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Assertions are a key element of all tests. The assertion are provided by the TestSuite interface that should be implemented by all PowerUnit Test. In general, with PowerUnit, all assertions method name start with assert.

Assertion on Object

In a most of case, assertion is done on some object. This is done by using the method assertThat method.


These assertion start by passing the object that must be check as parameter of the assertThat method. These methods can take an additionnal parameter (first one) that is a message to be used in case of failure. This method return a DSL object providing the various method to check the object. The more generic DSL method that can be used is the is one, which can be used to pass an hamcrest matcher or directly a value to be used for comparing (equality).

In case the object to be checked is a String, special assertion (like startsWith) can also be used.

Also, as a first step in the validation, the as method can be used to convert the received value into a sub class of the received one. For instance, if the assertion receive a Number, the as intermediate method can be used to transform this number into an Integer, if and only if the received instance is really an Integer.

Assertion on Iterable

In some case, the object to be check is an Iterable one. In this case, the method assertThatIterable can be used.

assertThatIterable(myIterable).contains(true, false);

In this case, the same DSL that for Object is available, but additionnal DSL method (like contains) can be used to validate the iterable.

Assertion on Function

In Java 8, the concept of Function is available and dedicated assertion integration is usefull. A Function is a piece of code that receive one parameter and return a result. In the context of assertion, the method assertThatFunction can be used to assert on the result of the execution of a Function, receiving some input.

assertThatFunction(Fibo::fibo, x).is(y);

The assertThatFunction take two parameters : the function itself and the value to be passed to the function. This method return a DSL object (same as for assertion on object) that will be used on the result of the execution of the function.

Also, a BiFunction concept exists in Java 8 (two parameters, one result). An assertThatBiFunction is also available.

Exception assertion

Finally, outside of validating some value, it can also be usefull to that on the outcome of the execution of a piece of code. PowerUnit provides a way to validate that a piece of code (a lambda), will raise an exception.

assertWhen((p) -> {throw new Throwable("tst");}).throwException(exceptionMessage("tst"));

The method assertWhen method receive a lambda (a one-argument method, void return), that can thrown exception. This method return DSL object that can be used to validate the thrown exception (using Matchers for instance) ; In case no-exception is thrown, the test is failed.