Formula formula = new Formula() {    @Override    public double calculate(int a) {        return sqrt(a * 100);    }};formula.calculate(100);     // 100.0formula.sqrt(16);           // 4.0

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

继续优化

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

对于函数体只有一行代码的，你可以去掉大括号{}以及return关键字，但是你还可以写得更短点：

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

Java编译器可以自动推导出参数类型。

3、函数式接口

Lambda 表达式是如何在java的类型系统中表示的呢？每一个lambda表达式都对应一个类型，通常是接口类型。而“函数式接口”是指仅仅只包含一个抽象方法的 接口，每一个该类型的lambda表达式都会被匹配到这个抽象方法。因为 默认方法 不算抽象方法，所以你也可以给你的函数式接口添加默认方法。

我们可以将lambda表达式当作任意只包含一个抽象方法的接口类型，确保你的接口一定达到这个要求，你只需要给你的接口添加 @FunctionalInterface 注解，编译器如果发现你标注了这个注解的接口有多于一个抽象方法的时候会报错的。

示例如下：

@FunctionalInterfaceinterface 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

需要注意如果@FunctionalInterface如果没有指定，上面的代码也是对的。

译者注 将lambda表达式映射到一个单方法的接口上，这种做法在Java 8之前就有别的语言实现，比如Rhino JavaScript解释器，如果一个函数参数接收一个单方法的接口而你传递的是一个function，Rhino 解释器会自动做一个单接口的实例到function的适配器，典型的应用场景有 org.w3c.dom.events.EventTarget 的addEventListener 第二个参数 EventListener。

四、方法与构造函数引用
前一节中的代码还可以通过静态方法引用来表示：

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

Java 8 允许你使用 :: 关键字来传递方法或者构造函数引用，上面的代码展示了如何引用一个静态方法，我们也可以引用一个对象的方法：

converter = something::startsWith;String converted = converter.convert("Java");System.out.println(converted);    // "J"

接下来看看构造函数是如何使用::关键字来引用的，首先我们定义一个包含多个构造函数的简单类：

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");

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

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

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);Built-in Functional Interfaces

Predicate<String> predicate = (s) -> s.length() > 0;predicate.test("foo");              // truepredicate.negate().test("foo");     // falsePredicate<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);             // > 0comparator.reversed().compare(p1, p2);  // < 0

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

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"

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"

并行Streams前面提到过Stream有串行和并行两种，串行Stream上的操作是在一个线程中依次完成，而并行Stream则是在多个线程上同时执行。下面的例子展示了是如何通过并行Stream来提升性能：首先我们创建一个没有重复元素的大表：

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));// 串行耗时: 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));// 并行排序耗时: 472 ms

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));

map.computeIfPresent(3, (num, val) -> val + num);map.get(3);             // val33map.computeIfPresent(9, (num, val) -> null);map.containsKey(9);     // falsemap.computeIfAbsent(23, num -> "val" + num);map.containsKey(23);    // truemap.computeIfAbsent(3, num -> "bam");map.get(3);             // val33

map.remove(3, "val3");map.get(3);             // val33map.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);             // val9map.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 idsZoneId 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));  // falselong hoursBetween = ChronoUnit.HOURS.between(now1, now2);long minutesBetween = ChronoUnit.MINUTES.between(now1, now2);System.out.println(hoursBetween);       // -3System.out.println(minutesBetween);     // -239

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

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);      // WEDNESDAYMonth month = sylvester.getMonth();System.out.println(month);          // DECEMBERlong minuteOfDay = sylvester.getLong(ChronoField.MINUTE_OF_DAY);System.out.println(minuteOfDay);    // 1439

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

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

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

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

Hint hint = Person.class.getAnnotation(Hint.class);System.out.println(hint);                   // nullHints hints1 = Person.class.getAnnotation(Hints.class);System.out.println(hints1.value().length);  // 2Hint[] hints2 = Person.class.getAnnotationsByType(Hint.class);System.out.println(hints2.length);          // 2

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