The document discusses object-oriented approaches to writing JUnit tests. It proposes that test methods should contain a single assertion statement and use immutable objects and matchers instead of procedural setup and teardown code. This reduces complexity, avoids issues with mutable state and mocking, and leads to better test and application design. Examples from OpenJDK and Cactoos libraries are presented to illustrate the approach.

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Yegor Bugayenko
Object-Oriented Flavor for
JUnit Tests

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F.I.R.S.T.:
Fast
Isolated/Independent
Repeatable
Self-validating
Thorough and Timely
— Robert Martin
Clean Code

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There are books:
Kent Beck, Test-Driven Development by Example, 2000.
Johannes Link, Unit testing in Java, 2003.
Ted Husted and Vincent Massol, JUnit in Action, 2003.
Andy Hunt and David Thomas, Pragmatic Unit Testing in C# with NUnit, 2003.
Gerard Meszaros, XUnit Test Patterns: Refactoring Test Code, 2007.
Nat Pryce and Steve Freeman, Growing Object-Oriented Software: Guided by Tests, 2009.
Lasse Koskela, Effective Unit Testing: A Guide for Java Developers, 2013.
J. B. Rainsberger, JUnit recipes, 2014.
Sujoy Acharya, Mastering Unit Testing Using Mockito and JUnit, 2014.

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Unit testing anti-patterns:
Happy Path Tests
Validation and Boundary
Easy Tests
The Giant
The Cuckoo
The Conjoined Twins
The Slow Poke
Anal Probe
Test It All
Line Hitter
Wait and See
The Silent Catcher
Chain Gang
The Mockery
The Free Ride
The Local Hero
Wet Floor
The Flickering Test
The Environmental Vandal
Second Class Citizens
The Secret Catcher
Logic in Tests
Code Matching
Misleading Tests
Not Asserting
Asserting on Not-Null

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I believe that one anti-pattern
is still missing:
The algorithm
Provided we’re talking about OOP

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xUnit test method is
basically a procedure.

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class BookTest {
@Test
void testWorksAsExpected() {
Book book = new Book();
book.setLanguage(Locale.RUSSIAN);
book.setEncoding(“UTF-8”);
book.setTitle(“Дон Кихот”);
assertTrue(book.getURL().contains(“%D0%94%D0%BE%D0”));
}
}
1.Algorithm
2.Output
3.Assertion

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A test method must have nothing
but a single statement:
assertThat()
Provided we’re talking about Java

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class BookTest {
@Test
void testWorksAsExpected() {
// nothing goes here
assertThat(
new Book(“Дон Кихот”, “UTF-8”, Locale.RUSSIAN),
new HasURL(new StringContains(“%D0%94%D0%BE%D0”))
);
}
}

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Assert that the book is similar
to a book that has a URL that
contains a string that is equal to
“%D0%94”.
assertThat(
new Book(“Дон Кихот”, “UTF-8”, Locale.RUSSIAN),
new HasURL(new StringContains(“%D0%94%”))
);

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Hamcrest
By the way, this is the anagram for “matchers”.

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Some of anti-patterns will just
disappear:
Happy Path Tests
Validation and Boundary
Easy Tests
The Giant
The Cuckoo
The Conjoined Twins
The Slow Poke
Anal Probe
Test It All
Line Hitter
Wait and See
The Silent Catcher
Chain Gang
The Mockery
The Free Ride
The Local Hero
Wet Floor
The Flickering Test
The Environmental Vandal
Second Class Citizens
The Secret Catcher
Logic in Tests
Code Matching
Misleading Tests
Not Asserting
Asserting on Not-Null

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Obvious benefits:
Reduced complexity
Immutable objects
Shared “matchers”
Fake objects and no mocking
Better design (more object-oriented)

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Reduced Complexity
14
void testWorksAsExpected() {
assertThat(
new Book(“Дон Кихот”, “UTF-8”, Locale.RUSSIAN),
new HasURL(new StringContains(“%D0%94%D0%BE%D0”))
);
}
No free ride
No multi asserts
No @Before/@After

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Immutable Objects
15
void testWorksAsExpected() {
assertThat(
new Book(“Дон Кихот”, “UTF-8”, Locale.RUSSIAN),
new HasURL(new StringContains(“%D0%94%D0%BE%D0”))
);
}
No setters
No temporal coupling
No procedures

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Shared Matchers
16
void testWorksAsExpected() {
assertThat(
new Book(“Дон Кихот”, “UTF-8”, Locale.RUSSIAN),
new HasURL(new StringContains(“%D0%94%D0%BE%D0”))
);
}
No private static methods
No duplication
Cleaner logs

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No Mocking
17
void testWorksAsExpected() {
assertThat(
new Book(“Дон Кихот”, “UTF-8”, Locale.RUSSIAN),
new HasURL(new StringContains(“%D0%94%D0%BE%D0”))
);
}
No assumptions
No false positives

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class BookTest {
@Test
void testWorksAsExpected() {
Book b = new Book(“Дон Кихот”, “UTF-8”, Locale.RUSSIAN);
Matcher m = new HasURL(new StringContains(“%D0%94%D0%BE%D0”));
assertThat(b, m);
}
}
1.Object
2.Matcher
3.Assertion

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OpenJDK
RandomStreamTest.java
java.util.Random
for

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@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);
}

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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;
}
}
@Test
public void testIntStream() {
final long seed = System.currentTimeMillis();
assertEquals(
new ArrayFromRandom(
new Random(seed)
).toArray(SIZE),
new Random(seed).ints().limit(SIZE).toArray()
);
}

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

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cactoos
www.cactoos.org

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new Pipe(
new TextAsInput(“Hello, world!”),
new FileAsOutput(new File(“test.txt”))
).push();

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@Test
public void canCopyTextToByteArray() {
ByteArrayOutputStream baos = new ByteArrayOutputStream();
new Pipe(
new TextAsInput(“Hello, world!”),
new OutputStreamAsOutput(baos)
).push();
assertThat(
new String(baos.toByteArray()),
Matchers.containsString(“Hello”)
);
}

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Input input = new TeeInput(
new TextAsInput(“Hello, world!”),
new FileAsOutput(new File(“test.txt”))
);
input.read();
new LengthOfInput(input).asValue();
6168af9

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@Test
public void canCopyTextToByteArray() {
ByteArrayOutputStream baos = new ByteArrayOutputStream();
assertThat(
new InputAsText(
new TeeInput(
new TextAsInput(“Hello, world!”),
new OutputStreamAsOutput(baos)
)
),
Matchers.allOf(
equalTo(new String(baos.toByteArray())),
containsString(“Hello”)
)
);
}

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@Test
public void canCopyTextToByteArray() {
ByteArrayOutputStream baos;
assertThat(
new InputAsText(
new TeeInput(
new TextAsInput(“Hello, world!”),
new OutputStreamAsOutput(baos = new ByteArrayOutputStream())
)
),
Matchers.allOf(
equalTo(new String(baos.toByteArray())),
containsString(“Hello”)
)
);
}

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@Test
public void canCopyTextToByteArray() {
assertThat(
new InputAsText(
new TeeInput(
new TextAsInput(“Hello, world!”),
new OutputStreamAsOutput(
new ByteArrayOutputStream() as baos
)
)
),
Matchers.allOf(
equalTo(new String(baos.toByteArray())),
containsString(“Hello”)
)
);
}

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The article on the blog:

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