Java Lambda Snippets

Java Lambda Snippets

/*
* This page is a running list of java lambda code snippets. 
* It lacks details on what each statement actually does. 
* If you are not comfortable with lambda then please
* refer to a good reference book on lambdas.
*/
//map and collect list of string to a comma seperated string
 list.stream()
  .map(i -> i % 2 == 0 ? "e" + i : "o" + i)
  .collect(joining(","));
 
//filter and collect into list
 list.stream()
  .filter(s -> s.startsWith("a"))
  .filter(s -> s.length() == 3)
  .collect(Collectors.toList());
 
//convert comma separated string pair to map
Map returnData = new HashMap<>();
rows.stream().map((row) -> row.split(",")).forEach((rowData) -> {
            returnData.put(rowData[keyColumn], rowData[valueColumn]);
        });
//-------------------------------------------
//Use lambda to find specific aggregate from list of objects
        Trader raoul = new Trader("Raoul", "Cambridge");
        Trader mario = new Trader("Mario", "Milan");
        Trader alan = new Trader("Alan", "Cambridge");
        Trader brian = new Trader("Brian", "Cambridge");
        List transactions = Arrays.asList(
                new Transaction(brian, 2011, 300),
                new Transaction(raoul, 2012, 1000),
                new Transaction(raoul, 2011, 400),
                new Transaction(mario, 2012, 710),
                new Transaction(mario, 2012, 700),
                new Transaction(alan, 2012, 950)
        );
        // Query 1: Find all transactions from year 2011 and sort them by value (small to high).
        List tr2011 = transactions.stream()
                .filter(transaction -> transaction.getYear() == 2011)
                .sorted(comparing(Transaction::getValue))
                .collect(toList());
        System.out.println(tr2011);
        // Query 2: What are all the unique cities where the traders work?
        List cities
                = transactions.stream()
                .map(transaction -> transaction.getTrader().getCity())
                .distinct()
                .collect(toList());
        System.out.println(cities);
        // Query 3: Find all traders from Cambridge and sort them by name.
        List traders
                = transactions.stream()
                .map(Transaction::getTrader)
                .filter(trader -> trader.getCity().equals("Cambridge"))
                .distinct()
                .sorted(comparing(Trader::getName))
                .collect(toList());
        System.out.println(traders);
        // Query 4: Return a string of all traders̢۪ names sorted alphabetically.
        String traderStr
                = transactions.stream()
                .map(transaction -> transaction.getTrader().getName())
                .distinct()
                .sorted()
                .reduce("", (n1, n2) -> n1 + n2);
        System.out.println(traderStr);
        // Query 5: Are there any trader based in Milan?
        boolean milanBased
                = transactions.stream()
                .anyMatch(transaction -> transaction.getTrader()
                        .getCity()
                        .equals("Milan")
                );
        System.out.println(milanBased);
        // Query 6: Update all transactions so that the traders from Milan are set to Cambridge.
        transactions.stream()
                .map(Transaction::getTrader)
                .filter(trader -> trader.getCity().equals("Milan"))
                .forEach(trader -> trader.setCity("Cambridge"));
        System.out.println(transactions);
        // Query 7: What's the highest value in all the transactions?
        int highestValue
                = transactions.stream()
                .map(Transaction::getValue)
                .reduce(0, Integer::max);
        System.out.println(highestValue);
//-------------------------------------------------------
//More example of comparator in lambda
List babies = BabyNamesParserUtil.getBabyNames(2000);
        List filtered = babies.stream().filter(
                p -> p.getGender().equalsIgnoreCase("F")).sorted(Comparator.comparing(Baby::getOccurance).reversed())
                .limit(10).collect(Collectors.toList());
        System.out.println("Top 10 female baby names >>>>>>");
        filtered.forEach(b -> System.out.println(b.toString()));
        List mfiltered = babies.stream().filter(
                p -> p.getGender().equalsIgnoreCase("M")).sorted(Comparator.comparing(Baby::getOccurance).reversed())
                .limit(10).collect(Collectors.toList());
        System.out.println("\n\nTop 10 male baby names >>>>>>");
        mfiltered.forEach(b -> System.out.println(b.toString()));
//---------------------------------------
// Regex that matches one or more consecutive whitespace characters
        Pattern pattern = Pattern.compile("\\s+");

