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Java8的新特性

2017-3-7 12:53| 发布者: zhangjf| 查看: 1326| 评论: 0

摘要: 来自： https://segmentfault.com/a/1190000004419611 java语言特性系列 Java5的新特性 Java6的新特性 Java7的新特性 Java8的新特性 Java9的新特性 ...

来自： https://segmentfault.com/a/1190000004419611

java语言特性系列

Java5的新特性

Java6的新特性

Java7的新特性

Java8的新特性

Java9的新特性

序

本文主要讲Java8的新特性，Java8也是一个重要的版本，在语法层面有更大的改动，支持了lamda表达式，影响堪比Java5的泛型支持。

特性列表

lamda表达式( 重磅 )

集合的stream操作

提升HashMaps的性能

Date-Time Package

java.lang and java.util Packages

Concurrency

lamda表达式( 重磅 )

方法引用

  1. /**
  2. * 静态方法引用：ClassName::methodName
  3. * 实例上的实例方法引用：instanceReference::methodName
  4. * 超类上的实例方法引用：super::methodName
  5. * 类型上的实例方法引用：ClassName::methodName
  6. * 构造方法引用：Class::new
  7. * 数组构造方法引用：TypeName[]::new
  8. * Created by patterncat on 2016-02-05.
  9. */
  10. public class MethodReference {
  12. @Test
  13. public void methodRef(){
  14. SampleData.getThreeArtists().stream()
  15. .map(Artist::getName)
  16. .forEach(System.out::println);
  17. }
  19. @Test
  20. public void constructorRef(){
  21. ArtistFactory<Artist> af = Artist::new;
  22. Artist a = af.create("patterncat","china");
  23. System.out.println(a);
  24. }
  25. }
复制代码
集合的stream操作
  1. /**
  2. * 主要接口
  3. * 1,predicate
  4. * 2,Unary/BinaryOperator:传入参数和返回值必然是同一种数据类型
  5. * 3,Int/Double/LongFunction/BiFunction:函数接口并不要求传入参数和返回值之间的数据类型必须一样
  6. * 4,Int/Long/DoubleConsumer/BiConsumer:消费数据
  7. * 5,Int/Long/DoubleSupplier:生产数据
  8. *
  9. * 主要方法:
  10. * 1,filter
  11. * 2,map
  12. * 3,reduce
  13. * 4,collect
  14. * 5,peek
  15. * -Djdk.internal.lambda.dumpProxyClasses
  16. * Created by patterncat on 2016-02-05.
  17. */
  18. public class LamdaDemo {
  20. int[] arr = {4,12,1,3,5,7,9};
  22. @Test
  23. public void filter(){
  24. Arrays.stream(arr).filter((x) -> x%2 !=0).forEach(System.out::println);
  25. }
  27. @Test
  28. public void map(){
  29. Arrays.stream(arr).map((x) -> x * x).forEach(System.out::println);
  30. }
  32. @Test
  33. public void reduce(){
  34. Arrays.stream(arr).reduce((x,y) -> x+y).ifPresent(System.out::println);
  35. System.out.println(Arrays.stream(arr).reduce(-10, (x, y) -> x + y));
  36. }
  38. @Test
  39. public void collect(){
  40. List<Integer> list = Arrays.stream(arr).collect(ArrayList::new,ArrayList::add,ArrayList::addAll);
  41. System.out.println(list);
  43. Set<Integer> set = list.stream().collect(Collectors.toSet());
  44. System.out.println(set);
  46. Map<String,Artist> map = SampleData.getThreeArtists().stream()
  47. .collect(Collectors.toMap(a -> a.getName(),a -> a));
  48. System.out.println(map);
  49. }
  51. @Test
  52. public void peek(){
  53. long count = Arrays.stream(arr).filter(x -> x > 2).peek(System.out::println).count();
