New to Java 8

Java is not dead yet - and people begin to understand this.

Welcome to the Java 8 Guide . This material will introduce you step by step with all the new features of the language. You will learn how to use default interface methods, lambda expressions, method references and repeatable annotations. All this will be accompanied by short and simple code examples. At the end of the article, you will get acquainted with the most recent changes in the API regarding streams, functional interfaces, extensions for associative arrays, as well as changes in the API for working with dates.

Default Interface Methods

Java 8 , default. , . :

interface Formula {
    double calculate(int a);

    default double sqrt(int a) {
        return Math.sqrt(a);
    }
}

calculate Formula sqrt. , , calculate. sqrt .

Formula formula = new Formula() {
    @Override
    public double calculate(int a) {
        return sqrt(a * 100);
    }
};

formula.calculate(100);     // 100.0
formula.sqrt(16);                // 4.0

formula . : 6 sqrt(a * 100). , Java 8 .

-

: .

List<String> names = Arrays.asList("peter", "anna", "mike", "xenia");

Collections.sort(names, new Comparator<String>() {
    @Override
    public int compare(String a, String b) {
        return b.compareTo(a);
    }
});

Collections.sort , . .

Java 8 — -, :

Collections.sort(names, (String a, String b) -> {
    return b.compareTo(a);
});

, . :

Collections.sort(names, (String a, String b) -> b.compareTo(a));

{} return. :

Collections.sort(names, (a, b) -> b.compareTo(a));

, . , -.

- Java? , . . - . , , .

-, . , , , @FunctionalInterface. , , .

:

@FunctionalInterface
interface Converter<F, T> {
    T convert(F from);
}

Converter<String, Integer> converter = (from) -> Integer.valueOf(from);
Integer converted = converter.convert("123");
System.out.println(converted);    // 123

, @FunctionalInterface.

, :

Converter<String, Integer> converter = Integer::valueOf;
Integer converted = converter.convert("123");
System.out.println(converted);   // 123

Java 8 . ::. . :

class Something {
    String startsWith(String s) {
        return String.valueOf(s.charAt(0));
    }
}

Something something = new Something();
Converter<String, String> converter = something::startsWith;
String converted = converter.convert("Java");
System.out.println(converted);    // "J"

, . :

class Person {
    String firstName;
    String lastName;

    Person() {}

    Person(String firstName, String lastName) {
        this.firstName = firstName;
        this.lastName = lastName;
    }
}

, :

interface PersonFactory<P extends Person> {
    P create(String firstName, String lastName);
}

:

PersonFactory<Person> personFactory = Person::new;
Person person = personFactory.create("Peter", "Parker");

Person::new. , PersonFactory.create.

- . , final, .

final int num = 1;
Converter<Integer, String> stringConverter = (from) -> String.valueOf(from + num);

stringConverter.convert(2);     // 3

, num final. :

int num = 1;
Converter<Integer, String> stringConverter = (from) -> String.valueOf(from + num);

stringConverter.convert(2);     // 3

num . :

int num = 1;
Converter<Integer, String> stringConverter = (from) -> String.valueOf(from + num);
num = 3;

num - .

, -. .

class Lambda4 {
    static int outerStaticNum;
    int outerNum;

    void testScopes() {
        Converter<Integer, String> stringConverter1 = (from) -> {
            outerNum = 23;
            return String.valueOf(from);
        };

        Converter<Integer, String> stringConverter2 = (from) -> {
            outerStaticNum = 72;
            return String.valueOf(from);
        };
    }
}

? Formula sqrt, , . -.

- . :

Formula formula = (a) -> sqrt( a * 100);

JDK 1.8 . , , Comparator Runnable. @FunctionalInterface.

Java 8 , . Google Guava. , , .

— , , boolean. , (and, or, negate).

Predicate<String> predicate = (s) -> s.length() > 0;

predicate.test("foo");              // true
predicate.negate().test("foo");     // false

Predicate<Boolean> nonNull = Objects::nonNull;
Predicate<Boolean> isNull = Objects::isNull;

Predicate<String> isEmpty = String::isEmpty;
Predicate<String> isNotEmpty = isEmpty.negate();

. (compose, andThen).

Function<String, Integer> toInteger = Integer::valueOf;
Function<String, String> backToString = toInteger.andThen(String::valueOf);

backToString.apply("123");     // "123"

(suppliers) . , .

Supplier<Person> personSupplier = Person::new;
personSupplier.get();   // new Person

(consumers) , .

Consumer<Person> greeter = (p) -> System.out.println("Hello, " + p.firstName);
greeter.accept(new Person("Luke", "Skywalker"));

Java. Java 8 .

Comparator<Person> comparator = (p1, p2) -> p1.firstName.compareTo(p2.firstName);

Person p1 = new Person("John", "Doe");
Person p2 = new Person("Alice", "Wonderland");

comparator.compare(p1, p2);             // > 0
comparator.reversed().compare(p1, p2);  // < 0

(optionals) , NullPointerException. , , , .

