Hamcrest is a framework for writing matcher objects allowing 'match' rules to be defined declaratively. There are a number of situations where matchers are invaluble, such as UI validation, or data filtering, but it is in the area of writing flexible tests that matchers are most commonly used. This tutorial shows you how to use Hamcrest for unit testing.

When writing tests it is sometimes difficult to get the balance right between overspecifying the test (and making it brittle to changes), and not specifying enough (making the test less valuable since it continues to pass even when the thing being tested is broken). Having a tool that allows you to pick out precisely the aspect under test and describe the values it should have, to a controlled level of precision, helps greatly in writing tests that are "just right". Such tests fail when the behaviour of the aspect under test deviates from the expected behaviour, yet continue to pass when minor, unrelated changes to the behaviour are made.

We'll start be writing a very simple JUnit 3 test, but instead of using JUnit's assertEquals methods, we use Hamcrest's assertThat construct and the standard set of matchers, both of which we statically import:

import static org.hamcrest.MatcherAssert.assertThat;

import static org.hamcrest.Matchers.*;



import junit.framework.TestCase;



public class BiscuitTest extends TestCase {

  public void testEquals() {

    Biscuit theBiscuit = new Biscuit("Ginger");

    Biscuit myBiscuit = new Biscuit("Ginger");

    assertThat(theBiscuit, equalTo(myBiscuit));

  }

}

The assertThat method is a stylized sentence for making a test assertion. In this example, the subject of the assertion is the object biscuit that is the first method parameter. The second method parameter is a matcher for Biscuit objects, here a matcher that checks one object is equal to another using the Object equals method. The test passes since the Biscuit class defines an equals method.

If you have more than one assertion in your test you can include an identifier for the tested value in the assertion:

assertThat("chocolate chips", theBiscuit.getChocolateChipCount(), equalTo(10));

assertThat("hazelnuts", theBiscuit.getHazelnutCount(), equalTo(3));

Hamcrest comes with a library of useful matchers. Here are some of the most important ones.

• Core
  • anything - always matches, useful if you don't care what the object under test is
  • describedAs - decorator to adding custom failure description
  • is - decorator to improve readability - see "Sugar", below
• Logical
  • allOf - matches if all matchers match, short circuits (like Java &&)
  • anyOf - matches if any matchers match, short circuits (like Java ||)
  • not - matches if the wrapped matcher doesn't match and vice versa
• Object
  • equalTo - test object equality using Object.equals
  • hasToString - test Object.toString
  • instanceOf, isCompatibleType - test type
  • notNullValue, nullValue - test for null
  • sameInstance - test object identity
• Beans
  • hasProperty - test JavaBeans properties
• Collections
  • array - test an array's elements against an array of matchers
  • hasEntry, hasKey, hasValue - test a map contains an entry, key or value
  • hasItem, hasItems - test a collection contains elements
  • hasItemInArray - test an array contains an element
• Number
  • closeTo - test floating point values are close to a given value
  • greaterThan, greaterThanOrEqualTo, lessThan, lessThanOrEqualTo - test ordering
• Text
  • equalToIgnoringCase - test string equality ignoring case
  • equalToIgnoringWhiteSpace - test string equality ignoring differences in runs of whitespace
  • containsString, endsWith, startsWith - test string matching

Hamcrest strives to make your tests as readable as possible. For example, the is matcher is a wrapper that doesn't add any extra behavior to the underlying matcher. The following assertions are all equivalent:

assertThat(theBiscuit, equalTo(myBiscuit));

assertThat(theBiscuit, is(equalTo(myBiscuit)));

assertThat(theBiscuit, is(myBiscuit));

The last form is allowed since is(T value) is overloaded to return is(equalTo(value)).

Hamcrest comes bundled with lots of useful matchers, but you'll probably find that you need to create your own from time to time to fit your testing needs. This commonly occurs when you find a fragment of code that tests the same set of properties over and over again (and in different tests), and you want to bundle the fragment into a single assertion. By writing your own matcher you'll elimate code duplication and make your tests more readable!

Let's write our own matcher for testing if a double value has the value NaN (not a number). This is the test we want to write:

public void testSquareRootOfMinusOneIsNotANumber() {

  assertThat(Math.sqrt(-1), is(notANumber()));

}

And here's the implementation:

package org.hamcrest.examples.tutorial;



import org.hamcrest.Description;

import org.hamcrest.Factory;

import org.hamcrest.Matcher;

import org.hamcrest.TypeSafeMatcher;



public class IsNotANumber extends TypeSafeMatcher<Double> {



  @Override

  public boolean matchesSafely(Double number) {

    return number.isNaN();

  }



  public void describeTo(Description description) {

    description.appendText("not a number");

  }



  @Factory

  public static <T> Matcher<Double> notANumber() {

    return new IsNotANumber();

  }



}

The assertThat method is a generic method which takes a Matcher parameterized by the type of the subject of the assertion. We are asserting things about Double values, so we know that we need a Matcher<Double>. For our Matcher implementation it is most convenient to subclass TypeSafeMatcher, which does the cast to a Double for us. We need only implement the matchesSafely method - which simply checks to see if the Double is NaN - and the describeTo method - which is used to produce a failure message when a test fails. Here's an example of how the failure message looks:

assertThat(1.0, is(notANumber()));

fails with the message

java.lang.AssertionError: 

Expected: is not a number

    got : <1.0>

The third method in our matcher is a convenience factory method. We statically import this method to use the matcher in our test:

import static org.hamcrest.MatcherAssert.assertThat;

import static org.hamcrest.Matchers.*;



import static org.hamcrest.examples.tutorial.IsNotANumber.notANumber;



import junit.framework.TestCase;



public class NumberTest extends TestCase {



  public void testSquareRootOfMinusOneIsNotANumber() {

    assertThat(Math.sqrt(-1), is(notANumber()));

  }

}