Stream实现List和Map互转总结(map集合转list集合)
前言
本篇介绍Stream流List和Map互转，同时在转换过程中遇到的问题分析。
一、Map转List
1.1 分析
按照默认顺序
mapToList.entrySet().stream().map(a -> new User(a.getKey(), a.getValue())).collect(Collectors.toList()); 复制代码
根据key排序
mapToList.entrySet().stream().sorted(Comparator.comparing(a -> a.getKey())).map(a -> new User(a.getKey(),a.getValue())).collect(Collectors.toList()); 复制代码
根据key排序
mapToList.entrySet().stream().sorted(Map.Entry.comparingByKey()).map(a -> new User(a.getKey(),a.getValue())).collect(Collectors.toList()); 复制代码
根据key倒序排序
mapToList.entrySet().stream().sorted(Map.Entry.comparingByKey(Comparator.reverseOrder())).map(a -> new User(a.getKey(),a.getValue())).collect(Collectors.toList()); 复制代码
根据value排序
mapToList.entrySet().stream().sorted(Comparator.comparing(Map.Entry::getValue)).map(a -> new User(a.getKey(), a.getValue())).collect(Collectors.toList()); 复制代码
根据value倒序排序
mapToList.entrySet().stream().sorted(Collections.reverseOrder(Map.Entry.comparingByValue())).map(a -> new User(a.getKey(),a.getValue())).collect(Collectors.toList()); 复制代码
1.2 完整代码
实体类
/** * @author lilinchao * @date 2021/7/28 * @description 1.0 **/ public class User { private Integer id; private String name; public User() { } public User(Integer id, String name) { this.id = id; this.name = name; } public Integer getId() { return id; } public void setId(Integer id) { this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; } @Override public String toString() { return "User{" + "id=" + id + ", name='" + name + '\'' + '}'; } } 复制代码Map转List代码
import java.util.*; import java.util.stream.Collectors; /** * @author lilinchao * @date 2021/7/28 * @description 1.0 **/ public class StreamMapToList { /** * 数据初始化 */ private static final Map<Integer, String> mapToList; static { mapToList = new HashMap<Integer, String>(); mapToList.put(1003, "Thymee"); mapToList.put(1001, "Leefs"); mapToList.put(1002, "Jeyoo"); } public static void main(String ages[]){ List<User> userList = defaultOrder(); System.out.println(userList); List<User> userList2and = orderByKeyMethodOne(); System.out.println(userList2and); List<User> userList3and = orderByKeyMethodTwo(); System.out.println(userList3and); List<User> userList4and = reverseOrderByKey(); System.out.println(userList4and); List<User> userList5and = orderByValue(); System.out.println(userList5and); List<User> userList6and = reverseOrderByValue(); System.out.println(userList6and); } /** * 按照默认顺序 */ public static List<User> defaultOrder(){ List<User> userList = mapToList.entrySet().stream().map(a -> new User(a.getKey(), a.getValue())).collect(Collectors.toList()); return userList; } /** *根据key排序，方法1 */ public static List<User> orderByKeyMethodOne(){ List<User> userList = mapToList.entrySet().stream().sorted(Comparator.comparing(a -> a.getKey())).map(a -> new User(a.getKey(),a.getValue())).collect(Collectors.toList()); return userList; } /** *根据key排序，方法2 */ public static List<User> orderByKeyMethodTwo(){ List<User> userList = mapToList.entrySet().stream().sorted(Map.Entry.comparingByKey()).map(a -> new User(a.getKey(),a.getValue())).collect(Collectors.toList()); return userList; } /** *根据key倒序排序 */ public static List<User> reverseOrderByKey(){ List<User> userList = mapToList.entrySet().stream().sorted(Map.Entry.comparingByKey(Comparator.reverseOrder())).map(a -> new User(a.getKey(),a.getValue())).collect(Collectors.toList()); return userList; } /** * 根据value排序 */ public static List<User> orderByValue(){ List<User> userList = mapToList.entrySet().stream().sorted(Comparator.comparing(Map.Entry::getValue)).map(a -> new User(a.getKey(), a.getValue())).collect(Collectors.toList()); return userList; } /** *根据value倒序排序 */ public static List<User> reverseOrderByValue(){ List<User> userList = mapToList.entrySet().stream().sorted(Collections.reverseOrder(Map.Entry.comparingByValue())).map(a -> new User(a.getKey(),a.getValue())).collect(Collectors.toList()); return userList; } } 复制代码运行结果
[User{id=1001, name='Leefs'}, User{id=1002, name='Jeyoo'}, User{id=1003, name='Thymee'}] [User{id=1001, name='Leefs'}, User{id=1002, name='Jeyoo'}, User{id=1003, name='Thymee'}] [User{id=1001, name='Leefs'}, User{id=1002, name='Jeyoo'}, User{id=1003, name='Thymee'}] [User{id=1003, name='Thymee'}, User{id=1002, name='Jeyoo'}, User{id=1001, name='Leefs'}] [User{id=1002, name='Jeyoo'}, User{id=1001, name='Leefs'}, User{id=1003, name='Thymee'}] [User{id=1003, name='Thymee'}, User{id=1001, name='Leefs'}, User{id=1002, name='Jeyoo'}] 复制代码二、List转Map
2.1 分析
指定key-value，value是对象中的某个属性值
userList.stream().collect(Collectors.toMap(User::getId, User::getName)); 复制代码
指定key-value，value是对象本身
User->User 是一个返回本身的lambda表达式
userList.stream().collect(Collectors.toMap(User::getId, User->User)); 复制代码
指定key-value，value是对象本身
Function.identity()是简洁写法，也是返回对象本身
userList.stream().collect(Collectors.toMap(User::getId, Function.identity())); 复制代码
指定key-value,key 冲突的解决办法
(key1,key2)->key2:第二个key覆盖第一个key (key1,key2)->key1:保留第一个key
userList.stream().collect(Collectors.toMap(User::getId, Function.identity(),(key1,key2)->key2)); 复制代码
2.2 完整代码
import java.util.Arrays; import java.util.List; import java.util.Map; import java.util.function.Function; import java.util.stream.Collectors; /** * @author lilinchao * @date 2021/7/28 * @description 1.0 **/ public class StreamListToMap { private static final List<User> userList; static { userList = Arrays.asList( new User(1003,"keko"), new User(1001,"jeek"), new User(1002,"mack") ); } public static void main(String ages[]){ Map<Integer, String> map = method01(); System.out.println(map); Map<Integer, User> map2and = method02(); System.out.println(map2and); Map<Integer, User> map3and = method03(); System.out.println(map3and); Map<Integer, User> map4and = method04(); System.out.println(map4and); } /** * 指定key-value，value是对象中的某个属性值 */ public static Map<Integer,String> method01(){ Map<Integer, String> userMap = userList.stream().collect(Collectors.toMap(User::getId, User::getName)); return userMap; } /** *指定key-value，value是对象本身，User->User 是一个返回本身的lambda表达式 */ public static Map<Integer,User> method02(){ Map<Integer, User> userMap = userList.stream().collect(Collectors.toMap(User::getId, User->User)); return userMap; } /** * 指定key-value，value是对象本身，Function.identity()是简洁写法，也是返回对象本身 */ public static Map<Integer,User> method03(){ Map<Integer, User> userMap = userList.stream().collect(Collectors.toMap(User::getId, Function.identity())); return userMap; } /** * 指定key-value,key 冲突的解决办法 * (key1,key2)->key2:第二个key覆盖第一个key * (key1,key2)->key1:保留第一个key */ public static Map<Integer,User> method04(){ Map<Integer, User> userMap = userList.stream().collect(Collectors.toMap(User::getId, Function.identity(),(key1,key2)->key2)); return userMap; } } 复制代码运行结果
