Home » Author Archives: Tomasz Nurkiewicz (page 2)

Author Archives: Tomasz Nurkiewicz

Java EE developer, Scala enthusiast. Enjoying data analysis and visualization. Strongly believes in the power of testing and automation.

Property-based testing with ScalaCheck – custom generators

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One of the core data structures provided by Hazelcast is IMap<K, V> extending java.util.concurrent.ConcurrentMap – which is basically a distributed map, often used as cache. You can configure such map to use custom MapLoader<K, V> – piece of Java code that will be asked every time you try to .get()something from that map (by key) which is not yet there. ...

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Hazelcast’s MapLoader pitfalls

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One of the core data structures provided by Hazelcast is IMap<K, V> extendingjava.util.concurrent.ConcurrentMap – which is basically a distributed map, often used as cache. You can configure such map to use custom MapLoader<K, V> – piece of Java code that will be asked every time you try to .get()something from that map (by key) which is not yet there. This ...

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Property-based testing with Spock

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Property based testing is an alternative approach to testing, complementing example based testing. The latter is what we’ve been doing all our lives: exercising production code against “examples” – inputs we think are representative. Picking these examples is an art on its own: “ordinary” inputs, edge cases, malformed inputs, etc. But why are we limiting ourselves to just few examples? ...

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Testing code for excessively large inputs

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When writing unit tests we mostly focus on business correctness. We do our best to exercise happy path and all edge cases. We sometimes microbenchmark and measure throughput. But one aspect that is often missed is how our code behaves when the input is excessively large? We test how we handle normal input files, malformed files, empty files, missing files… ...

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Grouping, sampling and batching – custom collectors in Java 8

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Continuing first article, this time we will write some more useful custom collectors: for grouping by given criteria, sampling input, batching and sliding over with fixed size window. Grouping (counting occurrences, histogram) Imagine you have a collection of some items and you want to calculate how many times each item (with respect toequals()) appears in this collection. This can be achieved ...

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Building extremely large in-memory InputStream for testing purposes

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For some reason I needed extremely large, possibly even infinite InputStream that would simply return the samebyte[] over and over. This way I could produce insanely big stream of data by repeating small sample. Sort of similar functionality can be found in Guava: Iterable<T> Iterables.cycle(Iterable<T>) and Iterator<T> Iterators.cycle(Iterator<T>). For example if you need an infinite source of 0 and 1, simply sayIterables.cycle(0, 1) and get 0, 1, 0, 1, 0, 1... infinitely. ...

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Introduction to writing custom collectors in Java 8

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Java 8 introduced the concept of collectors. Most of the time we barely use factory methods from Collectors class, e.g. collect(toList()), toSet() or maybe something more fancy like counting() or groupingBy(). Not many of us actually bother to look how collectors are defined and implemented. Let’s start from analysing what Collector<T, A, R> really is and how it works. Collector<T, A, R> works as a “sink” for streams – stream pushes ...

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Turning recursive file system traversal into Stream

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When I was learning programming, back in the days of Turbo Pascal, I managed to list files in directory usingFindFirst, FindNext and FindClose functions. First I came up with a procedure printing contents of a given directory. You can imagine how proud I was to discover I can actually call that procedure from itself to traverse file system recursively. Well, I didn’t know the ...

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Option.fold() considered unreadable

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We had a lengthy discussion recently during code review whether scala.Option.fold() is idiomatic and clever or maybe unreadable and tricky? Let’s first describe what the problem is. Option.fold does two things: maps a function f over Option‘s value (if any) or returns an alternative alt if it’s absent. Using simple pattern matching we can implement it as follows:     ...

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