Trying Java Streams

Streams in Java enables us to filter, sort, map, count and loop over elements in collections.

For this example, we use a Record type, a Java 14 feature to represent a static website generator.

        record StaticWebsiteGenerator(
            String name,
            String language,
            String license,
            int stars
        ) {}

We then create instances for each type of static website generator and store them within a list:

        StaticWebsiteGenerator swgNextJs = new StaticWebsiteGenerator("Next.js", "JavaScript", "MIT", 84087);
        StaticWebsiteGenerator swgHugo = new StaticWebsiteGenerator("Hugo", "Go", "Apache 2.0", 58055);
        StaticWebsiteGenerator swgGatsby = new StaticWebsiteGenerator("Gatsby", "JavaScript", "MIT", 52632);
        StaticWebsiteGenerator swgGatsby2 = new StaticWebsiteGenerator("Gatsby", "JavaScript", "MIT", 52632);
        StaticWebsiteGenerator swgJekyll = new StaticWebsiteGenerator("Jekyll", "Ruby", "MIT", 44409);

        List<StaticWebsiteGenerator> lstSwg = Arrays.asList(swgNextJs, swgHugo, swgGatsby, swgGatsby2, swgJekyll);

The .stream() method gives us a stream that we can use to process the elements. The .filter() method takes a lambda expression and only takes elements that evaluate to true through the lambda function. The .sorted() passes 2 object instances to the lambda and this is easily handled by calling .compareTo. The .map() method transforms the source elements - in this case, we get the names of the static website generators as strings in the resulting List.

        List<String> lstSwgNames = lstSwg.stream()
                .filter(swg -> swg.language == "JavaScript")
                .sorted( (x,y) -> x.name.compareTo(y.name) )
                .map(swg -> "[" + swg.name + "]")
                .collect(Collectors.toList())
                ;

We then process the List of String objects to keep only unique values and iterate over them in a loop that prints the Strings:

        lstSwgNames.stream()
                .distinct()
                .forEach(iterstr -> System.out.println(iterstr));

Streams can also be used for performing the equivalent of a SELECT-GROUP BY like this:

        Map<String, Long> swgByLicense = lstSwg.stream()
                .collect(
                        Collectors.groupingBy(
                                StaticWebsiteGenerator::license,
                                //Collectors.toList(),
                                Collectors.counting()
                        )
                )
                ;

        for(Map.Entry<String, Long> entry : swgByLicense.entrySet()) {
            System.out.println(
                entry.getKey() +
                " - " +
                entry.getValue().toString()
            );
        }

Data can also be reduced with the .reduce() or the short-hand methods: .min(), .max(), .average().