目录

一、概述

Stream 是 Java8 中处理集合的关键抽象概念，它可以指定你希望对集合进行的操作，可以执行非常复杂的查找、过滤和映射数据等操作。使用Stream API 对集合数据进行操作，就类似于使用 SQL 执行的数据库查询。也可以使用 Stream API 来并行执行操作。

简而言之，Stream API 提供了一种高效且易于使用的处理数据的方式。

特点：

二、分类

三、具体用法

1. 流的常用创建方法

1.1 使用​​Collection​​下的​​stream()​​和​​parallelStream()​​方法

List list = new ArrayList<>();
Stream stream = list.stream(); //获取一个顺序流
Stream parallelStream = list.parallelStream(); //获取一个并行流

1.2 使用Arrays 中的​​stream()​​方法，将数组转成流

Integer[] nums = new Integer[10];
Stream stream = Arrays.stream(nums);

1.3 使用Stream中的静态方法：​​of()​​、​​iterate()​​、​​generate()​​

Stream stream = Stream.of(1,2,3,4,5,6);

Stream stream2 = Stream.iterate(0, (x) -> x + 2).limit(6);
stream2.forEach(System.out::println); // 0 2 4 6 8 10

Stream stream3 = Stream.generate(Math::random).limit(2);
stream3.forEach(System.out::println);

1.4 使用​​BufferedReader.lines()​​方法，将每行内容转成流

BufferedReader reader = new BufferedReader(new FileReader("F:\\test_stream.txt"));
Stream lineStream = reader.lines();
lineStream.forEach(System.out::println);

1.5 使用​​Pattern.splitAsStream()​​方法，将字符串分隔成流

Pattern pattern = Pattern.compile(",");
Stream stringStream = pattern.splitAsStream("a,b,c,d");
stringStream.forEach(System.out::println);

2. 流的中间操作

2.1 筛选与切片

Stream stream = Stream.of(6, 4, 6, 7, 3, 9, 8, 10, 12, 14, 14);

Stream newStream = stream.filter(s -> s > 5) //6 6 7 9 8 10 12 14 14
.distinct() //6 7 9 8 10 12 14
.skip(2) //9 8 10 12 14
.limit(2); //9 8
newStream.forEach(System.out::println);

2.2 映射

List list = Arrays.asList("a,b,c", "1,2,3");

//将每个元素转成一个新的且不带逗号的元素
Stream s1 = list.stream().map(s -> s.replaceAll(",", ""));
s1.forEach(System.out::println); // abc 123

Stream s3 = list.stream().flatMap(s -> {
//将每个元素转换成一个stream
String[] split = s.split(",");
Stream s2 = Arrays.stream(split);
return s2;
});
s3.forEach(System.out::println); // a b c 1 2 3

2.3 排序

List list = Arrays.asList("aa", "ff", "dd");
//String 类自身已实现Compareable接口
list.stream().sorted().forEach(System.out::println);// aa dd ff

Student s1 = new Student("aa", 10);
Student s2 = new Student("bb", 20);
Student s3 = new Student("aa", 30);
Student s4 = new Student("dd", 40);
List studentList = Arrays.asList(s1, s2, s3, s4);

//自定义排序：先按姓名升序，姓名相同则按年龄升序
studentList.stream().sorted(
(o1, o2) -> {
if (o1.getName().equals(o2.getName())) {
return o1.getAge() - o2.getAge();
} else {
return o1.getName().compareTo(o2.getName());
}
}
).forEach(System.out::println);

2.4 消费

Student s1 = new Student("aa", 10);
Student s2 = new Student("bb", 20);
List studentList = Arrays.asList(s1, s2);

studentList.stream()
.peek(o -> o.setAge(100))
.forEach(System.out::println);

//结果：
Student{name="aa", age=100}
Student{name="bb", age=100}

3. 流的终止操作

3.1 匹配、聚合操作

List list = Arrays.asList(1, 2, 3, 4, 5);

boolean allMatch = list.stream().allMatch(e -> e > 10); //false
boolean noneMatch = list.stream().noneMatch(e -> e > 10); //true
boolean anyMatch = list.stream().anyMatch(e -> e > 4); //true

Integer findFirst = list.stream().findFirst().get(); //1
Integer findAny = list.stream().findAny().get(); //1

long count = list.stream().count(); //5
Integer max = list.stream().max(Integer::compareTo).get(); //5
Integer min = list.stream().min(Integer::compareTo).get(); //1