— , null. , , - , . , null, Java 8 .

Optional<String> optional = Optional.of("bam");

optional.isPresent();           // true
optional.get();                 // "bam"
optional.orElse("fallback");    // "bam"

optional.ifPresent((s) -> System.out.println(s.charAt(0)));     // "b"

java.util.Stream , . (intermediate) (terminal). , . . , , java.util.Collection, ( ). , .

, . :

List<String> stringCollection = new ArrayList<>();
stringCollection.add("ddd2");
stringCollection.add("aaa2");
stringCollection.add("bbb1");
stringCollection.add("aaa1");
stringCollection.add("bbb3");
stringCollection.add("ccc");
stringCollection.add("bbb2");
stringCollection.add("ddd1");

Java 8 , Collection.stream() Collection.parallelStream(). .

Filter

Filter , . , .. (, forEach) . ForEach , ( ) . ForEach . , .

stringCollection
    .stream()
    .filter((s) -> s.startsWith("a"))
    .forEach(System.out::println);

// "aaa2", "aaa1"

Sorted

Sorted , . , :

stringCollection
    .stream()
    .sorted()
    .filter((s) -> s.startsWith("a"))
    .forEach(System.out::println);

// "aaa1", "aaa2"

, sorted . stringCollection :

System.out.println(stringCollection);
// ddd2, aaa2, bbb1, aaa1, bbb3, ccc, bbb2, ddd1

Map

map . . map . , map.

stringCollection
    .stream()
    .map(String::toUpperCase)
    .sorted((a, b) -> b.compareTo(a))
    .forEach(System.out::println);

// "DDD2", "DDD1", "CCC", "BBB3", "BBB2", "AAA2", "AAA1"

Match

, , (match). boolean.

boolean anyStartsWithA = 
    stringCollection
        .stream()
        .anyMatch((s) -> s.startsWith("a"));

System.out.println(anyStartsWithA);      // true

boolean allStartsWithA = 
    stringCollection
        .stream()
        .allMatch((s) -> s.startsWith("a"));

System.out.println(allStartsWithA);      // false

boolean noneStartsWithZ = 
    stringCollection
        .stream()
        .noneMatch((s) -> s.startsWith("z"));

System.out.println(noneStartsWithZ);      // true

Count

Count . long.

long startsWithB = 
    stringCollection
        .stream()
        .filter((s) -> s.startsWith("b"))
        .count();

System.out.println(startsWithB);    // 3

Reduce

. .

Optional<String> reduced =
    stringCollection
        .stream()
        .sorted()
        .reduce((s1, s2) -> s1 + "#" + s2);

reduced.ifPresent(System.out::println);
// "aaa1#aaa2#bbb1#bbb2#bbb3#ccc#ddd1#ddd2"

, . , — .

, , .

:

int max = 1000000;
List<String> values = new ArrayList<>(max);
for (int i = 0; i < max; i++) {
    UUID uuid = UUID.randomUUID();
    values.add(uuid.toString());
}

.

long t0 = System.nanoTime();

long count = values.stream().sorted().count();
System.out.println(count);

long t1 = System.nanoTime();

long millis = TimeUnit.NANOSECONDS.toMillis(t1 - t0);
System.out.println(String.format("sequential sort took: %d ms", millis));

// sequential sort took: 899 ms

long t0 = System.nanoTime();

long count = values.parallelStream().sorted().count();
System.out.println(count);

long t1 = System.nanoTime();

long millis = TimeUnit.NANOSECONDS.toMillis(t1 - t0);
System.out.println(String.format("parallel sort took: %d ms", millis));

// parallel sort took: 472 ms

, , . , , stream() parallelStream().

, (maps) . , .

Map<Integer, String> map = new HashMap<>();

for (int i = 0; i < 10; i++) {
    map.putIfAbsent(i, "val" + i);
}

map.forEach((id, val) -> System.out.println(val));

: putIfAbsent null; forEach , .

:

map.computeIfPresent(3, (num, val) -> val + num);
map.get(3);             // val33

map.computeIfPresent(9, (num, val) -> null);
map.containsKey(9);     // false

map.computeIfAbsent(23, num -> "val" + num);
map.containsKey(23);    // true

map.computeIfAbsent(3, num -> "bam");
map.get(3);             // val33

, , :

map.remove(3, "val3");
map.get(3);             // val33

map.remove(3, "val33");
map.get(3);             // null

:

map.getOrDefault(42, "not found");  // not found

? :

map.merge(9, "val9", (value, newValue) -> value.concat(newValue));
map.get(9);             // val9

map.merge(9, "concat", (value, newValue) -> value.concat(newValue));
map.get(9);             // val9concat

Merge -. — .

