十五個常用經典的 Java8 Stream API 用法示例
- 2020 年 2 月 18 日
- 筆記
不出意外的話，再過幾天，Java 11就要正式對外發布了，不知各位同行都用上哪個版本了呢？先貼一張截取的調查圖，由此可見，目前Java 7/8的使用群體還是相當之大的。
下面列舉了十五個常用經典的Java8 Stream API用法示例。為了方便操作，首先抽取並定義公共數組變數array。
private Integer[] array = {10, 3, 3, 15, 9, 23};
- map
private void map() { List<Integer> collect = Stream.of(array).map(n -> n * 2).collect(Collectors.toList()); System.out.println("collect = " + collect); // [20, 6, 6, 30, 18, 46] collect = Stream.of(array).mapToInt(n -> n * 2).boxed().collect(Collectors.toList()); System.out.println("collect = " + collect); // [20, 6, 6, 30, 18, 46] ArrayList<Long> collect1 = Stream.of(array).mapToLong(Integer::longValue).boxed().collect(Collectors.toCollection(ArrayList::new)); System.out.println("collect1 = " + collect1); // [10, 3, 3, 15, 9, 23] TreeSet<Object> collect2 = Stream.of(array).mapToDouble(Integer::doubleValue).collect(TreeSet::new, TreeSet::add, TreeSet::addAll); System.out.println("collect2 = " + collect2); // [3.0, 9.0, 10.0, 15.0, 23.0] }
- filter
private void filter() { Object[] objects = Stream.of(array).filter(n -> n >= 10).toArray(); System.out.println("objects = " + Arrays.toString(objects));// [10, 15, 23] }
- sort
private void sort() { // naturalOrder List<Integer> collect = Stream.of(array).sorted().collect(Collectors.toList()); System.out.println("collect = " + collect); // [3, 3, 9, 10, 15, 23] // reverseOrder collect = Stream.of(array).sorted(Comparator.reverseOrder()).collect(Collectors.toList()); System.out.println("collect = " + collect); // [23, 15, 10, 9, 3, 3] collect = Stream.of(array).sorted(Comparator.comparingInt(Integer::intValue).reversed()).collect(Collectors.toList()); System.out.println("collect = " + collect); // [23, 15, 10, 9, 3, 3] }
- skip
private void skip() { TreeSet<Integer> collect = Stream.of(array).skip(3).collect(Collectors.toCollection(TreeSet::new)); System.out.println("collect = " + collect); // [9, 15, 23] }
- distinct
private void distinct() { LinkedList<Integer> collect = Stream.of(array).distinct().collect(Collectors.toCollection(LinkedList::new)); System.out.println("collect = " + collect); // [10, 3, 15, 9, 23] }
- sum and count
private void sumAndCount() { int sum = Stream.of(array).mapToInt(Integer::intValue).sum(); System.out.println("sum = " + sum); // 63 long sum1 = Stream.of(array).mapToLong(Integer::intValue).sum(); System.out.println("sum1 = " + sum1); // 63 double sum2 = Stream.of(array).mapToDouble(Integer::intValue).sum(); System.out.println("sum2 = " + sum2); // 63.0 // array.length is equal 6 long count = Stream.of(array).count(); System.out.println("sumAndCount = " + count); // 6 }
- limit
private void limit() { Set<Integer> collect = Stream.of(array).limit(3).collect(Collectors.toSet()); System.out.println("collect = " + collect); // [3, 10] }
- match
boolean allMatch = Stream.of(array).allMatch(n -> n > 5); System.out.println("allMatch = " + allMatch); // false boolean anyMatch = Stream.of(array).anyMatch(n -> n > 5); System.out.println("anyMatch = " + anyMatch); // true boolean noneMatch = Stream.of(array).noneMatch(n -> n > 5); System.out.println("noneMatch = " + noneMatch); // false
- find
private void find() { Optional<Integer> any = Stream.of(array).filter(n -> n * 2 > 10 && n * 2 < 20).findAny(); any.ifPresent(n -> System.out.println("any = " + n)); // 9 any = Stream.of(array).filter(n -> n < 5).findFirst(); any.ifPresent(n -> System.out.println("any = " + n)); // 3 }
- min and max
private void minAndMax() { Stream.of(array).min(Comparator.naturalOrder()).ifPresent(n -> System.out.println("n = " + n)); // 3 Stream.of(array).max(Comparator.comparingInt(Integer::intValue)).ifPresent(n -> System.out.println("n = " + n)); // 23 }
- peek
private void peek() { LinkedHashSet<Integer> collect = Stream.of(array).peek(n -> System.out.print(n + " ")).collect(Collectors.toCollection(LinkedHashSet::new)); System.out.println(); System.out.println("collect = " + collect); // [10, 3, 15, 9, 23] }
- reduce
private void reduce() { Integer reduce = Stream.of(array).reduce(100, Integer::sum); System.out.println("reduce = " + reduce); // 163 }
- flatMap
private void flatMap() { List<Integer> collect = Stream.of(array, array).flatMap(Arrays::stream).collect(Collectors.toList()); System.out.println("collect = " + collect); // [10, 3, 3, 15, 9, 23, 10, 3, 3, 15, 9, 23] }
- present
private void present() { boolean isPresent = Stream.of(array).findAny().isPresent(); System.out.println("isPresent = " + isPresent); Stream.of(array).filter(n -> n == 20).findFirst().ifPresent(n -> System.out.println("n = " + n)); }
- summaryStatistics
private void summaryStatistics() { IntSummaryStatistics collect = Stream.of(array).collect(Collectors.summarizingInt(Integer::intValue)); System.out.println(collect.getCount() + " " + collect.getSum() + " " + collect.getMin() + " " + collect.getAverage() + " " + collect.getMax()); System.out.println("collect = " + collect); // IntSummaryStatistics{sumAndCount=6, sum=63, min=3, average=10.500000, max=23} IntSummaryStatistics intSummaryStatistics = Stream.of(array).mapToInt(Integer::intValue).summaryStatistics(); System.out.println("intSummaryStatistics = " + intSummaryStatistics); // IntSummaryStatistics{sumAndCount=6, sum=63, min=3, average=10.500000, max=23} }
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