3.2 规约操作

//经过测试，当元素个数小于24时，并行时线程数等于元素个数，当大于等于24时，并行时线程数为16
List list = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24);

Integer v = list.stream().reduce((x1, x2) -> x1 + x2).get();
System.out.println(v); // 300

Integer v1 = list.stream().reduce(10, (x1, x2) -> x1 + x2);
System.out.println(v1); //310

Integer v2 = list.stream().reduce(0,
(x1, x2) -> {
System.out.println("stream accumulator: x1:" + x1 + " x2:" + x2);
return x1 - x2;
},
(x1, x2) -> {
System.out.println("stream combiner: x1:" + x1 + " x2:" + x2);
return x1 * x2;
});
System.out.println(v2); // -300

Integer v3 = list.parallelStream().reduce(0,
(x1, x2) -> {
System.out.println("parallelStream accumulator: x1:" + x1 + " x2:" + x2);
return x1 - x2;
},
(x1, x2) -> {
System.out.println("parallelStream combiner: x1:" + x1 + " x2:" + x2);
return x1 * x2;
});
System.out.println(v3); //197474048

Student s1 = new Student("aa", 10,1);
Student s2 = new Student("bb", 20,2);
Student s3 = new Student("cc", 10,3);
List list = Arrays.asList(s1, s2, s3);

//装成list
List ageList = list.stream().map(Student::getAge).collect(Collectors.toList()); // [10, 20, 10]

//转成set
Set ageSet = list.stream().map(Student::getAge).collect(Collectors.toSet()); // [20, 10]

//转成map,注:key不能相同，否则报错
Map studentMap = list.stream().collect(Collectors.toMap(Student::getName, Student::getAge)); // {cc=10, bb=20, aa=10}

//字符串分隔符连接
String joinName = list.stream().map(Student::getName).collect(Collectors.joining(",", "(", ")")); // (aa,bb,cc)

//聚合操作
//1.学生总数
Long count = list.stream().collect(Collectors.counting()); // 3
//2.最大年龄 (最小的minBy同理)
Integer maxAge = list.stream().map(Student::getAge).collect(Collectors.maxBy(Integer::compare)).get(); // 20
//3.所有人的年龄
Integer sumAge = list.stream().collect(Collectors.summingInt(Student::getAge)); // 40
//4.平均年龄
Double averageAge = list.stream().collect(Collectors.averagingDouble(Student::getAge)); // 13.333333333333334
// 带上以上所有方法
DoubleSummaryStatistics statistics = list.stream().collect(Collectors.summarizingDouble(Student::getAge));
System.out.println("count:" + statistics.getCount() + ",max:" + statistics.getMax() + ",sum:" + statistics.getSum() + ",average:" + statistics.getAverage());

//分组
Map> ageMap = list.stream().collect(Collectors.groupingBy(Student::getAge));
//多重分组,先根据类型分再根据年龄分
Map>> typeAgeMap = list.stream().collect(Collectors.groupingBy(Student::getType, Collectors.groupingBy(Student::getAge)));

//分区
//分成两部分，一部分大于10岁，一部分小于等于10岁
Map> partMap = list.stream().collect(Collectors.partitioningBy(v -> v.getAge() > 10));

//规约
Integer allAge = list.stream().map(Student::getAge).collect(Collectors.reducing(Integer::sum)).get(); //40

//toList 源码
public static  Collector> toList() {
return new CollectorImpl<>((Supplier>) ArrayList::new, List::add,
(left, right) -> {
left.addAll(right);
return left;
}, CH_ID);
}

//为了更好地理解，我们转化一下源码中的lambda表达式
public  Collector> toList() {
Supplier> supplier = () -> new ArrayList();
BiConsumer, T> accumulator = (list, t) -> list.add(t);
BinaryOperator> combiner = (list1, list2) -> {
list1.addAll(list2);
return list1;
};
Function, List> finisher = (list) -> list;
Set characteristics = Collections.unmodifiableSet(EnumSet.of(Collector.Characteristics.IDENTITY_FINISH));

return new Collector, List>() {
@Override
public Supplier supplier() {
return supplier;
}

@Override
public BiConsumer accumulator() {
return accumulator;
}

@Override
public BinaryOperator combiner() {
return combiner;
}

@Override
public Function finisher() {
return finisher;
}

@Override
public Set characteristics() {
return characteristics;
}
};

}