Java Stream API 实战-优快云博客

本文链接：https://blog.youkuaiyun.com/summer_du/article/details/116565059

一、创建对象实例

/**
 * 学生实体类
 */
@NoArgsConstructor
@AllArgsConstructor
@Data
public class Student {
    private String name;
    private Integer age;
    private String sex;
}

		List<Student> list = new ArrayList<>();
		list.add(new Student("张三", 21, "男"));
		list.add(new Student("李四", 20, "女"));
		list.add(new Student("王五", 18, "男"));
		list.add(new Student("王宝强", 19, "男"));
		list.add(new Student("宋老虎", 22, "男"));
		list.add(new Student("唐僧", 20, "男"));
		list.add(new Student("张飞", 23, "女"));
		list.add(new Student("刘备", 21, "女"));
		list.add(new Student("关羽", 19, "男"));
		list.add(new Student("萨尔", 21, "女"));

二、中间操作

1、过滤

1.1 filter

//获取姓张的学生集合
List<Student> students = list.stream().filter(item -> item.getName().startsWith("张")).collect(Collectors.toList());
students.forEach(item-> System.out.println(item));

Student(name=张三, age=21, sex=男)
Student(name=张飞, age=23, sex=女)

1.2 distinct

//获取年龄集合，然后去重排序
Stream<Integer> distinct = list.stream().map(item -> item.getAge()).collect(Collectors.toList()).stream().distinct().sorted();
distinct.forEach(item-> System.out.println(item + "岁"));

18岁
19岁
20岁
21岁
22岁
23岁

1.3 limit

//获取前3个。长度不足，则获取实际长度
List<Student> students = list.stream().limit(3).collect(Collectors.toList());
students.forEach(item-> System.out.println(item));

Student(name=张三, age=21, sex=男)
Student(name=李四, age=20, sex=女)
Student(name=王五, age=18, sex=男)

1.4 sorted

//按年龄排序
List<Student> students = list.stream().sorted((a, b) -> a.getAge() - b.getAge()).collect(Collectors.toList());
students.forEach(item-> System.out.println(item));

Student(name=王五, age=18, sex=男)
Student(name=王宝强, age=19, sex=男)
Student(name=关羽, age=19, sex=男)
Student(name=李四, age=20, sex=女)
Student(name=唐僧, age=20, sex=男)
Student(name=张三, age=21, sex=男)
Student(name=刘备, age=21, sex=女)
Student(name=萨尔, age=21, sex=女)
Student(name=宋老虎, age=22, sex=男)
Student(name=张飞, age=23, sex=女)

1.5 skip

//跳过前几条数据
List<Student> students = list.stream().skip(6L).collect(Collectors.toList());
students.forEach(item-> System.out.println(item));

Student(name=张飞, age=23, sex=女)
Student(name=刘备, age=21, sex=女)
Student(name=关羽, age=19, sex=男)
Student(name=萨尔, age=21, sex=女)

2、映射

2.1 map

//获取名字集合
List<String> names= list.stream().map(Student::getName).collect(Collectors.toList());
names.forEach(item-> System.out.println(item));

张三
李四
王五
王宝强
宋老虎
唐僧
张飞
刘备
关羽
萨尔

2.2 flatMap

flatMap是将一个流中的每个值都转成一个个流，然后再将这些流扁平化成为一个流

//名字包含的汉字去重
List<String> collect = list.stream().map(item -> item.getName().split("")).flatMap(Arrays::stream).distinct().collect(Collectors.toList());
collect.forEach(item-> System.out.print(item+","));

张,三,李,四,王,五,宝,强,宋,老,虎,唐,僧,飞,刘,备,关,羽,萨,尔,

三、终端操作

1、查找

1.1 allMatch

//年龄是否都大于20
boolean b = list.stream().allMatch(item -> item.getAge() > 20);
System.out.println(b);

false

1.2 anyMatch

//有一个或者多个匹配
boolean b = list.stream().anyMatch(item -> item.getName().equals("李四"));
System.out.println(b);

true

1.3 noneMathch

//都不满足
boolean b = list.stream().noneMathch(item -> item.getName().equals("李四"));
System.out.println(b);

false

1.4 findFirst

//获取第一个元素
Optional<Student> b = list.stream().filter(item->item.getSex().equals("女")).findFirst();
System.out.println(b);

