java8 lambda 求list最大值、最小值、平均值、求和、中位数、属性排序(空指针异常，空值排前、排后)、去重
import org.junit.Test;
import java.text.SimpleDateFormat;
import java.util.*;
import java.util.stream.Collectors;
import static java.util.Comparator.comparingLong;
import static java.util.stream.Collectors.*;
/**
* @Author:
* @Date: 2018/12/12 13:08
* @Description:
*/
public class test {
public static void main(String[] args) {
List<User> list = new ArrayList<>();
list.add(new User(21L, "张三"));
list.add(new User(25L, "李四"));
list.add(new User(22L, "王五"));
list.add(new User(19L, "赵柳"));
list.add(new User(32L, "王5"));
list.add(new User(29L, "王6"));
list.add(new User(21L, "王7"));
// 对象根据年龄属性升序排序
List<User> newList = list.stream().sorted(Comparator.comparing(User::getAge)).collect(toList());
// 对象根据年龄属性降序排序
List<User> newList = list.stream().sorted(Comparator.comparing(User::getAge).reversed()).collect(toList());
// 标识升序，再按创建日期降序
// List<BhAnnouncement> newList = announcementList.stream().sorted(Comparator.comparing(BhAnnouncement::getReadFlag).thenComparing(BhAnnouncement::getSendTime).reversed()).collect(toList());
// list遍历
newList.forEach(System.out::println);
// 平均数
double asDouble = list.stream().mapToLong(User::getAge).average().getAsDouble();
System.out.println("average:" + asDouble);
double avg = list.stream().collect(Collectors.averagingLong(User::getAge));
System.out.println("average:" + avg);
// 最大值
long asLong = list.stream().mapToLong(User::getAge).max().getAsLong();
System.out.println("max:" + asLong);
// 最小值
long asLong1 = list.stream().mapToLong(User::getAge).min().getAsLong();
System.out.println("min:" + asLong1);
// 求和
long sum1 = list.stream().mapToLong(User::getAge).sum();
System.out.println("sum:" + sum1);
// 提取对象属性生成list
List<Long> ids = list.stream().map(User::getAge).collect(toList());
System.out.println(ids);
// list升序排序
Collections.sort(ids);
System.out.println(ids);
// 生成中位数
Long j;
if (ids.size() % 2 == 0) {
j = (ids.get(ids.size() / 2 - 1) + ids.get(ids.size() / 2)) / 2;
System.out.println("中位数为" + j);
} else {
j = ids.get(ids.size() / 2);
System.out.println("中位数为" + j);
}
// list倒序排序
ids.sort(Comparator.reverseOrder());
System.out.println(ids);
//初始化：
Student student1 = new Student("1","2",90,new User("1","2",10,"11"),"");
Student student2 = new Student("2","3",840,new User("4","2",10,"11"),"");
Student student3 = new Student("3","4",80,new User("3","2",10,"11"),"");
Student student4 = new Student("4","7",90,new User("2","2",10,"11"),"");
List<Student> students = new ArrayList<>();
students.add(student1);
students.add(student2);
students.add(student3);
students.add(student4);
//根据对象的子对象中的字段排序
List<Student> studentList = students.stream()
.sorted(Comparator.comparing((Function<Student,String>)student -> student.getUser().getId()).reversed())
.collect(Collectors.toList());
// 根据对象的子对象中的字段排序 排序字段值为空，空值排在前面
List<Student> studentList1 = students.stream()
.sorted(Comparator.comparing((Function<Student,String>)student -> student.getUser().getId()
,Comparator.nullsLast(String::compareTo)).reversed())
.collect(Collectors.toList());
//根据对象的子对象中的字段排序 排序字段值为空，空值排在后面
List<Student> studentList2 = students.stream()
.sorted(Comparator.comparing((Function<Student,String>)student -> student.getUser().getId()
,Comparator.nullsFirst(String::compareTo)).reversed())
.collect(Collectors.toList());
// 去重
List<User> users = list.stream().collect(collectingAndThen(toCollection(() -> new TreeSet<>(comparingLong(User::getAge))), ArrayList::new));
System.out.println("去重:"+users);
/**
* List -> Map
* 需要注意的是：toMap 如果集合对象有重复的key，会报错Duplicate key ....