  54. System.out.println(count);
  55. }
  57. @Test
  58. public void average(){
  59. Arrays.stream(arr).average().ifPresent(System.out::println);
  60. }
  62. @Test
  63. public void sum(){
  64. System.out.println(Arrays.stream(arr).sum());
  65. }
  67. @Test
  68. public void max(){
  69. Arrays.stream(arr).max().ifPresent(System.out::println);
  70. }
  72. @Test
  73. public void min(){
  74. Arrays.stream(arr).min().ifPresent(System.out::println);
  75. }
  77. @Test
  78. public void sorted(){
  79. Comparator<Artist> asc = (x,y) -> x.getName().compareTo(y.getName());
  80. SampleData.getThreeArtists().stream().sorted(asc).forEach(System.out::println);
  81. SampleData.getThreeArtists().stream().sorted(asc.reversed()).forEach(System.out::println);
  82. SampleData.getThreeArtists().stream().sorted(Comparator.comparing(Artist::getName)).forEach(System.out::println);
  83. SampleData.getThreeArtists().stream().sorted(Comparator.comparing(Artist::getName).reversed()).forEach(System.out::println);
  85. SampleData.getThreeArtists().stream().sorted(Comparator.comparing(Artist::getName).thenComparing(Artist::getNationality)).forEach(System.out::println);
  86. }
  88. @Test
  89. public void groupBy(){
  90. Map<String,List<Artist>> rs = SampleData.getThreeArtists().stream().collect(Collectors.groupingBy(Artist::getNationality));
  91. System.out.println(rs);
  92. }
  94. @Test
  95. public void join(){
  96. String joinedNames = SampleData.getThreeArtists().stream().map(Artist::getName).collect(Collectors.joining(","));
  97. System.out.println(joinedNames);
  98. joinedNames.chars().mapToObj(c -> (char) Character.toUpperCase(c)).forEach(System.out::println);
  99. }
  101. @Test
  102. public void flatMap(){
  103. Set<Artist> rs = SampleData.getThreeArtists().stream().flatMap(a -> a.getMembers()).collect(Collectors.toSet());
  104. rs.stream().forEach(System.out::println);
  105. }
  107. @Test
  108. public void arrStream(){
  109. Arrays.stream(arr).forEach(System.out::println);
  110. }
  112. @Test
  113. public void then(){
  114. // IntConsumer out = System.out::println;
  115. // IntConsumer err = System.err::println;
  116. IntConsumer out = (x) -> System.out.println("out consume:"+x);
  117. IntConsumer err = (x) -> System.err.println("err consume:"+x);
  118. // Arrays.stream(arr).forEach(out.andThen(err));
  119. Arrays.stream(arr).forEach(err.andThen(out));
  120. }
  123. @Test
  124. public void foreach(){
  125. List<Integer> numbers = Arrays.asList(1,2,3,4,5,6);
  126. numbers.forEach(System.out::println);
  127. }
  129. @Test
  130. public void visitOuterVar(){
  131. final int num = 2;
  132. Function<Integer,Integer> fun = (from) -> from * num;
  133. System.out.println(fun.apply(3));
  134. }
  135. }
复制代码
提升HashMaps的性能

