Posts Tagged development

Writing your own custom Java stream collector

One of the common operations of the Collectors API introduced in Java 8 is the possibility to collect results into a result container like List, Set or Map. The following example uses the collect() method to generate a HashSet containing unique numbers:

Now suppose a given string and the need to compute some summaries on it, like the number of uppercase, lowercase, invalid chars and how many digits are present on that string.

Applying a reduce operation on each needed summary operation would result in more than one pass through the data, like this (here I am using the var keyword introduced in Java 10):

In the above code excerpt we are iterating three times through the string to compute the summaries. If we want to iterate just one time on the give string and get the desired results, we could fallback to the traditional imperative approach:

Despite the solution above, there is another one: writing your own custom stream collector. This custom collector can compute the number of uppercase, lowercase, invalid chars and how many digits are present on the given string, in a single pass through the data. It is possible to make it run in parallel with the Streams API as well.

The custom stream collector shown here uses the chars() method of the String class which returns an IntStream. The IntStream class contains a collect() method that computes a mutable reduction on the elements and returns its result in a container class.

The next example shows the container class code. It receives and accumulates each char of the String in the accept() method, thus categorizing it as a digit, uppercase char, lowercase char or as an invalid char.

The complete implementation of the collect() method which produces the result of the reduction in the CharSummaryStatistics class is shown below. As a new char arrives, it is categorized in the CharSummaryStatistics.accept() method. The CharSummaryStatistics.combine() method is used to merge partial results.

Have you ever written a custom Java stream collector? Please drop your comments here.

The complete source code can be found on GitHub.

As a reference for writing this post, this article is part of a series about Streams and it is a good reference for learning how to aggregate and collect with Streams. All the articles from this series about Streams can be found here.

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Jcombiner: Combinations of collections for Java

JCombiner is a framework to generate combinations of collections for Java. I have written it in Java 11 using Java 9 modules (JPMS) and Gradle as build tool. JUnit 5 and Mockito are used for unit testing and Jacoco for code coverage. Streams and the Collectors API are extensively used throughout the development of JCombiner project.

Jcombiner’s source code is available under GitHub.

Code examples of its usage can be found on GitHub here. More examples can be found on this module inside JCombiner.

Share your comments about this framework here! Please feel free to contribute to it, more features are welcome!

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Refactoring large conditional method using method references

Some years ago I wrote junit-parameters, which is basically a custom JUnit test runner that make it possible to add parameters to JUnit 4 test methods.

Browsing its source code SonarLint pointed me a large conditional if/else method from the ParameterTypeConverterFactory class:

This method converts the method parameter to its specific type based on its Class object. As it is few lines long, it showed me a good opportunity to refactor this code a little bit with a more elegant solution. This project has unit tests, so I could start refactoring it in small steps and start over again whether I have made a mistake.

I started by defining a functional interface called ParameterConverter:

and I created an immutable map which maps each Class object to its associated ParameterConverter instance (making use of method references):

Then I refactored the original conditional method to get the ParameterConverter instance from the convertersByClass map and mapping to an Optional instance in case it didn’t exist.

After those refactorings, SonarLint stopped warning me. Below is the modified version of the original method with some helper methods:

The complete code can be found at my GitHub here.

This was the first change of breaking this complicated conditional method into a more readable one. It was safe to do so because after each change I could run my unit tests suite to assert I haven’t broken anything. The next refactoring could be towards a more object-oriented approach like Replace Conditional with Polymorphism.

What did you think about this refactoring? Have you ever had such a situation? Drop your comments here!

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Streams in JDK 8: The Good, the Bad, and the Ugly

Great session in JavaOne 2017 about Streams and lambdas introduced in JDK8.

The session shows many examples of Java code using forEach() with side effects and how to refactor them to a functional approach using streams and the Collectors API.

What are your experiences using Streams and lambdas in JDK 8? Are you correctly using the Collectors API?

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3 Things Every Java Developer Should Stop Doing

My friends Andre and Leonnardo have sent me an interesting article about some bad habits every Java developer should stop doing in their code.

Basically the author picked up the following points and discussed each one of them, showing code examples why they aren’t good practices at all:

  1. Returning Null
  2. Defaulting to Functional Programming
  3. Creating Indiscriminate Getters and Setters

I agree with all these points as being bad practices in Java code. How about you? What do you think about it?

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Java 8: Converting Optional Collection to the Streams API

Although Java 9 has already been released, this post is about converting an optional collection to the Streams API introduced in Java 8.

Suppose some person could have zero, one or more cars and it is represented by the Person class below (some code omitted).

Now we create a list of people and we want to get Mark’s cars.

How can we do that using the Streams API, since the getCars() method return an Optional?

One possibility is to filter people’s list by Mark’s name, filter the Optional if it is present or not and map its wrapped value (our cars list):

At this moment we reached the reason of this blog post. And how can we get all people’s cars? The idea here is to use the flatMap() operation unwrapping the Optional to the collection’s stream when it is present or getting an empty stream when it isn’t present:

We can do better and replace the above solution to be more functional using method references:

If you use IntelliJ IDEA as your favorite IDE, it has an inspection that helps replacing Optional.isPresent() with a functional expression:

P.S. In Java 9, the stream() method was added to the Optional API, so we can rewrite the above stream pipeline into the following one:

In case you are interested, this post on the IntelliJ IDEA blog has some good tips when working with Java 8.

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About the Kotlin programming language

Kotlin is a statically typed language which is fully interoperable with Java.

Recently my friend Andre showed me Kotlin‘s nice syntax and I considered giving it a try.

In the meantime, my friend Leonnardo sent me this nice link which helps migrating from Java to Kotlin easily.

Let’s compare some syntax from Java and Kotlin and see the differences. Suppose we have some employees and we want to group them by their departments.

In Java we create an Employee class, build some employees and use the Streams API to group them by their departments:

In Kotlin we create a data class Employee, build some employees and use the collection built-in groupBy method to group them by their departments:

As you can see, Kotlin has some syntactic sugar that makes it less verbose than Java.

If you haven’t considered trying Kotlin yet, I think it is worth giving it a try.

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35 programming habits that make your code smell

Interesting article about some bad programming practices, including topics like code organization, teamwork, testing and maintenance.

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Thinking Iterative, but doing Waterfall development

Some companies think they’re applying iterative and incremental development, but in practice are doing waterfall development. Here are some tips described in Applying UML and Patterns, by Craig Larman, which can help you discover if your company doesn’t understand the Unified Process and iterative development (these tips can clarify all stakeholders misunderstanding about the difference of the two approaches):

  1. You think that inception = requirements, elaboration = design, and construction = implementation (that is, superimposing a waterfall lifecycle onto the UP).
  2. You think that the purpose of elaboration is to fully and carefully define models, which are translated into code during construction.
  3. You try to define most of the requirements before starting design or implementation.
  4. You try to define most of the design before starting implementation; you try to fully define and commit to an architecture before iterative programming and testing.
  5. A “long time” is spent doing requirements or design work before programming starts.
  6. You believe that a suitable iteration length is four months long, rather than four weeks long (excluding projects with hundreds of developers).
  7. You think UML diagramming and design activities are a time to fully and accurately define designs and models in great detail, and of programming as a simple mechanical translation of these into code.
  8. You think that adopting the UP means to do many of the possible activities and create many documents, and thinks of or experiences the UP as a formal, fussy process with many steps to be followed.
  9. You try to plan a project in detail from start to finish; you try to speculatively predict all the iterations, and what should happen in each one.
  10. You want believable plans and estimates for projects before the elaboration phase is finished.
  11. Are you involved in such a situation at your company?

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