Stream常用操作
一 、list 加法
BigDecimal 类型:
BigDecimal bb =list.stream().map(Plan::getAmount).reduce(BigDecimal.ZERO,BigDecimal::add);
int、double、long 类型:
double max = list.stream().mapToDouble(User::getHeight).sum();
二、合并list
public static void main(String[] args) throws InterruptedException, ExecutionException {
List<Integer> listA = Arrays.asList(new Integer[]{1, 2});
List<Integer> listB = Arrays.asList(new Integer[]{3, 4});
List<Integer> res = Stream.of(listA, listB).flatMap(Collection::stream).collect(Collectors.toList());
System.out.println(res.size());
res.forEach(System.out::println);
}
三、将list中对象属性取出放到另一个list
list result = list.stream().map(Student:: getName).collect(collectors.tolist());
四 、 代码
import java.io.Serializable; import java.util.ArrayList; import java.util.Comparator; import java.util.Map; import java.util.concurrent.ConcurrentHashMap; import java.util.function.Function; import java.util.function.Predicate; import java.util.stream.Collectors; public class User implements Serializable { private static final long serialVersionUID = 1220193146286930256L; private String id; private String name; private String groupId; private int num; private int age; public String getId() { return id; } public void setId(String id) { this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; } public String getGroupId() { return groupId; } public void setGroupId(String groupId) { this.groupId = groupId; } public int getNum() { return num; } public void setNum(int num) { this.num = num; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } public User(String id, String name, String groupId, int num, int age) { this.id = id; this.name = name; this.groupId = groupId; this.num = num; this.age = age; } @Override public String toString() { return "User{" + "id='" + id + '\'' + ", name='" + name + '\'' + ", groupId='" + groupId + '\'' + ", num=" + num + ", age=" + age + '}'; } private static <T> Predicate<T> distinctByKey(Function<? super T, ?> keyExtractor) { Map<Object, Boolean> seen = new ConcurrentHashMap<>(); return t -> seen.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null; } public static void main(String[] args) { ArrayList<User> list = new ArrayList<>(10); list.add(new User("1", "1号", "A", 10, 10)); list.add(new User("2", "2号", "A", 20, 10)); list.add(new User("3", "3号", "E", 50, 12)); list.add(new User("4", "4号", "B", 60, 20)); list.add(new User("5", "5号", "C", 80, 40)); list.add(new User("6", "6号", "B", 220, 20)); list.add(new User("7", "7号", "A", 110, 2)); list.add(new User("8", "8号", "D", 110, 68)); //去重复获取组 System.out.println("----------去重复获取组-------------------"); list.stream().filter(distinctByKey(User::getGroupId)).map(User::getGroupId).forEach(s -> { System.out.println(s); }); //从第2个开始(下标) System.out.println("-------------从第2个开始----------------"); list.stream().skip(2).forEach(s -> { System.out.println(s); }); //取10以内的 System.out.println("-------------取10以内的----------------"); list.stream().filter(user -> user.getAge() <= 10).forEach(s -> { System.out.println(s); }); //取最大 System.out.println("----------------取最大-------------"); User user = list.stream().max(Comparator.comparing(User::getNum)).get(); System.out.println(user); //分组求合 System.out.println("---------------分组求合--------------"); list.stream().collect(Collectors.groupingBy(User::getGroupId, Collectors.summingInt(User::getNum))).forEach((k, v) -> { System.out.println("K:" + k + "-V:" + v); }); //分组取组内最大 