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

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

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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