Optional[Student(name=李四, age=20, sex=女)]

1.5 findAny

//获取任意一个元素
Optional<Student> b = list.stream().filter(item->item.getSex().equals("女")).findAny();
System.out.println(b);

Optional[Student(name=李四, age=20, sex=女)]

2、规约

2.1 reduce

//-------求年龄总和-----------

//之前方法
Integer sum = list.stream().mapToInt(Student::getAge).sum();
//规约操作
Integer reduce = list.stream().map(Student::getAge).reduce(0, (a, b) -> a + b);
//进一步简化
Integer reduce1 = list.stream().map(Student::getAge).reduce(0, Integer::sum);
//无初始值
Optional<Integer> reduce2 = list.stream().map(Student::getAge).reduce(Integer::sum);
System.out.println("sum:"+sum);
System.out.println("reduce:"+reduce);
System.out.println("reduce1:"+reduce1);
System.out.println("reduce2:"+reduce2);

sum:204
reduce:204
reduce1:204
reduce2:Optional[204]

3、收集

3.1 规约

//学生总数
Long collect = list.stream().collect(Collectors.counting());
//最大年龄
Optional<Student> collect1 = list.stream().collect(Collectors.maxBy((a, b) -> a.getAge() - b.getAge()));
//最小年龄
Optional<Student> collect2 = list.stream().collect(Collectors.minBy(Comparator.comparingInt(Student::getAge)));
//年龄总和
Integer collect3 = list.stream().collect(Collectors.summingInt(Student::getAge));
//平均年龄
Double collect4 = list.stream().collect(Collectors.averagingInt(Student::getAge));
//名字拼接
String collect5 = list.stream().map(Student::getName).collect(Collectors.joining(","));
System.out.println("学生总数：" + collect);
System.out.println("最大年龄：" + collect1);
System.out.println("最小年龄：" + collect2);
System.out.println("年龄总和：" + collect3);
System.out.println("平均年龄：" + collect4);
System.out.println("名字拼接：" + collect5);

学生总数：10
最大年龄：Optional[Student(name=张飞, age=23, sex=女)]
最小年龄：Optional[Student(name=王五, age=18, sex=男)]
年龄总和：204
平均年龄：20.4
名字拼接：张三,李四,王五,王宝强,宋老虎,唐僧,张飞,刘备,关羽,萨尔

3.2 分组

//按性别分组
Map<String, List<Student>> collect = list.stream().collect(Collectors.groupingBy(Student::getSex));
collect.keySet().forEach(item-> System.out.println("key:"+ item + ";value:" +collect.get(item)));

key:女;value:[Student(name=李四, age=20, sex=女), Student(name=张飞, age=23, sex=女), Student(name=刘备, age=21, sex=女), Student(name=萨尔, age=21, sex=女)]
key:男;value:[Student(name=张三, age=21, sex=男), Student(name=王五, age=18, sex=男), Student(name=王宝强, age=19, sex=男), Student(name=宋老虎, age=22, sex=男), Student(name=唐僧, age=20, sex=男), Student(name=关羽, age=19, sex=男)]

//按性别分组统计个数
Map<String, Long> collect = list.stream().collect(Collectors.groupingBy(Student::getSex, Collectors.counting()));
collect.keySet().forEach(item -> System.out.println("key:" + item + ";value:" + collect.get(item)));

key:女;value:4
key:男;value:6

3.3 分区

//按姓张和非姓张分组。key只有true和false两种
Map<Boolean, List<Student>> collect = list.stream().collect(Collectors.partitioningBy(item->item.getName().startsWith("张")));
collect.keySet().forEach(item-> System.out.println("key:"+ item + ";value:" +collect.get(item)));

key:false;value:[Student(name=李四, age=20, sex=女), Student(name=王五, age=18, sex=男), Student(name=王宝强, age=19, sex=男), Student(name=宋老虎, age=22, sex=男), Student(name=唐僧, age=20, sex=男), Student(name=刘备, age=21, sex=女), Student(name=关羽, age=19, sex=男), Student(name=萨尔, age=21, sex=女)]
key:true;value:[Student(name=张三, age=21, sex=男), Student(name=张飞, age=23, sex=女)]