当hash冲突时，以前都是用链表存储，在java8里头，当节点个数>=TREEIFY_THRESHOLD - 1时，HashMap将采用红黑树存储，这样最坏的情况下即所有的key都Hash冲突，采用链表的话查找时间为O(n)，而采用红黑树为O(logn)。

Date-Time Package

Java 8新增了LocalDate和LocalTime接口，一方面把月份和星期都改成了enum防止出错，另一方面把LocalDate和LocalTime变成不可变类型，这样就线程安全了。

  1. @Test
  2. public void today(){
  3. LocalDate today = LocalDate.now();
  4. System.out.println(today);
  5. }
  7. @Test
  8. public void parseString(){
  9. // 严格按照ISO yyyy-MM-dd验证，02写成2都不行，当然也有一个重载方法允许自己定义格式
  10. LocalDate date = LocalDate.parse("2016-02-05");
  11. System.out.println(date);
  12. }
  14. @Test
  15. public void calculate(){
  16. LocalDate today = LocalDate.now();
  17. LocalDate firstDayOfThisMonth = today.with(TemporalAdjusters.firstDayOfMonth());
  18. System.out.println(firstDayOfThisMonth);
  20. // 取本月第2天：
  21. LocalDate secondDayOfThisMonth = today.withDayOfMonth(2);
  22. System.out.println(secondDayOfThisMonth);
  24. // 取本月最后一天，再也不用计算是28，29，30还是31：
  25. LocalDate lastDayOfThisMonth = today.with(TemporalAdjusters.lastDayOfMonth());
  26. System.out.println(lastDayOfThisMonth);
  28. // 取下一天：
  29. LocalDate nextDay = lastDayOfThisMonth.plusDays(1);
  30. System.out.println(nextDay);
  32. // 取2015年1月第一个周一，这个计算用Calendar要死掉很多脑细胞：
  33. LocalDate firstMondayOf2015 = LocalDate.parse("2015-01-01").with(TemporalAdjusters.firstInMonth(DayOfWeek.MONDAY));
  34. System.out.println(firstMondayOf2015);
  35. }
  37. @Test
  38. public void getTime(){
  39. LocalTime now = LocalTime.now();
  40. System.out.println(now);
  41. }
  43. @Test
  44. public void getTimeWithoutMillis(){
  45. LocalTime now = LocalTime.now().withNano(0);
  46. System.out.println(now);
  47. }
  49. @Test
  50. public void parseTime(){
  51. LocalTime zero = LocalTime.of(0, 0, 0); // 00:00:00
  52. System.out.println(zero);
  54. LocalTime mid = LocalTime.parse("12:00:00"); // 12:00:00
  55. System.out.println(mid);
  56. }
复制代码
java.lang and java.util Packages

比如数组的并行排序

  1. public class UtilDemo {
  3. int[] data = {4,12,1,3,5,7,9};
  5. @Test
  6. public void parallelSort(){
  7. Arrays.parallelSort(data);
  8. System.out.println(Arrays.toString(data));
  9. }
  11. @Test
  12. public void testCollectPrallel() {
  13. //[4, 16, 17, 20, 25, 32, 41]
  14. Arrays.parallelPrefix(data, Integer::sum);
  15. System.out.println(Arrays.toString(data));
  16. }
  17. }
复制代码

比如文件遍历

  1. @Test
  2. public void list() throws IOException {
  3. Files.list(Paths.get(".")).filter(Files::isDirectory).forEach(System.out::println);
  4. }
  6. @Test
  7. public void walk() throws IOException {
  8. Files.walk(Paths.get("."), FileVisitOption.FOLLOW_LINKS).forEach(System.out::println);
  9. }
复制代码
Concurrency

StampedLock

  1. public class BankAccountWithStampedLock {
  3. private final StampedLock lock = new StampedLock();
  4. private double balance;
  6. public void deposit(double amount) {
  7. long stamp = lock.writeLock();
  8. try {
  9. balance = balance + amount;
  10. } finally {
  11. lock.unlockWrite(stamp);
  12. }
  13. }
  15. public double getBalance() {
  16. long stamp = lock.readLock();
  17. try {
  18. return balance;
  19. } finally {
  20. lock.unlockRead(stamp);
  21. }
  22. }
  23. }
复制代码

测试

  1. @Test
  2. public void bench() throws InterruptedException {
  3. BankAccountWithStampedLock account = new BankAccountWithStampedLock();
  4. ExecutorService pool = Executors.newCachedThreadPool();
  5. List<Callable<Double>> callables = IntStream.range(1,5)
  6. .mapToObj(x -> (Callable<Double>) () -> {
  7. // if (x % 2 == 0) {
  8. // return account.getBalance();
  9. // } else {
  10. // account.deposit(x);
  11. // return 0d;
  12. // }
  13. account.deposit(x);
  14. return 0d;
  15. })
  16. .collect(Collectors.toList());
  17. pool.invokeAll(callables).forEach(x -> {
  18. try {
  19. System.out.println(x.get());
  20. } catch (InterruptedException e) {
  21. e.printStackTrace();
  22. } catch (ExecutionException e) {
  23. e.printStackTrace();
  24. }
  25. });
  26. pool.shutdown();
  27. System.out.println(account.getBalance());
  28. }
复制代码

ConcurrentHashMap的stream支持

参考

What's New in JDK 8

如何在Java 8中愉快地处理日期和时间

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