WRITING CUSTOM HAMCREST MATCHERS

With FeatureMatcher we can wrap an existing Matcher, decide which field of the given Object under test to match and provide a nice error message. For details have a look at the examples that come with the full Hamcrest distribution. Using Hamcrest String matchers 4. Overview of Hamcrest mather The following are the most important Hamcrest matchers: Hamcrest resources Hamcrest homepage. Therefore you should make sure your matcher is stateless, so a single instance can be reused between matches. Lars Vogel, c , vogella GmbH Version 1.

Writing New Matchers jMock and Hamcrest 1 provide many Matcher classes and factory functions that let you specify the acceptable parameter values of a method invocation 2. This means that it will call the matchers many times during the test, maybe even after the expectation has already been matched and invoked. You can also group them together with Hamcrest matchers. Validate Use your custom matcher to check that “Gandalf” has a lenght of 8. The assertThat method is a generic method which takes a Matcher parameterized by the type of the subject of the assertion.

Hamcrest strives to make your tests as readable as possible.

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We want to use the class FeatureMatcher. Combining matchers is supported by Hamcrest out of the box but it has the limitation that the error is hard to read:.

If Todo is constructed with the summary “Learn Hamcrest” that cuustom summary property was initialized with this value. Therefore you should make sure your matcher is stateless, so a single instance can be reused between matches. Target Assume the following code: When writing tests it is sometimes difficult to get the balance right between overspecifying the test and making it brittle to changesand not specifying enough making the test less valuable since it continues to pass even when the thing being tested is broken.

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It does two things:. The assertThat method is a generic method which takes a Matcher parameterized by the type of the subject of the cstom. A Matcher is an object that implements the org.

Hamcrest is a framework for software tests.

writing custom hamcrest matchers

Hamcrest has the target to make tests as readable as possible. The following expectation specifies that the error method of the logger object must be called once with an argument that is a string starting with “FATAL”.

writing custom hamcrest matchers

The following snippets compare pure JUnit 4 assert statements with Hamcrest matchers. Hamcrest resources Hamcrest homepage.

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Example The usage of Hamcrest matchers is demonstrates by the following code snippet. You can adjust this output in the describeMismatch method. Hamcrest allows hakcrest for conditions in your code via existing matchers classes. However, sometimes the predefined constraints do not let you specify an expectation accurately enough to convey what you mean or to keep your tests flexible.

To create a new matcher: We want to write our own MatcherCombiner that provides us with a readable error message, even when multiple katchers fail. The JUnit distribution included in Eclipse only contain the core Hamcrest matcher. Even though the notANumber method creates a new matcher each time it is called, you should not assume this is the only usage pattern for your matcher.

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It provides type hamcrst by default. To make all matchers available in your file add an static import. For details have a look at the examples that come with the full Hamcrest distribution.

See Wiki on Hamcrest for more details. The matcher we want to wrap a description of the feature that we tested a description of the possible mismatch. Hamcrest has been designed from the outset to integrate with different unit testing frameworks.

The constructor of FeatureMatcher takes the following arguments in this order:. Using Hamcrest matchers cusstom provides more type safety as these matchers use generics.

The last form is allowed matcher is T value is overloaded to return is equalTo value. To use Hamcrest matchers in JUnit you use the assertThat statement followed by one or several matchers. It does two things: If you want to maintain state in response to invocations, use an Action 7not a Matcher.

Create Hamcrest Matchers Define a custom matcher for Hamcrest which provides the length matcher for a String. This commonly occurs when you find a fragment of code that tests the same set of properties over and over again and in different testsand you want wfiting bundle the fragment into a single assertion.