# Single Statement Unit Tests

Source: https://www.yegor256.com/2017/05/17/single-statement-unit-tests.html

Many articles and books have already been written about unit testing patterns
and anti-patterns. I want to add one more recommendation which, I believe,
can help us make our tests, _and_ our production code, more _object-oriented_.
Here it is: a test method must contain nothing but a single `assert`.


{% jb_picture_body %}

Look at this test method from `RandomStreamTest` from OpenJDK 8,
created by Brian Goetz:

```java
@Test
public void testIntStream() {
  final long seed = System.currentTimeMillis();
  final Random r1 = new Random(seed);
  final int[] a = new int[SIZE];
  for (int i=0; i < SIZE; i++) {
    a[i] = r1.nextInt();
  }
  final Random r2 = new Random(seed);
  final int[] b = r2.ints().limit(SIZE).toArray();
  assertEquals(a, b);
}
```

{% youtube 1bAixLaOCSA %}

There are two parts in this method: the algorithm and the assertion. The
algorithm prepares two arrays of integers and the assertion compares them
and throws `AssertionError` if they are not equal.

I'm saying that the first part, the algorithm, is the one we should try
to avoid. The only thing we must have is the assertion. Here is
how I would re-design this test method:

```java
@Test
public void testIntStream() {
  final long seed = System.currentTimeMillis();
  assertEquals(
    new ArrayFromRandom(
      new Random(seed)
    ).toArray(SIZE),
    new Random(seed).ints().limit(SIZE).toArray()
  );
}
private static class ArrayFromRandom {
  private final Random random;
  ArrayFromRandom(Random r) {
    this.random = r;
  }
  int[] toArray(int s) {
    final int[] a = new int[s];
    for (int i=0; i < s; i++) {
      a[i] = this.random.nextInt();
    }
    return a;
  }
}
```

If Java had [monikers]({% pst 2017/may/2017-05-16-monikers %})
this code would look even more elegant:

```java
@Test
public void testIntStream() {
  assertEquals(
    new ArrayFromRandom(
      new Random(System.currentTimeMillis() as seed)
    ).toArray(SIZE),
    new Random(seed).ints().limit(SIZE).toArray()
  );
}
```

As you can see, there is only one "statement" in this method: `assertEquals()`.

[Hamcrest](http://hamcrest.org/) with its
[`assertThat()`](http://hamcrest.org/JavaHamcrest/javadoc/2.0.0.0/org/hamcrest/MatcherAssert.html)
and its
[collection](http://hamcrest.org/JavaHamcrest/javadoc/2.0.0.0/allclasses-frame.html)
of basic matchers is a perfect instrument to make our
single-statement test methods even more cohesive and readable.

{% youtube 1QTfi9kLzrM %}

There are a number of practical benefits of this principle, if we
agree to follow it:

  * **Reusability**. The classes we will have to create for test
    assertions will be reusable in other test methods and test cases.
    Just as, in the example above, `ArrayFromRandom` could be used
    somewhere else. Similarly, Hamcrest matchers may and will constitute
    a library of reusable test components.

  * **Brevity**. Since it will be rather difficult to create a long
    test method when it only has a single `assert`, you and your fellow programmers
    will inevitably write shorter and more readable code.

  * **Readability**. With a single `assert` it will always be obvious
    what the intent of the test method is. It will start with the intent
    declaration while all other lower level details will be indented.

  * **Immutability**. It will be almost impossible to have
    [setters]({% pst 2014/sep/2014-09-16-getters-and-setters-are-evil %}) in the
    production code if test methods have no place for algorithmic code. You
    inevitably will create
    [immutable objects]({% pst 2014/jun/2014-06-09-objects-should-be-immutable %})
    to make them testable with a single `assert`.

The biggest benefit we get when this principle is applied to our tests
is that they become declarative and object-oriented, instead of being
algorithmic, imperative, and procedural.