Assert Optional Value in Java

In this article, we will learn how to assert Optional in Java using JUnit 5 and AssertJ assertion libraries.

Overview

We usually write a unit test to verify the expected output from a method. When a method returns an Optional object, we have to write test cases to check if the Optional is present, empty or has an expected value. We can do that using Junit 5 and AssertJ libraries.

Assert Optional

Let’s look at quick examples to write test cases for returning Optional Object using Junit 5 and AssertJ libraries.

JUnit 5

JUnit 5 doesn’t provide great support for Optional like AssertJ library though we managed to do all basic Optional assertions in the below examples:-

1. Test that a value is present in Optional:-

   Optional<String> optional = Optional.of("foo");
   assertTrue(optional.isPresent()); // pass
   

2. Test that a value is not present or empty in Optional:-

   Optional<String> optional = Optional.empty();
   assertFalse(optional.isPresent()); // pass
   

3. Test an Optional String:-

   Optional<String> optional = Optional.of("foo");
   assertTrue(optional.isPresent());
   assertEquals("foo", optional.get());
   

4. Test an Optional Object having multiple properties:-

   Optional<User> userOptional = Optional.of(new User("Jack", 21, true));
   assertTrue(userOptional.isPresent());
   userOptional.ifPresent(user -> {
     assertEquals("Jack", user.getFirstName());
     assertTrue(user.getAge() > 18);
     assertTrue(user.getIsPremiumUser());
   });
   

   Note that Junit 5 doesn’t provide any assertion to test multiple properties of an Optional Object so we made use of Optional.ifPresent() to assert all the properties.

Also Read Unit Test with JUnit 5 in Java for more details.

AssertJ

AssertJ has good support for Optional objects and provides many fluent assertions specific to Optional. Let’s look at the below examples:-

1. Test that a value is present in Optional:-

   Optional<String> optional = Optional.of("foo");
   assertThat(optional).isPresent(); // pass
   assertThat(optional).isNotEmpty(); // pass
   

   Note that isPresent() and isNotEmpty() are alias and can be used interchangeably.

2. Test that a value is not present or empty in Optional:-

   Optional<String> optional = Optional.empty();
   assertThat(optional).isEmpty(); // pass
   assertThat(optional).isNotPresent(); // pass
   

   Note that isEmpty() and isNotPresent() are alias and anyone can be used interchangeably.

3. Test an Optional String:-

   Optional<String> optional = Optional.of("foo");
   assertThat(optional)
           .isPresent()
           .isNotEmpty()
           .containsInstanceOf(String.class)
           .hasValue("foo")
           .contains("foo");
   

   Note that hasValue() and contains() are alias and anyone can be used interchangeably.

4. Test an Optional Object having multiple properties:-

   Optional<User> userOptional = Optional.of(new User("Jack", 21, true));
   assertThat(userOptional)
           .isPresent()
           .isNotEmpty()
           .containsInstanceOf(User.class)
           .hasValueSatisfying(user -> {
               assertThat(user.getFirstName()).isEqualTo("Jack");
               assertThat(user.getAge()).isGreaterThan(18);
               assertThat(user.getIsPremiumUser()).isTrue();
           });
   

   We have tested all the properties of an Optional Object using hasValueSatisfying() in the above example.

Also Read Unit Test with AssertJ in Java for more details.