{1001=jeek, 1002=mack, 1003=keko} {1001=User{id=1001, name='jeek'}, 1002=User{id=1002, name='mack'}, 1003=User{id=1003, name='keko'}} {1001=User{id=1001, name='jeek'}, 1002=User{id=1002, name='mack'}, 1003=User{id=1003, name='keko'}} {1001=User{id=1001, name='jeek'}, 1002=User{id=1002, name='mack'}, 1003=User{id=1003, name='keko'}} 复制代码三、List转Map常见问题
3.1 常见问题
问题一
报错Duplicate key xxxx
该问题是因为在生成Map集合时key值重复造成的
解决方案
1. 后面的value覆盖前面的value
userList.stream().collect(Collectors.toMap(User::getId, User::getName, (key1, key2) -> key2)); 复制代码
也可以保留前面的value,将key2换成key1即可
2. value进行拼接
userList.stream().collect(Collectors.toMap(User::getId, User::getName, (key1, key2) -> key1+","+key2)); 复制代码
3. key重复时，返回集合
userList2and.stream().collect(Collectors.toMap(User::getId, p -> { List<String> getNameList = new ArrayList<>(); getNameList.add(p.getName()); return getNameList; }, (List<String> value1, List<String> value2) -> { value1.addAll(value2); return value1; } )); 复制代码问题二
报错：Exception in thread "main" java.lang.NullPointerException
该问题是因为在存入Map集合时value值为null
解决方案
在转换流中加上判空，即便value为空,依旧输出。(与上面方法三相同)
3.2 完整代码
import java.util.ArrayList; import java.util.Arrays; import java.util.List; import java.util.Map; import java.util.stream.Collectors; /** * @author lilinchao * @date 2021/7/28 * @description 1.0 **/ public class StreamListToMapAnalyze { private static final List<User> userList; private static final List<User> userList2and; static { //重复的key userList = Arrays.asList( new User(1003,"keko"), new User(1001,"jeek"), new User(1001,"teek"), new User(1002,"mack") ); //重复的key,value为null userList2and = Arrays.asList( new User(1003,"keko"), new User(1001,"jeek"), new User(1001,"teek"), new User(1002,null) ); } public static void main(String ages[]){ Map<Integer, String> map = keyRepeatMethod01(); System.out.println(map); Map<Integer, String> map2and = keyRepeatMethod02(); System.out.println(map2and); Map<Integer,List<String>> map3and = keyRepeatMethod03(); System.out.println(map3and); } /** * key重复时，后面的value覆盖前面的value */ public static Map<Integer,String> keyRepeatMethod01(){ Map<Integer, String> userMap = userList.stream().collect(Collectors.toMap(User::getId, User::getName, (key1, key2) -> key2)); return userMap; } /** * key重复时，value进行拼接 */ public static Map<Integer,String> keyRepeatMethod02(){ Map<Integer, String> userMap = userList.stream().collect(Collectors.toMap(User::getId, User::getName, (key1, key2) -> key1+","+key2)); return userMap; } /** * key重复时，返回集合 * value值为空，在转换流中加上判空，即便value为空,依旧输出 */ public static Map<Integer,List<String>> keyRepeatMethod03(){ Map<Integer, List<String>> userListMap = userList2and.stream().collect(Collectors.toMap(User::getId, p -> { List<String> getNameList = new ArrayList<>(); getNameList.add(p.getName()); return getNameList; }, (List<String> value1, List<String> value2) -> { value1.addAll(value2); return value1; } )); return userListMap; } } 复制代码运行结果
{1001=teek, 1002=mack, 1003=keko} {1001=jeek,teek, 1002=mack, 1003=keko} {1001=[jeek, teek], 1002=[null], 1003=[keko]}
作者：Leefs
链接：https://juejin.cn/post/7036240821607202824
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