        // count occurrences of each word in a Stream sorted by word
        Map wordCounts
                = Files.lines(Paths.get("C:\\java8\\LambdaPresentation\\data\\LambdaPresentation\\data\\MobyDick.txt")) //Util function returns stream of all the lines in a file
                .map(line -> line.replaceAll("(?!')\\p{P}", ""))
                .flatMap(line -> pattern.splitAsStream(line)) //Pattern got new method splitAsStream to split as CharSequence
                .collect(Collectors.groupingBy(String::toLowerCase,
                                TreeMap::new, Collectors.counting()));

        // display the words grouped by starting letter
        wordCounts.entrySet()
                .stream().filter(entry -> StringUtils.isNotBlank(entry.getKey()) && Character.isLetter(entry.getKey().trim().charAt(0)))
                .collect(Collectors.groupingBy(entry -> entry.getKey().trim().charAt(0),
                                TreeMap::new, Collectors.toList()
                        ))
                .forEach((letter, wordList)
                        -> {
                    System.out.printf("%n%C%n", letter);
                    wordList.stream().forEach(word -> System.out.printf(
                                    "%13s: %d%n", word.getKey(), word.getValue()));
                });
//----------------------------------------
//Stream. Iterate
Stream.iterate(5.0, p -> p * 2)
                .peek(e -> System.out.println("Fetching " + e)) // peek is a good tool for debugging
                .limit(5).toArray();
//parallel stream
Map airports = CSVUtil.readAsMap("C:\\java8\\LambdaPresentation\\data\\airports.csv", 0, 1);
            airports.keySet().parallelStream().forEach(p -> { //now try with parallelStream
                System.out.println("Airport Code : " + p);
                System.out.println("Airport Name : " + airports.get(p));
                System.out.println("Is Delay : " + FAAUtil.contactFAA(p).isIsDelay());
            });
List coldAirports
                    = airports.stream().filter(a -> a.getTemperatue() < 40) //try parallel
                    .sorted(comparing(Airport::getTemperatue).reversed())
                    .map(a -> a.getCode())
                    .collect(Collectors.toList());
//IntStream
List list = IntStream.of(100,300,500,600,700,800).boxed().collect(Collectors.toList());
        list.forEach(p->System.out.println(p));
//Secure random
SecureRandom random = new SecureRandom();

        // roll a die 1,000,000 times and summarize the results
        System.out.printf("%-6s%s%n", "Face", "Frequency");
        random.ints(6_000_000, 1, 7) //created infinite stream
                .boxed() //important otherwise collect wont work on these streams
                .collect(Collectors.groupingBy(Function.identity(),
                                Collectors.counting()))
                .forEach((face, frequency)
                        -> System.out.printf("%-6d%d%n", face, frequency));

//Java 9 features for Optional and Stream
List integerList = List.of(1,2,45,55,78,98);
  System.out.println("Take while output");
  integerList.stream().takeWhile(i -> i< 55).forEach(System.out::println);
  
  System.out.println("Drop while output");
  integerList.stream().dropWhile(i -> i< 55).forEach(System.out::println);
  
  System.out.println("Mimic for loop");
  IntStream.iterate(0, i -> i+1).limit(4).forEach(System.out::println);
  
  //get orElse methods in stream can be replaced with Optional ifPresentElse
  Optional mayBeInteger = integerList.stream().filter(s -> s > 100).findAny();
  mayBeInteger.ifPresentOrElse(System.out::println, () -> System.out.println("Empty Result!"));
  
  //Map method in Optional is also smarter now
  Optional mayBeAnotherInteger = integerList.stream().filter(s -> s > 100).findAny();
  Optional definitelyInteger = mayBeAnotherInteger.or(()-> Optional.of(-999));
  System.out.println(definitelyInteger.get());
  
  //Optional can also be converted to stream now, it will always have at most one value.
  Stream optionalStream = integerList.stream().filter(s -> s < 100).findFirst().stream();

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