Jasmine-style expectations for Java
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pom.xml [maven-release-plugin] prepare for next development iteration Feb 26, 2013



Jasmine-style expectations for Java


JUnit 4's assertThat() is kinda awesome and kinda sucks. It's awesome cuz it encourages you to write your own matchers. It sucks cuz it makes it really painful to write your own matchers. And it makes it really hard to find existing matchers.

great-expectations makes it dead-simple to find existing relevant matchers using autocomplete in your favorite IDE, and it makes it very easy to declare your own type-safe matchers.

Quick Start

  1. Download great-expectations. If you're not using Maven or something like it for dependency management, don't forget to grab asm and asm-commons too, and put them all in your test classpath. (And think about using Maven.)
  2. Write your first expectation in a test:


Writing asserts:

Here's a JUnit assertion:

import static org.hamcrest.CoreMatchers.not;
import static org.junit.Assert.assertThat;
import static org.junit.matchers.JUnitMatchers.containsString;

@Test public void testMyAttitude() {
  assertThat("team", not(containsString("me")));

Here's the equivalent great-expectations assertion:

import static com.example.Expect.expect;

@Test public void testMyAttitude() {

Not a huge difference, but note that there's just a single import; once you get "expect" imported, everything else is autocompletable.

Writing matchers:

Here's how the matcher looks:

public class StringMatcher<T extends String, M extends StringMatcher<T, M>> extends ObjectMatcher<T, M> {
  public boolean toContain(String expected) {
    return actual.indexOf(expected) != -1;

Create a class with a generic signature from hell. Then just create a method which returns true or false. Done. Boom.

Custom failure messages from your matchers

If you don't like the default failure message that you see in your red tests, you can customize the message right inside the implementation of each matcher method by assigning strings to variables called descriptionOfActual and/or descriptionOfExpected. Remember that your test can fail even when your matcher returns true if the test used not to invert your matcher, so if you want to customize the failure message go ahead and do it unconditionally.

Or, if you'd like to customize the entire message, assign a string to the variable called failureMessage. You should do so unconditionally. It will automatically get prefixed with either "Failure: Expected" or "Failure: Did not expect" depending on whether the test used not to invert your matcher.

Generating Expect.java

If you want to include your own custom matchers, we need to generate some java glue code too. Bummer. Use ExpectGenerator to spew out your Expect class. You can add your own matchers by extending ExpectGenerator.matcherClasses() and adding to the list.

We do this so you'll only ever have to import a single Expect class in any test.


great-expectations is available through maven:


or download directly:



0.14 (2013-02-25)

  • Updated Expect class and Expect example to work with Java 1.7 [Thanks Chris Heisterkamp!]
  • Added Expect method for long primitive.

0.13 (2012-04-19)

  • Added matchers: toContain, toContainExactly, toBeEmpty, and toNumber for Sets, toNumber for Iterables.
  • Added default Expect class for quick and easy initial setup.

0.12 (2011-11-23)

  • Added matchers: toStartWith and toEndWith for Strings, toContainExactlyInAnyOrder for Iterables, and nicer handling of Long comparisons. [Thanks Ryan Richard @ Pivotal Labs!]

0.11 (2011-11-08)

  • Iterables can now contain nulls. [Thanks Pascal and Taylor @ Square!]

0.10 (2011-06-06)

  • Matchers can override the default messages returned when an expectation fails. [Thanks Ryan Richard @ Pivotal Labs!]

0.9 (2011-04-14)

  • Added String toMatch(), which matches against regular expressions.
  • Iterable's toContain() now doesn't care about order. Added toContainInOrder().
  • Fixes compilation errors and warnings.

0.8 (2011-04-10)

  • Initial packaged release.


  • Thanks to Tyler Schultz @ Pivotal Labs for the name!
  • Thanks to Phil Goodwin @ Pivotal Labs for his expertise navigating the darkest corners of Java generics syntax!

great-expectations was written by Christian Williams and released under the MIT License.