Java 集合之间的转换

java集合 list转map一些常用的方式（Stream流，，，）

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前言

这片文章是用来整理开发中经常用到的一些集合之间的转换方法，作为笔记，提高开发效率，有需要的小伙伴可以参考一下，这片文章会慢慢补充完整。

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提示：以下是本篇文章正文内容，下面案例可供参考

一、List转换为Map？

示例：代码案例（JAVA版）。

public class People (private Integer id;private String name;private Integer ageprivate String address;//TODO 构造方法和get和set方法省略
)

final static List<People> peopleList = new ArrayList<People>();// 初始化集合数据static {People p1 = new People(0001, "张三", 12, "江苏南京");People p2 = new People(0002, "李四", 14, "上海");People p3 = new People(0003, "王二", 11, "浙江台州");People p4 = new People(0004, "李五", 12, "河南郑州");peopleList.add(p1);peopleList.add(p2);peopleList.add(p3);peopleList.add(p4);
}

1.1、List<Object>转化为Map<Integer,String>

Map<Integer, String> map = peopleList.stream().collect(Collectors.toMap
(People::getId, People::getAddress, (value1, value2) -> value1));

1.2 、List<Object>转化为Map<Integer,Object>

Map<Integer, People> map = peopleList.stream().collect(Collectors.toMap
(People::getId, each -> each, (value1, value2) -> value1));

1.3、List<Object>转化为Map<Integer,List<Object>>

Map<Integer, List<People>> map = peopleList.stream().collect(Collectors
.groupingBy(People::getId));

1.4 List<Object>转化为Map<Integer,List<String>>

Map<Integer, List<String>> map3 = peopleList.stream().collect(Collectors.
toMap(People::getId, each -> Collections.singletonList(each.getName()), 
(value1, value2) -> {List<String> union = new ArrayList<>(value1);union.addAll(value2);return union;
}));

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1.5、 List<Map<String,Object>> 转化为Map<String,Object>

final static List<Map<String, Object>> mapStudentList = new ArrayList<>();
public static void main(String[] args) {Map<String, Object> map4 = mapStudentList.stream().collect(Collectors.toMap(each -> Objects.toString(each.get("id"), ""), each -> each.get("student"), (key1, key2) -> key1));}/*** 初始化集合数据*/static {Student stu1 = new Student("0001", "张三", 12, "江苏南京");Student stu2 = new Student("0002", "李四", 14, "江苏无锡");Student stu3 = new Student("0003", "王二", 11, "浙江台州");Student stu4 = new Student("0004", "李五", 12, "浙江温州");Map<String, Object> map1 = new HashMap<>();map1.put("id", "0001");map1.put("student", stu1);Map<String, Object> map2 = new HashMap<>();map2.put("id", "0002");map2.put("student", stu2);Map<String, Object> map3 = new HashMap<>();map3.put("id", "0003");map3.put("student", stu3);Map<String, Object> map4 = new HashMap<>();map4.put("id", "0004");map4.put("student", stu4);mapStudentList.add(map1);mapStudentList.add(map2);mapStudentList.add(map3);mapStudentList.add(map4);}

1.6、List<Map<String,String>> 转化为Map<String,Map<String,String>>

    final static List<Map<String, String>> listMapList = new ArrayList<>();public static void main(String[] args) {Map<String, Map<String, String>> map5 = listMapList.stream().collect(Collectors.toMap(each -> each.get("id"), each -> each, (key1, key2) -> key1));System.out.println("map5 = " + map5);}/*** 初始化集合数据*/static {Map<String, String> map1 = new HashMap<>();map1.put("id", "0001");map1.put("name", "张三");map1.put("age", "12");map1.put("address", "江苏南京");Map<String, String> map2 = new HashMap<>();map2.put("id", "0002");map2.put("name", "李四");map2.put("age", "14");map2.put("address", "江苏无锡");Map<String, String> map3 = new HashMap<>();map3.put("id", "0003");map3.put("name", "王二");map3.put("age", "11");map3.put("address", "浙江台州");Map<String, String> map4 = new HashMap<>();map4.put("id", "0004");map4.put("name", "李五");map4.put("age", "12");map4.put("address", "浙江温州");listMapList.add(map1);listMapList.add(map2);listMapList.add(map3);listMapList.add(map4);}

1.7、List<Map<String,String>> 转化为Map<String,String>

     final static List<Map<String, String>> listmapstringlist = new ArrayList<>();public static void main(String[] args) {Map<String, String> map6 = listmapstringlist.stream().collect(Collectors.toMap(each -> each.get("id"), each -> each.get("name"), (key1, key2) -> key1));}/*** 初始化集合数据*/static {Map<String, String> map1 = new HashMap<>();map1.put("id", "0001");map1.put("name", "张三");map1.put("age", "12");map1.put("address", "江苏南京");Map<String, String> map2 = new HashMap<>();map2.put("id", "0002");map2.put("name", "李四");map2.put("age", "14");map2.put("address", "江苏无锡");Map<String, String> map3 = new HashMap<>();map3.put("id", "0003");map3.put("name", "王二");map3.put("age", "11");map3.put("address", "浙江台州");Map<String, String> map4 = new HashMap<>();map4.put("id", "0004");map4.put("name", "李五");map4.put("age", "12");map4.put("address", "浙江温州");listmapstringlist.add(map1);listmapstringlist.add(map2);listmapstringlist.add(map3);listmapstringlist.add(map4);}

总结

先总结这么多吧；
日积月累，小溪汇大海