/*
* 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 traders’ 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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