System.out.println("--------------分组取组内最大---------------"); list.stream().collect(Collectors.groupingBy(User::getGroupId, Collectors.maxBy(Comparator.comparingInt(User::getNum)))).forEach((k, v) -> { System.out.println("K:" + k + "-V:" + v.get()); }); //分组取组内最小 System.out.println("------------分组取组内最小-----------------"); list.stream().collect(Collectors.groupingBy(User::getGroupId, Collectors.minBy(Comparator.comparingInt(User::getNum)))).forEach((k, v) -> { System.out.println("K:" + k + "-V:" + v.get()); }); //分组聚合多列 System.out.println("---------------分组聚合多列计算--------------"); list.stream().collect(Collectors.groupingBy(User::getGroupId, Collectors.collectingAndThen(Collectors.toList(), users -> { Double[] ints = new Double[2]; Double asDouble1 = users.stream().mapToInt(User::getNum).average().getAsDouble(); Double asDouble2 = users.stream().mapToInt(User::getAge).average().getAsDouble(); ints[0] = asDouble1; ints[1] = asDouble2; return ints; }))).forEach((k, v) -> { System.out.println("K:" + k + "-V:" + v[0] + "," + v[1]); }); //分组后组内排序 System.out.println("---------------分组后组内排序--------------"); list.stream().collect(Collectors.groupingBy(User::getGroupId, Collectors.collectingAndThen(Collectors.toList(), users -> { users.sort(Comparator.comparing(User::getNum).reversed()); return users; }))).forEach((k, v) -> { System.out.println("k:" + k); System.out.println("v----->:"); v.forEach(s -> { System.out.println(s); }); }); } }
运行结果
----------去重复获取组-------------------
A
E
B
C
D
-------------从第2个开始----------------
User{id=‘3’, name=‘3号’, groupId=‘E’, num=50, age=12}
User{id=‘4’, name=‘4号’, groupId=‘B’, num=60, age=20}
User{id=‘5’, name=‘5号’, groupId=‘C’, num=80, age=40}
User{id=‘6’, name=‘6号’, groupId=‘B’, num=220, age=20}
User{id=‘7’, name=‘7号’, groupId=‘A’, num=110, age=2}
User{id=‘8’, name=‘8号’, groupId=‘D’, num=110, age=68}
-------------取10以内的----------------
User{id=‘1’, name=‘1号’, groupId=‘A’, num=10, age=10}
User{id=‘2’, name=‘2号’, groupId=‘A’, num=20, age=10}
User{id=‘7’, name=‘7号’, groupId=‘A’, num=110, age=2}
----------------取最大-------------
User{id=‘6’, name=‘6号’, groupId=‘B’, num=220, age=20}
---------------分组求合--------------
K:A-V:140
K:B-V:280
K:C-V:80
K:D-V:110
K:E-V:50
--------------分组取组内最大---------------
K:A-V:User{id=‘7’, name=‘7号’, groupId=‘A’, num=110, age=2}
K:B-V:User{id=‘6’, name=‘6号’, groupId=‘B’, num=220, age=20}
K:C-V:User{id=‘5’, name=‘5号’, groupId=‘C’, num=80, age=40}
K:D-V:User{id=‘8’, name=‘8号’, groupId=‘D’, num=110, age=68}
K:E-V:User{id=‘3’, name=‘3号’, groupId=‘E’, num=50, age=12}
------------分组取组内最小-----------------
K:A-V:User{id=‘1’, name=‘1号’, groupId=‘A’, num=10, age=10}
K:B-V:User{id=‘4’, name=‘4号’, groupId=‘B’, num=60, age=20}
K:C-V:User{id=‘5’, name=‘5号’, groupId=‘C’, num=80, age=40}
K:D-V:User{id=‘8’, name=‘8号’, groupId=‘D’, num=110, age=68}
K:E-V:User{id=‘3’, name=‘3号’, groupId=‘E’, num=50, age=12}
---------------分组聚合多列计算--------------
K:A-V:46.666666666666664,7.333333333333333
K:B-V:140.0,20.0
K:C-V:80.0,40.0
K:D-V:110.0,68.0
K:E-V:50.0,12.0
---------------分组后组内排序--------------
Disconnected from the target VM, address: ‘127.0.0.1:55118’, transport: ‘socket’
k:A
v----->:
User{id=‘7’, name=‘7号’, groupId=‘A’, num=110, age=2}
User{id=‘2’, name=‘2号’, groupId=‘A’, num=20, age=10}
User{id=‘1’, name=‘1号’, groupId=‘A’, num=10, age=10}
k:B
v----->:
User{id=‘6’, name=‘6号’, groupId=‘B’, num=220, age=20}
User{id=‘4’, name=‘4号’, groupId=‘B’, num=60, age=20}
k:C
v----->:
User{id=‘5’, name=‘5号’, groupId=‘C’, num=80, age=40}
k:D
v----->:
User{id=‘8’, name=‘8号’, groupId=‘D’, num=110, age=68}
k:E
v----->:
User{id=‘3’, name=‘3号’, groupId=‘E’, num=50, age=12}