Java8 lambda表达式 list操作：转map，分组、过滤、求和、最值、排序、去重

柔情只为你懂 2023-06-16 01:53 2阅读 0赞

演示代码如下：

package com.dalingjia.LambdaStream;
    
    import lombok.Data;
    
    import java.math.BigDecimal;
    
    /**
     * Created by Administrator on 2018/7/9.
     */
    @Data
    public class User {
    
        private Integer id;
        private String name;
        private Integer age;
        private BigDecimal account;
        /**
         * 年级
         */
        private Integer grade;
    
    
        public User(Integer id, String name, Integer age, BigDecimal account, Integer grade) {
            this.id = id;
            this.name = name;
            this.age = age;
            this.account = account;
            this.grade = grade;
        }
    
        @Override
        public String toString() {
            return "User{" +
                    "id=" + id +
                    ", name='" + name + '\'' +
                    ", age=" + age +
                    ", grade=" + grade +
                    '}';
        }
    }

测试类：

package com.dalingjia.LambdaStream;
    
    import java.math.BigDecimal;
    import java.util.ArrayList;
    import java.util.Comparator;
    import java.util.List;
    import java.util.Map;
    import java.util.stream.Collectors;
    
    /**
     * @author tanhq
     * @Description java 8 lambda表达式 list操作：转map，分组、过滤、求和、最值、排序、去重
     * @Date 2019/11/23 下午2:11
     * @Version 1.0
     **/
    public class ListOper {
    
        private static List<User> list = new ArrayList<>();
    
        private static void addArticles() {
            list.add(new User(1, "tanhq", 23, new BigDecimal(40000), 3));
            list.add(new User(2, "xiaohong", 33, new BigDecimal(20000), 2));
            list.add(new User(3, "maoge", 26, new BigDecimal(10000), 3));
            list.add(new User(4, "fanfan", 27, new BigDecimal(30000), 1));
            list.add(new User(5, "suning", 25, new BigDecimal(50000), 2));
    
        }
    
        public static void main(String[] args) {
            addArticles();
    
            //1,list转map
            Map<Integer, String> map = list.stream().collect(Collectors.toMap(User::getId, User::getName));
            for (Map.Entry<Integer, String> entry : map.entrySet()) {
                System.out.println(entry.getKey() + "---" + entry.getValue());
            }
    
            Map<Integer, User> map2 = list.stream().collect(Collectors.toMap(User::getId, a -> a));
            for (Map.Entry<Integer, User> entry : map2.entrySet()) {
                System.out.println(entry.getKey() + "---" + entry.getValue());
            }
            /**
             * 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 ....
             *  User1,User2的id都为1。
             *  可以用 (u1, u2) -> u1 来设置，如果有重复的key,则保留key1,舍弃key2
             */
            Map<Integer, User> map3 = list.stream().collect(Collectors.toMap(User::getId, a -> a, (u1, u2) -> u1));
            for (Map.Entry<Integer, User> entry : map3.entrySet()) {
                System.out.println(entry.getKey() + "---" + entry.getValue());
            }
    
            //2,list分组
            Map<Integer, List<User>> groupByGrade = list.stream().collect(Collectors.groupingBy(User::getGrade));
            for (Map.Entry<Integer, List<User>> entry : groupByGrade.entrySet()) {
                System.out.println(entry.getKey());
                for (User user : entry.getValue()) {
                    System.out.println(user.toString());
                }
            }
    
            //3,list过滤
            List<User> filterList = list.stream().filter(user -> !user.getName().equals("xiaohong")).collect(Collectors.toList());
            filterList.stream().forEach(user -> System.out.println("filterList：" + user.toString()));
    
            //4,list求和
                //a,基本类型求和
            int sum = list.stream().mapToInt(User::getAge).sum();
                //b,大数类型求和
            BigDecimal result = list.stream().map(user -> user.getAccount()).reduce(BigDecimal.ZERO, BigDecimal::add);
    
            //5,list最值
            Integer minAge = list.stream().map(User::getAge).min(Integer::compareTo).get();
            Integer maxAge = list.stream().map(user -> user.getAge()).max(Integer::compareTo).get();
    
            //6,list排序,多字段排序，根据grade，年龄，账号排序
            list.sort(Comparator.comparing(User::getGrade).thenComparing(User::getAge).thenComparing(User::getAccount));
    
            //7,list去重
            List<Integer> idList = new ArrayList<Integer>();
            idList.add(1);
            idList.add(1);
            idList.add(2);
            List<Integer> distinctIdList = idList.stream().distinct().collect(Collectors.toList());
    
        }
    }

[Java8之Consumer、Supplier、Predicate、Function][Java8_Consumer_Supplier_Predicate_Function]

[Java8_Consumer_Supplier_Predicate_Function]: https://www.cnblogs.com/SIHAIloveYAN/p/11288064.html