The Problems with null References
If you are a Java developer then you must have used null to denote as "the absence of a value". And it might not be the case that you never got a NullPointerException (NPE) in any of your Java program. If you have written code in Java 8 or later, then you might have known that Java has introduced java.util.Optional<T> as a better alternative to null. In this article, I'm going to explain what is Optional and how you can use it effectively. But before going too far, I want you to think about the problems of using null and how null has created a useful(!) exception (NPE) to a nightmare to all the existing Java developers.
I would like to mention the most common problems you may face while using null like the following:
nullis meaningless. When an object refer tonullreference, it indicates the absence of value. But it's hard to tell whether the value is actually empty (intentionally/logically) or not initialized yet or empty due to a bug in your algorithm/logic.- Unnecessarily null checking compromises readability by increasing nested indentations and code verbosity.
- A generous(!) source of errors. As Java doesn't enforce null checking, so when you forget to check nullability of an object it may produce the annoying
NullPointerException. nullcarries no information about the intention of use - type information, if missing value is permitted or not etc.- and more...
Let's understand the problems by some code examples. Say, we have the following domains from an imaginary university management system:
public class Student {
private String name;
private Account account;
// other fields, getters, and setters are omitted for simplicity
}
public class Account {
private double balance;
private Loan loan;
// other fields, getters, and setters are omitted for simplicity
}
public class Loan {
private double amount;
// other fields, getters, and setters are omitted for simplicity
}Now if you want to retrieve loan information of a student then you may write a method simply like this:
public Double getLoanAmountOfStudent(Student student){
return student.getAccount().getLoan().getAmount();
}The problem with the code is that if any of the reference in the call is null then, it will cause train wreck by throwing the NPE. To solve this you may rewrite the method like below:
public Double getLoanAmountOfStudent(Student student) {
if (student != null) {
if (student.getAccount() != null) {
Account account = student.getAccount();
if (account.getLoan() != null) {
Loan loan = account.getLoan();
if (loan.getAmount() != null) {
return loan.getAmount();
}
}
}
}
return 0d;
}Now this method has become hard to read because everytime you have a doubt on an object reference, you need to check the nullability. If you miss one null check then it may produce NulPointerException exception. Another problem is with the return type, by observing the returned value of the method you can't say whether the student does not have a loan or the loan amount has become 0 (maybe paid off the loan already).
Introduction to Optional<T>
ava 8 introduces a new class called java.util.Optional that is inspired by Haskell and Scala. The class may contain an optional value otherwise empty value. The intention of the Optional class isn’t to replace every single null reference, rather help you design comprehensible APIs, bring better readability, and obviously help to avoid the NPE.
Using Optionals can bring the following benefits to your code:
- Declaring a variable of type
Optional<T>indicates that the variable of that type may contain missing value. - Enforces "null checking" by encapsulating the actual value.
- Can be used in functional way.
Creating Optional Objects
We can think of three types of values an Optional object can hold - empty, nullable, and non-null. Let's see how you can create different types of Optionals using static factory methods of java.lang.Optional<T>.
Empty Optional indicates absence of value and can be created like below -
Optional.empty()
Nullable Optional indicates the value in it is permitted to be null and can be created like below -
Optional.ofNullable(value)Non-null Optional indicates the value in it is mandatory to be present and can be created like below -
Optional.of(value)
Code Example Using Optional
So, I think you have got enough about Optional and how to create an object of Optional. Let's redesign the above classes using Optionals.
public class Student {
private String name;
private Account account;
// other fields, getters, and setters are omitted for simplicity
}
public class Account {
private Double balance;
private Optional<Loan> loan;
// other fields, getters, and setters are omitted for simplicity
}
public class Loan {
private Double amount;
// other fields, getters, and setters are omitted for simplicity
}
You can see that I have only changed the Loan type to Optional<Loan> in the Account class. That is a clear indication for a developer that an account may not have a loan against it, and it's planned that way. Other fields remain the same as earlier and indicate composition that means being pointing to null references for these variables indicates either missing data or bugs in your code.
The designers of the Optional class developed it with the purpose to support the optional-return idiom only. Thus Optional does not implement the Serializable interface and may break applications that need domains/classes to be serialized. That's why using Optional as a field type is an anti-pattern.
Optional is primarily intended for use as a method return type where there is a clear need to represent "no result, "and where using null is likely to cause errors. A variable whose type is Optional should never itself be null; it should always point to an Optional instance. see at Java SE documentation
The problem could be solved by adding Optional to the return type of getters. Let's refactor the classes to avoid the problem as well as having the benefits of using Optionals.
public class Student {
private String name;
private Account account;
public Optional<Account> getAccount() {
return Optional.of(account);
}
// other fields, getters, and setters are omitted for simplicity
}
public class Account {
private Double balance;
private Loan loan;
public Optional<Loan> getLoan() {
return Optional.ofNullable(loan);
}
// other fields, getters, and setters are omitted for simplicity
}
public class Loan {
private Double amount;
// other fields, getters, and setters are omitted for simplicity
}
Now, let's rewrite the getLoanAmountOfStudent method using Optional with the updated domains.
public Double getLoanAmountOfStudent(Student student) {
Optional<Student> opStudent = Optional.ofNullable(student);
if (opStudent.isPresent() && opStudent.get().getAccount().isPresent()) {
Account account = opStudent.get().getAccount().get();
if (account.getLoan().isPresent()) {
return account.getLoan().get().getAmount();
}
}
return 0d;
}
This is much better in terms of code quality, at least we're not going to get unexpected NPE. But it still suffers in terms of readability because we're still producing the nested logical conditions unnecessarily. That's the problem of imperative style of programming - easy to implement but hard to read in many cases.
