十五个常用经典的 Java8 Stream API 用法示例

不出意外的话，再过几天，Java 11就要正式对外发布了，不知各位同行都用上哪个版本了呢？先贴一张截取的调查图，由此可见，目前Java 7/8的使用群体还是相当之大的。


下面列举了十五个常用经典的Java8 Stream API用法示例。为了方便操作，首先抽取并定义公共数组变量`array`。

``` java

private Integer[] array = {10, 3, 3, 15, 9, 23};

```

1. map

``` java

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]

}

```

2. filte

``` java

private void filter() {

Object[] objects = Stream.of(array).filter(n -> n >= 10).toArray();

System.out.println("objects = " + Arrays.toString(objects));// [10, 15, 23]

}

```

3. sort

``` java

private void sort() {

// naturalOrde

List<Integer> collect = Stream.of(array).sorted().collect(Collectors.toList());

System.out.println("collect = " + collect); // [3, 3, 9, 10, 15, 23]

// reverseOrde

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]

}

```

4. skip

``` java

private void skip() {

TreeSet<Integer> collect = Stream.of(array).skip(3).collect(Collectors.toCollection(TreeSet::new));

System.out.println("collect = " + collect); // [9, 15, 23]

}

```

5. distinct

``` java

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]

}

```

6. sum and count

``` java

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

}

```

7. limit

``` java

private void limit() {

Set<Integer> collect = Stream.of(array).limit(3).collect(Collectors.toSet());

System.out.println("collect = " + collect); // [3, 10]

}

```

8. match

``` java

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

```

9. find

``` java

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

}

```

10. min and max

``` java

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

}

```

11. peek

``` java

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]

}

```

12. reduce

``` java

private void reduce() {

Integer reduce = Stream.of(array).reduce(100, Integer::sum);

System.out.println("reduce = " + reduce); // 163

}

```

13. flatMap

``` java

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]

}

```

14. present

``` java

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));

}

```

15. summaryStatistics

``` java

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}

}

```

> 文章已授权转载，原文链接：[十五个常用经典的 Java8 Stream API 用法示例](https://blog.mariojd.cn/15-examples-of-java8-stream-usage.html)