pattern for Task because non-existence of task in the repository is an exception situation. Also, by adding TaskNotFoundException in the throws clause we have made it explicit for the client that this code can throw exception.
public String taskAssignedTo(String taskId) throws TaskNotFoundException {
Task task = taskRepository.find(taskId);
if (task != null) {
return task.getAssignedTo().getUsername();
}
throw new TaskNotFoundException(String.format("No task exist with id '%s'", taskId));
}Java 8 introduced a new data type java.util.Optional which encapsulates an empty value. It makes intent of the API clear. If a function returns a value of type Optional then it tells the clients that value might not be present. Use of Optional data type makes it explicit to the API client when it should expect an optional value. When you use Optional type then you as a developer makes it visible via the type system that value may not be present and client can cleanly work with it. The purpose of using Optional type is to help API designers design APIs that makes it visible to their clients by looking at the method signature whether they should expect optional value or not.
Let's update our domain model to reflect optional values.
import java.util.Optional;
import java.util.UUID;
public class Task {
private final String id;
private final String title;
private final TaskType type;
private Optional<User> assignedTo;
public Task(String title, TaskType type) {
this.id = UUID.randomUUID().toString();
this.title = title;
this.type = type;
this.assignedTo = Optional.empty();
}
public Optional<User> getAssignedTo() {
return assignedTo;
}
public void setAssignedTo(User assignedTo) {
this.assignedTo = Optional.of(assignedTo);
}
}
public class User {
private final String username;
private Optional<String> fullname;
public User(String username) {
this.username = username;
this.fullname = Optional.empty();
}
public String getUsername() {
return username;
}
public Optional<String> getFullname() {
return fullname;
}
public void setFullname(String fullname) {
this.fullname = Optional.of(fullname);
}
}Use of Optional data type in the data model makes it explicit that Task refers to an Optional and User has an Optional username. Now whoever tries to work with assignedTo User would know that it might not be present and they can handle it in a declarative way. We will talk about Optional.empty and Optional.of methods in the next section.
In the domain model shown above, we used couple of creational methods of the Optional class but I didn't talked about them. Let's now discuss three creational methods which are part of the Optional API.
-
Optional.empty: This is used to create an Optional when value is not present like we did above
this.assignedTo = Optional.empty();in the constructor. -
Optional.of(T value): This is used to create an Optional from a non-null value. It throws
NullPointerExceptionwhen value is null. We used it in the code shown abovethis.fullname = Optional.of(fullname);. -
Optional.ofNullable(T value): This static factory method works for both null and non-null values. For null values it will create an empty Optional and for non-null value it will create Optional using the value.
Below is a simple example of how you can write API using Optional.
public class TaskRepository {
private static final Map<String, Task> TASK_STORE = new ConcurrentHashMap<>();
public Optional<Task> find(String taskId) {
return Optional.ofNullable(TASK_STORE.get(taskId));
}
public void add(Task task) {
TASK_STORE.put(task.getId(), task);
}
}Optional can be thought as a Stream with one element. It has methods similar to Stream API like map, filter, flatmap that we can use to work with values contained in the Optional.
To read the value of title for a Task we would write code as shown below. The map function was used to transform from Optional to Optional. The orElseThrow method is used to throw a custom business exception when no Task is found.
public String taskTitle(String taskId) {
return taskRepository.
find(taskId).
map(Task::getTitle).
orElseThrow(() -> new TaskNotFoundException(String.format("No task exist for id '%s'",taskId)));
}There are three variants of orElse* method:
-
orElse(T t): This is used to return a value when exists or returns the value passed as parameter like
Optional.ofNullable(null).orElse("NoValue"). This will returnNoValueas no value exist. -
orElseGet: This will return the value if present otherwise invokes the
Suppliersgetmethod to produce a new value. For example,Optional.ofNullable(null).orElseGet(() -> UUID.randomUUID().toString()could be used to lazily produce value only when no value is present. -
orElseThrow: This allow clients to throw their own custom exception when value is not present.
The find method shown above returns an Optional<Task> that the client can use to get the value. Suppose we want to get the task's title from the Optional, we can do that by using the map function as shown below.
To get the username of the user who is assigned a task we can use the flatMap method as shown below.
public String taskAssignedTo(String taskId) {
return taskRepository.
find(taskId).
flatMap(task -> task.getAssignedTo().map(user -> user.getUsername())).
orElse("NotAssigned");
}The third Stream API like operation supported by Optional is filter, which allows you to filter an Optional based on property as shown in example below.
public boolean isTaskDueToday(Optional<Task> task) {
return task.flatMap(Task::getDueOn).filter(d -> d.isEqual(LocalDate.now())).isPresent();
}