Assert that Optional has certain value

Question

I have a Java method that returns an Optional. I'd like to write an easy-to-read unit test for it that asserts that

1. the returned Optional has a value (i.e., the Optional is not empty) and that

2. the returned value is equal to an expected value.

Let's say my tested method is

Optional<String> testedMethod(){
  return Optional.of("actual value");
}

Solution 1

You can also use AssertJ for fluent assertions

@Test
public void testThatOptionalIsNotEmpty() {
    assertThat(testedMethod()).isNotEmpty();
}

@Test
public void testThatOptionalHasValue() {
    assertThat(testedMethod()).hasValue("hello");
}

Solution 2

TL;DR the best overall approach was suggested by Ole V. V.:

assertEquals(Optional.of("expected"), opt);

Other alternatives are discussed below.

There are several different ways to do this, depending on your taste for clarity of test results vs. conciseness of test writing. For this answer I'll stick to "stock" Java 8 and JUnit 4 with no additional dependencies.

One way, as suggested in a comment by Ole V.V., is simply to write

assertEquals("expected", opt.get());

This mostly works but if the Optional is empty, then get() will throw NoSuchElementException. This in turn causes JUnit to signal an error instead of a failure, which might not be what you want. It's also not very clear what's going on unless you already know that get() throws NSEE in this case.

An alternative is

assertTrue(opt.isPresent() && "expected".equals(opt.get()));

This also mostly works but it doesn't report the actual value if there's a mismatch, which might make debugging inconvenient.

Another alternative is

assertEquals("expected", opt.orElseThrow(AssertionFailedError::new));

This gives the right failures and reports the actual value when there's a mismatch, but it's not very explicit about why AssertionFailedError is thrown. You might have to stare at it a while until you realize that AFE gets thrown when the Optional is empty.

Still another alternative is

assertEquals("expected", opt.orElseThrow(() -> new AssertionFailedError("empty")));

but this is starting to get verbose.

You could split this into two assertions,

assertTrue(opt.isPresent());
assertEquals("expected", opt.get());

but you had previously objected to this suggestion because of verbosity. This isn't really terribly verbose in my opinion, but it does have some cognitive overhead since there are two separate assertions, and it relies on the second only being checked if the first succeeds. This is not incorrect but it is a bit subtle.

Finally, if you're willing to create a bit of your own infrastructure, you could create a suitably-named subclass of AssertionFailedError and use it like this:

assertEquals("expected", opt.orElseThrow(UnexpectedEmptyOptional::new));

Finally, in another comment, Ole V. V. suggested

assertEquals(Optional.of("correct"), opt);

This works quite well, and in fact this might be the best of all.

Solution 3

I use Hamcrest Optional for that:

import static com.github.npathai.hamcrestopt.OptionalMatchers.hasValue;
import org.junit.Test;

public class MyUnitTests {

  @Test
  public void testThatOptionalHasValue(){
    String expectedValue = "actual value";
    assertThat(testedMethod(), hasValue(expectedValue));
  }
}

You can add Hamcrest Optional to your dependencies by including it in your build.gradle:

dependencies {
  testCompile 'junit:junit:4.12'
  testCompile 'com.github.npathai:hamcrest-optional:1.0'
}

Solution 4

Why don’t you use isPresent() and get()?

Solution 5

The below approach uses the fact that you can specify a default return for an optional. So your test method could be something like this:

@test
public void testThatOptionalHasValue() {
    String expectedValue = "actual value";
    String actualValue = Optional.ofNullable(testedMethod()).orElse("not " + expectedValue);
    assertEquals("The values are not the same", expectedValue, actualValue);
}

This guarentees that if your method returns null, then the result can not be the same as the expected value.

View more solutions

Comments

[Ole V.V.]

I’d simply use assertEquals("actual value", testedMethod().get()); It will fail with an exception if the optional is empty, which is enough for your unit test to fail,so you really need nothing more.

[Matthias Braun]

That's two assertions and results in a quite verbose unit test. I was looking for something terser.

[Ole V.V.]

If you accept null as another conventional representation of the absence of a value, there is also assertEquals("correct", opt.orElse(null));. It’s not my personal preference, but should it be yours, go ahead.

[Ole V.V.]

Thinking again, there’s assertEquals(Option‌​al.of("corre‌​ct"), opt); too. It’s terse, and while it took me a while to think of writing it, it’s OK to read, isn’t it? I would also expect it to give nice, clear messages on failures, both in the case where opt holds the wrong value and when it’s empty.

[shmosel]

I think the last way is definitely the clearest way and most "correct" approach. Consider how you would test the contents of a list or map. No reason this should be any different.

[clausavram]

I think it would be nicer if the terse last update from Ole V. V. got moved to the top of the answer. It's the most important part and shouldn't be last in such a long answer

[borjab]

@MatthiasBraun There is nothing bad about having two assertions if they test a single concept. I agree that it is not very terse

[Marcos]

If you try to use "get()" without a "isPresent()" check, the IDE will throw a warning which is not good for me