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个人简介
作者是一个来自河源的大三在校生,以下笔记都是作者自学之路的一些浅薄经验,如有错误请指正,将来会不断的完善笔记,帮助更多的Java爱好者入门。
文章目录
JDK8新特性-Stream流
教程概述
- 本教程附有非常多的例子,看完肯定能懂Stream流!
- 看完本教程,对于Stream api基本的使用完全没有问题,底层原理则不会深究!
- 本教程借鉴过很多其他大佬的教程,并进行总结创新,难免会有相同之处。
Stream流的创建
用集合创建流
//创建普通顺序流
Stream<Integer> stream = asList.stream();
//创建并行流
Stream<Integer> parallelStream = asList.parallelStream();
用数组创建流
int arr[]={
1,2,3,4,5};
IntStream intStream = Arrays.stream(arr);
使用Stream的静态方法创建流
Stream<Integer> integerStream = Stream.of(1, 2, 3, 4, 5, 6);
顺序流转换成并行流
List<Integer> asList = Arrays.asList(1, 2, 3, 4, 5);
//创建顺序流
Stream<Integer> integerStream = asList.stream();
//把顺序流转换成并行流
Stream<Integer> parallel = integerStream.parallel();
实体类Person:
class Person {
private String name; // 姓名
private double salary; // 薪资
private int age; // 年龄
public Person(String name, double salary, int age) {
this.name = name;
this.salary = salary;
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public double getSalary() {
return salary;
}
public void setSalary(double salary) {
this.salary = salary;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
@Override
public String toString() {
return "Person{" +
"name='" + name + '\'' +
", salary=" + salary +
", age=" + age +
'}';
}
}
流的遍历和查找元素(forEach、find)
遍历Stream顺序流
List<Person> list=new ArrayList<>();
list.add(new Person("z1",2000.0,18));
list.add(new Person("z2",3200.0,15));
list.add(new Person("z3",1500.0,27));
list.add(new Person("z4",7000.0,36));
list.add(new Person("z5",5000.0,22));
list.add(new Person("z6",4200.0,42));
//创建Stream顺序流
Stream<Person> stream = list.stream();
//遍历Stream顺序流
stream.forEach(System.out::println);
遍历并行流(多线程,输出顺序会不一样)
//创建并行流(多线程)
Stream<Person> parallelStream = list.parallelStream();
//遍历并行流(多线程,输出顺序会不一样)
parallelStream.forEach(System.out::println);
找出流中第一个元素
Optional<Person> first = stream.findFirst();
System.out.println(first.get());
流的筛选(filter)
案例1:集合中大于5的元素,并打印出来
List<Integer> list = Arrays.asList(7, 6, 9, 3, 8, 2, 1);
Stream<Integer> stream = list.stream();
//1:集合中大于5的元素,并打印出来
stream.filter(el->el>5).forEach(System.out::println);
案例2:筛选年龄大于25岁的人,并形成一个只有name的新的集合
List<Person> personList = new ArrayList<Person>();
personList.add(new Person("z1",2000.0,18));
personList.add(new Person("z2",3200.0,15));
personList.add(new Person("z3",1500.0,27));
personList.add(new Person("z4",7000.0,36));
personList.add(new Person("z5",5000.0,22));
personList.add(new Person("z6",4200.0,42));
Stream<Person> personStream = personList.stream();
List<String> collect = personStream
.filter(person -> person.getAge() > 25)
//只筛选出name,如果直接collect则返回的是person对象
.map(Person::getName)
.collect(Collectors.toList());
collect.forEach(System.out::println);
聚合(max、min、count)
案例3:获取String集合中最长的元素
List<String> list1 = Arrays.asList("qdiq", "sdji", "aaa", "ihduxdswaa", "qwer");
Stream<String> stream1 = list1.stream();
Optional<String> maxString = stream1.max(Comparator.comparing(String::length));
System.out.println("maxString="+maxString.get());
案例4:比较集合中数字最大的并输出
List<Integer> integerList = Arrays.asList(20, 10, 30, 52, 42, 15, 11, 13, 19, 30);
Stream<Integer> integerStream = integerList.stream();
Optional<Integer> optionalInteger = integerStream.max(Comparator.comparing(Integer::intValue));
System.out.println(optionalInteger.get());
案例5:计算Integer集合中大于6的元素的个数
List<Integer> list2 = Arrays.asList(7, 6, 4, 8, 2, 11, 9);
long count = list2.stream().filter(integer -> integer > 6).count();
System.out.println("集合中大于6的个数="+count);
映射(map)
- 可以直接操作每一个流的元素
- 凡是需要操作流中元素的都用map,filter只是起到筛选的作用
案例6:英文字符串数组的元素全部改为大写
Stream<String> stream = Arrays.stream(strArr);
//s就是每一个元素
List<String> collect = stream.map(s -> s.toUpperCase()).collect(Collectors.toList());
collect.forEach(System.out::println);
案例7:整数数组每个元素+3
List<Integer> intList = Arrays.asList(1, 3, 5, 7, 9, 11);
Stream<Integer> stream1 = intList.stream();
stream1.map(integer -> integer+=3).collect(Collectors.toList()).forEach(System.out::println);
案例8:将员工的薪资全部增加1000
List<Person> personList = new ArrayList<Person>();
personList.add(new Person("z1",2000.0,18));
personList.add(new Person("z2",3200.0,15));
personList.add(new Person("z3",1500.0,27));
