Since Java 16 we can use the method mapMulti(BiConsumer) of the Stream API. This method allows us to map each element of the stream to multiple elements. We can also do that with the flatMap(Function) method, but if we want to map a limited set of elements, mapMulti is more convenient. Internally a shared stream is used and we don’t have the cost of creating a new stream for each element. Another use case is if the logic to map an element to multiple elements is complex and is hard to implement by returning a stream. Then mapMulti allows us to write that logic in a BiConsumer instead of a Function.

For a lot of types AssertJ has special assertion methods. Also for the type Optional. If we want to assert an Optional value we can use several methods that AssertJ provides. For example to check if an Optional is present we can use isPresent() or the alias isNotEmpty(). To check if the Optional is empty we can use isEmpty() or the alias isNotPresent(). Checking the value of an Optional (if it is indeed set) can be done with hasValue() or contains(). For more fine grained assertions on the value we can use hasValueSatisfying(Condition) or hasValueSatisfying(Consumer). With the map(Function) and flatMap(Function) methods we can map the Optional, if not empty, to another value and assert that value.

AssertJ has a lot of custom assertion methods for different types. For example to assert an URL object AssertJ gives us some specific methods. We can check for different components of the URL instance with different methods. For example we can check if the protocol is equal to the protocol we expect with hasProtocol(String). Similarly we can write assertions for the host, port, authority, path and anchor. To assert query parameters we can use hasQueryParameter(String) to check if query parameter is set and with hasQueryParameter(String, String) we can check if the query parameter has an expected value. To check the whole query string we can use hasQueryString(String).

Each of the assertion methods also has version to assert a component is not present. For example hasNoQuery() to assert a query is not defined for an URL.

AssertJ has some nice methods to verify string values. If we want to verify a string value is Base64 encoded we can use the isBase64String() method. We can leave out the padding of the value as it is optional. With the method asBase64Decoded() we can decode the value and write our assertions for the decoded value. The method asBase64Decoded() returns a byte[] object and we can use the asString() to convert it into a string value again.

To compare string values we can use the isEqualTo(String) method in AssertJ. But if we want to verify that a string contains a certain variable value we can use string templates. This makes the assertion more readable as we can see what value we expect in the string. To use string templates we must the method isEqualTo(String, Object…​). The first argument is the string template and the following arguments will be the actual values that should be used in the template. Actually the String.format(String, Object…​) method is used behind the scenes to format the string template, but we don’t have to clutter our assertions with that call.

With the returns method in AssertJ we can verify an object using a function. This allows us to verify an object in a very flexible way. We can chain multiple returns method calls to verify multiple aspects of our object. The first argument of the returns method is the expected value of the function call. And the second argument is a function that calls a method on the object we want to verify. A simple function call would be a method reference using the class of the object. But we can also write our own function, where the argument of the function is actual object we are writing the assertion for. To verify the function doesn’t return an expected value we can use the method doesNotReturn.
We can also pass the function to the from method, available in the Assertions class. It can make the assertion more readeable as we can now read the code as: we expect the following value from calling this function.

Writing assertions using the nice fluent API of AssertJ is a joy. Besides some of the basic assertions like isEqualTo AssertJ also has specific assertions for specific types. For example if we want write an assertion to check if a String value starts or ends with an expected value we can use the startsWith(String) or endsWith(String) methods. If we don’t care that a character is upper or lower case we can also use startsWithIgnoringCase(String) or endsWithIgnoringCase(String). Each of the methods also has a counterpart method to check the String value doesn’t start or end with an expected value. For example we can use doesNotStartWith(String) to assert a value does not start with the expected value.

AssertJ has many useful methods to write assertions using a fluid API. If we want to test the toString() method implementation of an object we can of course invoke the toString() method inside an assertThat expression and check with the assert method isEqualTo(String) the value. But AssertJ can make that easier so we don’t have to invoke toString() method ourselves. We can use the assert method hasToString(String) on the object we defined in the assertThat expression and specify our expected value as argument to the method. If we want to assert the toString() method doesn’t return an expected value we can use the assert method doesNotHaveToString(String).

GINQ (Groovy-INtegerate Query) is part of Groovy since version 4. With GINQ we can use SQL-like queries to work with in-memory data collections. If we want to sort the data we can use orderby followed by the property of the data we want to sort just like in SQL we can use order by. By default the sort ordering is ascending and null values are put last. We can change the sort ordering by specifying in desc with the orderby clause. Or to make the ascending order explicitly we use the statement in asc. Each of asc and desc also can take an argument to specify how we want null values to be sorted. The default way is to keep null values last in the ordering. If we want to make this explicit we use nullslast as argument to asc or desc. To have null values in the sorted result first we use the argument nullsfirst.

Since Groovy 4 we can use SQL like queries on in-memory collections with GINQ (Groovy-Integrated Query). GINQ provides some built-in aggregate functions like min, max, sum and others. One of these functions is median. With median we can get the value that is in the middle of the sorted list of values we want to calculate the median for. If the list has an uneven number of elements the element in the middle is returned, but if the list has an even number of elements the average of the two numbers in the middle is returned.

In the following example we see the use of the median function with GINQ:

Groovy supports a tuple type. A tuple is an immutable object to store elements of potentially different types. In Groovy there is a separate Tuple class based on how many elements we want to store in the tuple. The range starts at Tuple0 and ends with Tuple16. So we can store a maximum of 16 elements in a Groovy tuple.
Each of the classes has a constructor that takes all elements we want to store. But the Tuple class also has static factory methods to create those classes. We can use the tuple method and based on how many elements we provide to this method we get the corresponding Tuple object.

In a previous blog post we learned how to use DataVariable and DataVariables to test asynchronous code. Spock also provides PollingConditions as a way to test asynchronous code. The PollingConditions class has the methods eventually and within that accept a closure where we can write our assertions on the results of the asynchronous code execution. Spock will try to evaluate conditions in the closure until they are true. By default the eventually method will retry for 1 second with a delay of 0.1 second between each retry. We can change this by setting the properties timeout, delay, initialDelay and factor of the PollingConditions class. For example to define the maximum retry period of 5 seconds and change the delay between retries to 0.5 seconds we create the following instance: new PollingConditions(timeout: 5, initialDelay: 0.5).
Instead of changing the PollingConditions properties for extending the timeout we can also use the method within and specify the timeout in seconds as the first argument. If the conditions can be evaluated correctly before the timeout has expired then the feature method of our specification will also finish earlier. The timeout is only the maximum time we want our feature method to run.