Avoid using isPresent() and get() pairs, they are not elegant
Optional as Monad
Let's think of a declarative way of implementation where you can write programs using functional programming principles. You can think of Optional as a Monad. A monad is a type that wraps another type and gives some form of quality to the underlying type. An Optional Monad does work similar to a monad that wraps a potentially null value, allows to perform some transformations if the value has not become null in between operations and give a way to pull the resultant value. java.lang.Optional provides transformation functions like map, flatMap, and filter (see documentation) similar to Stream APIs to compose a sequence of function calls (a "pipeline") and each steps returns a monadic value which can be fed into the next step in the pipeline.
public Double getLoanAmountOfStudent(Student student) {
return Optional.ofNullable(student)
.flatMap(Student::getAccount)
.flatMap(Account::getLoan)
.map(Loan::getAmount)
.orElse(0d);
}
Voila!, the code is now super readable with less complexity and that's the beauty of Optional as a Monad. But if you notice carefully the code still may throw the NullPointerException if account field of Student object has null reference and that's expected because we wrote the getter as non-null Optional.
But we still have the problem with the return-type of getLoanAmountOfStudent method. As it returns 0 if there's no loan against an account while it should have given the indication of absence of value. You can rewrite the method like below using Optional to clear the intention as:
public Optional<Double> getLoanAmountOfStudent(Student student) {
return Optional.ofNullable(student)
.flatMap(Student::getAccount)
.flatMap(Account::getLoan)
.map(Loan::getAmount)
.or(Optional::empty);
}
Using Optional as method arguments is not recommended as it creates an extra layer of wrapping.
Streaming with Optional
Let's say, we want to find the count of students who have a loan. We can write a method like below for that:
public long countStudentHavingLoan(List<Student> students) {
return students.stream()
.map(Student::getAccount)
.map(acc -> acc.flatMap(Account::getLoan))
.filter(Optional::isPresent)
.map(Optional::get)
.count();
}
See, here we are streaming over the list of students, transforming to account and then extracting loan from each account. The problem is that every account of students doesn't have loans. So in the stream of Stream<Optional<Loan>> we may get an empty optional. To get rid of empty optionals, we have used a filter and map to get non-null optionals. And finally filter and count the number of students having a loan.
From Java 9, the stream() method has been introduced in the Optional class that can be used to transform a Stream of optional elements to a Stream of present value elements. It may seem convenient in this case. See the code below:
public long countStudentHavingLoan(List<Student> students) {
return students.stream()
.map(Student::getAccount)
.map(acc -> acc.flatMap(Account::getLoan))
.flatMap(Optional::stream)
.count();
}
Here, you can see we've used Optional::stream that is converting Stream<Optional<Loan>> to Stream<Loan> directly with a single operation and removing empty Optionals.
Use Optional Lazily while Returning Computed Value
Suppose, you're writing a method to find students by id. For that, at first you try to find in cache and if not found then query to the database and retrieve students, otherwise throw exception. The method look like below:
public Student findStudent(String id) {
return studentCache.getStudent(id)
.orElse(studentService.getStudent(id)
.orElseThrow(() -> new NotFoundException("Student is not found with id" + id))
);
}
The above code will query both cache and database even if it finds the student in the cache. orElse() get called even if the value is present. To avoid call to the database you can use orElseGet(Supplier<? extends T> supplier) that will be evaluated only when the value is empty.
public Student findStudentById(String id) {
return studentCache.getStudent(id)
.orElseGet(() ->
studentService.getStudent(id)
.orElseThrow(() -> new NotFoundException("Student is not found with id" + id))
);
}
From Java 9, Optional has been enhanced with or(Supplier<? extends Optional<? extends T>> supplier) method, which can execute an action and return Optional instead of direct value. So, the above method can be further refactored like below:
public Optional<Student> findStudentById(String id) {
return cacheService.GetStudent(id)
.or(() -> studentServiceGetStudent(id));
}
See here to know more about Optional.
Things to Keep in Mind While Using Optional
- Optional primarily intended to be used as method return type only.
- A variable whose type is Optional should never itself be null; it should always point to an Optional instance.
- Optional should be avoided on field types because of serialization issues, alternatively can be used in getter/setter.
- Client side is responsible for handling the empty Optionals. (Don’t call get() directly)
- Do not overuse Optionals, wrapping values into an extra instance can degrade performance.
Please, don't forget to leave your feedback. Happy coding 😊