personList.add(new Person("z4",7000.0,36));
personList.add(new Person("z5",5000.0,22));
personList.add(new Person("z6",4200.0,42));
Stream<Person> personStream = personList.stream();
personStream.map(person -> {
double old = person.getSalary();
person.setSalary(old+1000); //增加1000
return person; //返回原对象
}).collect(Collectors.toList()).forEach(System.out::println);
归约(reduce)
- 把一个流缩减成一个值
- 作用:实现一个流的加法、乘法、求最值等计算
整个流的加法、乘法、最大值
List<Integer> list = Arrays.asList(1, 3, 2, 8, 11, 4);
Stream<Integer> stream = list.stream();
//整个流的加法
Optional<Integer> add = stream.reduce((x, y) -> x + y);
System.out.println(add.get());
//整个流的乘法
Optional<Integer> num = stream.reduce((x, y) -> x * y);
System.out.println(num.get());
//最大值
Optional<Integer> maxNumber = stream.reduce((x, y) -> x > y ? x : y);
System.out.println(maxNumber.get());
案例9:求所有员工的工资之和。
List<Person> personList = new ArrayList<Person>();
personList.add(new Person("z1",2000.0,18));
personList.add(new Person("z2",3200.0,15));
personList.add(new Person("z3",1500.0,27));
personList.add(new Person("z4",7000.0,36));
personList.add(new Person("z5",5000.0,22));
personList.add(new Person("z6",4200.0,42));
Stream<Person> personStream1 = personList.parallelStream();
Optional<Double> sum = personStream1
//指定选择操作薪资
.map(person -> person.getSalary())
.reduce((x, y) -> x + y);
System.out.println(sum.get());
案例10:最高工资
List<Person> personList = new ArrayList<Person>();
personList.add(new Person("z1",2000.0,18));
personList.add(new Person("z2",3200.0,15));
personList.add(new Person("z3",1500.0,27));
personList.add(new Person("z4",7000.0,36));
personList.add(new Person("z5",5000.0,22));
personList.add(new Person("z6",4200.0,42));
Stream<Person> personStream2 = personList.parallelStream();
Optional<Double> max = personStream2
//指定操作salary
.map(person -> person.getSalary())
.reduce((x, y) -> x > y ? x : y);
System.out.println(max.get());
收集(collect)
Stream流转List(toList)
List<Integer> list = Arrays.asList(1, 6, 3, 4, 6, 7, 9, 6, 20);
Stream<Integer> parallel1 = list.stream().parallel();
List<Integer> list1 = parallel1.collect(Collectors.toList());
list1.forEach(System.out::println);
Stream流转Set(toSet)
List<Integer> list = Arrays.asList(1, 6, 3, 4, 6, 7, 9, 6, 20);
Stream<Integer> parallel1 = list.stream().parallel();
Set<Integer> set = parallel2.collect(Collectors.toSet());
set.forEach(System.out::println);
Stream流转Map(toMap)
List<Person> personList = new ArrayList<Person>();
personList.add(new Person("z1", 2000.0, 18));
personList.add(new Person("z2", 3200.0, 15));
personList.add(new Person("z3", 1500.0, 27));
personList.add(new Person("z4", 7000.0, 36));
personList.add(new Person("z5", 5000.0, 22));
personList.add(new Person("z6", 4200.0, 42));
Stream<Person> personStream = personList.stream().parallel();
Map<String, Person> map =
personStream
// toMap(k, v)
.collect(Collectors.toMap(x -> x.getName(), y -> y));
// lambda遍历map
map.forEach(
(k, v) -> {
System.out.println("key=" + k + ",v=" + v);
});
案例11:求流的平均值
List<Integer> list2 = Arrays.asList(1, 2, 3, 4, 10);
Stream<Integer> integerStream = list2.stream();
Double averag = integerStream.collect(Collectors.averagingInt(x -> x));
System.out.println(averag);
排序(sorted)
排序注意点
- 排序不能用并行流,否则将失效
案例12:对纯数字进行排序
List<Integer> list = Arrays.asList(12, 5, 6, 3, 2, 9, 22, 17, 15, 13, 6, 5, 1);
Stream<Integer> integerStream1 = list.stream();
//1:对纯数字进行排序
integerStream1.sorted(Comparator.comparingInt(x->x)).forEach(System.out::println);
案例13:将对象的薪资属性进行排序
List<Person> personList = new ArrayList<Person>();
personList.add(new Person("z1",2000.0,18));
personList.add(new Person("z2",3200.0,15));
personList.add(new Person("z3",1500.0,27));
personList.add(new Person("z4",7000.0,36));
personList.add(new Person("z5",5000.0,22));
personList.add(new Person("z6",4200.0,42));
Stream<Person> stream = personList.stream();
stream.sorted(Comparator.comparing(el->el.getSalary())).forEach(System.out::println);
去重和限制(distinct、limit)
List<Integer> list = Arrays.asList(3, 6, 6, 2, 3, 1, 2, 9, 12, 15);
Stream<Integer> stream = list.stream();
stream
//去重
.distinct()
//分页,限制最多输出前几个
.limit(3)
.forEach(System.out::println);
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