API

Java 8 API , java.time. API Joda-Time, . API.

Clock

Clock . System.currentTimeMillis() . Instant.
java.util.Date.

Clock clock = Clock.systemDefaultZone();
long millis = clock.millis();

Instant instant = clock.instant();
Date legacyDate = Date.from(instant);   // legacy java.util.Date

ZoneId. . , .

System.out.println(ZoneId.getAvailableZoneIds());
// prints all available timezone ids

ZoneId zone1 = ZoneId.of("Europe/Berlin");
ZoneId zone2 = ZoneId.of("Brazil/East");
System.out.println(zone1.getRules());
System.out.println(zone2.getRules());

// ZoneRules[currentStandardOffset=+01:00]
// ZoneRules[currentStandardOffset=-03:00]

LocalTime

LocalTime , , 10pm 17:30:15. , . , .

LocalTime now1 = LocalTime.now(zone1);
LocalTime now2 = LocalTime.now(zone2);

System.out.println(now1.isBefore(now2));  // false

long hoursBetween = ChronoUnit.HOURS.between(now1, now2);
long minutesBetween = ChronoUnit.MINUTES.between(now1, now2);

System.out.println(hoursBetween);       // -3
System.out.println(minutesBetween);     // -239

LocalTime , , .

LocalTime late = LocalTime.of(23, 59, 59);
System.out.println(late);       // 23:59:59

DateTimeFormatter germanFormatter =
    DateTimeFormatter
        .ofLocalizedTime(FormatStyle.SHORT)
        .withLocale(Locale.GERMAN);

LocalTime leetTime = LocalTime.parse("13:37", germanFormatter);
System.out.println(leetTime);   // 13:37

LocalDate

LocalDate , , 2014-03-11. LocalDate LocalTime. , . , .

LocalDate today = LocalDate.now();
LocalDate tomorrow = today.plus(1, ChronoUnit.DAYS);
LocalDate yesterday = tomorrow.minusDays(2);

LocalDate independenceDay = LocalDate.of(2014, Month.JULY, 4);
DayOfWeek dayOfWeek = independenceDay.getDayOfWeek();
System.out.println(dayOfWeek);    // FRIDAY

LocalDate :

DateTimeFormatter germanFormatter =
    DateTimeFormatter
        .ofLocalizedDate(FormatStyle.MEDIUM)
        .withLocale(Locale.GERMAN);

LocalDate xmas = LocalDate.parse("24.12.2014", germanFormatter);
System.out.println(xmas);   // 2014-12-24

LocalDateTime

LocalDateTime . LocalDateTime LocalTime LocalDate. -:

LocalDateTime sylvester = LocalDateTime.of(2014, Month.DECEMBER, 31, 23, 59, 59);

DayOfWeek dayOfWeek = sylvester.getDayOfWeek();
System.out.println(dayOfWeek);      // WEDNESDAY

Month month = sylvester.getMonth();
System.out.println(month);          // DECEMBER

long minuteOfDay = sylvester.getLong(ChronoField.MINUTE_OF_DAY);
System.out.println(minuteOfDay);    // 1439

Instant.

Instant instant = sylvester
        .atZone(ZoneId.systemDefault())
        .toInstant();

Date legacyDate = Date.from(instant);
System.out.println(legacyDate);     // Wed Dec 31 23:59:59 CET 2014

- , . .

DateTimeFormatter formatter =
    DateTimeFormatter
        .ofPattern("MMM dd, yyyy - HH:mm");

LocalDateTime parsed = LocalDateTime.parse("Nov 03, 2014 - 07:13", formatter);
String string = formatter.format(parsed);
System.out.println(string);     // Nov 03, 2014 - 07:13

java.text.NumberFormat, DateTimeFormatter .

.

Java 8 . , , .

-, :

@interface Hints {
    Hint[] value();
}

@Repeatable(Hints.class)
@interface Hint {
    String value();
}

Java 8 @Repeatable.

1: - ( )

@Hints({@Hint("hint1"), @Hint("hint2")})
class Person {}

2: ( )

@Hint("hint1")
@Hint("hint2")
class Person {}

2 @Hints. .

Hint hint = Person.class.getAnnotation(Hint.class);
System.out.println(hint);                   // null

Hints hints1 = Person.class.getAnnotation(Hints.class);
System.out.println(hints1.value().length);  // 2

Hint[] hints2 = Person.class.getAnnotationsByType(Hint.class);
System.out.println(hints2.length);          // 2

@Hints Person, getAnnotation(Hints.class). getAnnotationsByType, @Hint.

, Java 8 :

@Target({ElementType.TYPE_PARAMETER, ElementType.TYPE_USE})
@interface MyAnnotation {}

Java 8 . JDK 1.8, , Arrays.parallelSort, StampedLock, CompletableFuture .

GitHub.