欢迎阅读我的java8新特性介绍教程。本教程将一步一步的引领你浏览所有新的语言特性。伴随着一些简单且简短的代码示例,你将学习如何使用默认接口方法,lambda表示式,方法引用和可重复的annotation。 在阅读完本篇文章,你将对新加入的和更新的API有着更详细的了解。这些API包括流、功能接口、扩展的map以及新的Date API。
interface Formula { double calculate(int a); default double sqrt(int a) { return Math.sqrt(a); } }在抽象方法calculator之外,接口Formula还定义了一个默认方法sqrt。实现类只需要实现抽象方法calculate。默认方法sqrt可以在定义之外使用。如:
Formula formula = new Formula() { @Override public double calculate(int a) { return sqrt(a * 100); } }; formula.calculate(100); // 100.0 formula.sqrt(16); // 4.0formula被实现为一个匿名类。代码有点啰嗦:六行代码里就就只有一句简单的计算:sqrt(a*100)。我们将在下面部分看到一种用java8实现的更加简洁的办法。
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); } });这个静态工具方法Collections.sort接受一个列表和一个用于元素比较的比较器。你会发现自己会经常创建匿名类并把它们传递给排序方法。
Collections.sort(names, (String a, String b) -> { return b.compareTo(a); });现在的代码已经变得简短和便于阅读。但是,实际上,它可以变得更加简短:
Collections.sort(names, (String a, String b) -> b.compareTo(a));
Collections.sort(names, (a, b) -> b.compareTo(a));java编译器能够探测到这些参数的类型,这样使得的你可以直接跳过它们。下面我们来解答为什么lambda表示式可以这样随意的使用。
@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如果FunctionInterface注解被添加,你定义的接口将总会被检测。
Converter<String, Integer> converter = Integer::valueOf; Integer converted = converter.convert("123"); System.out.println(converted); // 123java8可以让你通过关键字::来传递方法和构造函数的引用。上面的示例展示了如何引用一个静态方法。我们同样也可以引用对象方法。
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"现在我们将看到关键字::如何为构造函数工作。首先我们定义一个拥有不同构造函数的bean类:
class Person { String firstName; String lastName; Person() {} Person(String firstName, String lastName) { this.firstName = firstName; this.lastName = lastName; } }
</pre><pre code_snippet_id="265470" snippet_file_name="blog_20140330_13_2612710" name="code" class="java">interface PersonFactory<P extends Person> { P create(String firstName, String lastName); }
PersonFactory<Person> personFactory = Person::new; Person person = personFactory.create("Peter", "Parker");我们通过Person::new创建一个指向Person构造函数的引用。java编译器自动的选择正确的构造函数来匹配PersonFactory.create的函数签名。
final int num = 1; Converter<Integer, String> stringConverter = (from) -> String.valueOf(from + num); stringConverter.convert(2); // 3
int num = 1; Converter<Integer, String> stringConverter = (from) -> String.valueOf(from + num); stringConverter.convert(2); // 3但是实际上,变量num在编译期是被隐式的转换为fianl类型的。下面的代码是不能被成功的编译的:
int num = 1; Converter<Integer, String> stringConverter = (from) -> String.valueOf(from + num); num = 3;在lambda表达式内部向变量num写入值同样是不允许的。
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 formula = (a) -> sqrt( a * 100);
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();
Function<String, Integer> toInteger = Integer::valueOf; Function<String, String> backToString = toInteger.andThen(String::valueOf); backToString.apply("123"); // "123"
Supplier<Person> personSupplier = Person::new; personSupplier.get(); // new Person
Consumer<Person> greeter = (p) -> System.out.println("Hello, " + p.firstName); greeter.accept(new Person("Luke", "Skywalker"));
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
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"
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");Java8的Collections类已经被扩展了,你可以简单的调用Collection.stream()或者Collection.parallelSteam()来创建流。下面部分将介绍大部分流操作。
stringCollection .stream() .filter((s) -> s.startsWith("a")) .forEach(System.out::println); // "aaa2", "aaa1"
stringCollection .stream() .sorted() .filter((s) -> s.startsWith("a")) .forEach(System.out::println); // "aaa1", "aaa2"
System.out.println(stringCollection); // ddd2, aaa2, bbb1, aaa1, bbb3, ccc, bbb2, ddd1
stringCollection .stream() .map(String::toUpperCase) .sorted((a, b) -> b.compareTo(a)) .forEach(System.out::println); // "DDD2", "DDD1", "CCC", "BBB3", "BBB2", "AAA2", "AAA1"
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
long startsWithB = stringCollection .stream() .filter((s) -> s.startsWith("b")) .count(); System.out.println(startsWithB); // 3
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 msParallel Sort
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你会观察到这两种模式的代码基本上市一致的,但是并行排序所花费的时间大约是顺序排序的一半。
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));
</pre>上述的代码应该很清晰了:putIfAbsent使得我们不用写是否为null值的检测语句;forEach使用consumer来对map中的每个元素进行操作。</div><div></div><div>下面的例子向我们展示使用功能性函数在map里执行代码:</div><div><pre code_snippet_id="265470" snippet_file_name="blog_20140330_37_9422552" name="code" class="java">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
Clock clock = Clock.systemDefaultZone(); long millis = clock.millis(); Instant instant = clock.instant(); Date legacyDate = Date.from(instant); // legacy java.util.Date
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 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); // -239LocalTime包含了多个工厂方法用来简化创建过程,其中也包括通过字符串来创建时间:
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 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从字符串解析出LocalDate和解析LocalTime一样简单:
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 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在一些额外的时区信息帮助下,它可以被转换为instant。Instants可以被容易的转换为遗留的java.util.Date类型。
Instant instant = sylvester .atZone(ZoneId.systemDefault()) .toInstant(); Date legacyDate = Date.from(instant); System.out.println(legacyDate); // Wed Dec 31 23:59:59 CET 2014格式date-time的过程和格式date和time基本上是一样的。在使用系统自带的定义格式时,我们也可以定义我们自己的格式:
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和java.text.NumberFormat不一样的是DateTimeFormatter是不可变的并且是类型安全的。
@interface Hints { Hint[] value(); } @Repeatable(Hints.class) @interface Hint { String value(); }
@Hints({@Hint("hint1"), @Hint("hint2")}) class Person {}
@Hint("hint1") @Hint("hint2") class Person {}在第二种情景下,java编译器隐式的在该注解使用中加入@Hints。这种后期处理在通过反射获取注解是十分重要的。
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虽然我们从来没有在类Person上声明@Hints注解,但该信息还是可以通过getAnnotation(Hint.class)获得。 此外,getAnnotationsByType是一种更加便利的方法,它可以保证我们访问所有使用的@Hint注解。
@Target({ElementType.TYPE_PARAMETER, ElementType.TYPE_USE}) @interface MyAnnotation {}