* apple1,apple12的id都为1。可以用 (k1,k2)->k1 来设置，如果有重复的key,则保留key1,舍弃key2
*/
Map<Long, User> userMap = list.stream().collect(Collectors.toMap(User::getAge, a -> a, (k1, k2) -> k1));
System.out.println(userMap);
//过滤出符合条件的数据
List<User> filterList = list.stream().filter(a -> a.getName().equals("李四")).collect(toList());
System.out.println("filterList:" + filterList);
List<Integer> list2 = Arrays.asList(1, 2, 3, 4, 5);
int sum = list2.stream().reduce(0, (acc, value) -> acc + value);
System.out.println(sum);
List<Integer> result = list2.stream().filter((value) -> value > 2).collect(toList());
result.forEach(System.out::println);
List<String> result2 = list2.stream().map(value -> String.format("String:%s", value)).collect(toList());
result2.forEach(System.out::println);
// 用于收集统计数据的状态对象，例如count，min，max，sum和平均。
IntSummaryStatistics stats = list2.stream().mapToInt((x) -> x).summaryStatistics();
System.out.println("Max : " + stats.getMax());
System.out.println("Min: " + stats.getMin());
System.out.println("Sun: " + stats.getSum());
System.out.println("Average : " + stats.getAverage());
System.out.println("Count : " + stats.getCount());
System.out.println("toString : " + stats.toString());
}
}
class User {
private Long age;
private String name;
public User(Long i, String s) {
this.age = i;
this.name = s;
}
public Long getAge() {
return age;
}
public void setAge(Long age) {
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
@Override
public String toString() {
return "User [age=" + age + ", name=" + name + "]";
}
}
List<Map> maps = Lists.newArrayList(
Maps.newHashMap("aa", 10, "bb", DateUtil.formatStringToDate("2020-07-30", "yyyy-MM-dd")),
Maps.newHashMap("aa", 10, "bb", DateUtil.formatStringToDate("2020-07-29", "yyyy-MM-dd")),
Maps.newHashMap("aa", 16, "bb", DateUtil.formatStringToDate("2020-07-28", "yyyy-MM-dd")),
Maps.newHashMap("aa", 16, "bb", DateUtil.formatStringToDate("2020-07-30", "yyyy-MM-dd")),
Maps.newHashMap("aa", 20, "bb", DateUtil.formatStringToDate("2020-07-31", "yyyy-MM-dd"))
);
// List<Map> 使用sorted排序时，单个字段排序没有问题, 但是使用thenComparing排序多个字段就会出现问题
maps = maps.stream().sorted(Comparator.comparing(a -> Long.parseLong(a.get("aa") + ""))).collect(Collectors.toList());
maps.forEach(System.err::println);
System.err.println("===================5======================");
// maps = maps.stream().sorted(Comparator.comparing(a -> Long.parseLong(a.get("aa") + "")).thenComparing(Comparator.comparing(a -> (Date) a.get("bb"), Comparator.reverseOrder()))).collect(Collectors.toList());
// 上面那种方式没法排序，但是直接使用list的sort排序却是可以的
Comparator<Map> aa = Comparator.comparing(a -> Long.parseLong(a.get("aa") + ""));
Comparator<Map> bb = Comparator.comparing(a -> (Date) a.get("bb"), Comparator.reverseOrder());
maps.sort(aa.thenComparing(bb));
maps.forEach(System.err::println);
//既然按照说是Java8的方式来排序，那就给一个按照java8的方式进行排序的代码吧
public class Test {
public static void main(String[] args) {
Map<String, Object> map = new HashMap<String, Object>();
map.put("name", "ZK");
map.put("age", 13);
Map<String, Object> map2 = new HashMap<String, Object>();
map2.put("name", "ZA");
map2.put("age", 15);
Map<String, Object> map3 = new HashMap<String, Object>();
map3.put("name", "CX");
map3.put("age", 20);
Map<String, Object> map4 = new HashMap<String, Object>();
map4.put("name", "CX");
map4.put("age", 18);
List<Map<String, Object>> list = new ArrayList<Map<String,Object>>();
list.add(map);
list.add(map2);
list.add(map3);
list.add(map4);
// 排序代码如下
List<Map<String, Object>> collect = list.stream().sorted(Comparator.comparing(Test::comparingByName)
.thenComparing(Comparator.comparing(Test::comparingByAge).reversed()))
.collect(Collectors.toList());
}
private static String comparingByName(Map<String, Object> map){
return (String) map.get("name");
}
private static Integer comparingByAge(Map<String, Object> map){
return (Integer) map.get("age");
}
用到的一些Java8的东西
主要首先是stream了，list.stream()这里是把map的List集合变成map的流
然后就是Test::comparingByName这种中间加::表示方法引用
其次就是关键的stream.sorted()方法，参数是传一个比较器Comparator，这里由JDK自带的Comparator.comparing工具方法可以帮你构建一个按照xx属性进行比较的比较器，默认是升序
然后是比较器Comparator支持thenComparing方法，表示按照一定的比较顺序把各个比较连接起来比较
其次是比较器Comparator的reversed方法，可以让比较器的原始顺序逆序，这也正好满足题主需要按照age逆序排列的要求
最后就是collect()方法，把流的数据按照一定的方式收集起来，参数是一个收集器collector，这里用的是JDK自带的工具方法Collectors.toList把流的数据收集为集合