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Java使用Stream流的Lambda语法进行List转Map的操作方式

作者：Armyyyyy丶

这篇文章主要介绍了Java使用Stream流的Lambda语法进行List转Map的操作方式,具有很好的参考价值,希望对大家有所帮助,如有错误或未考虑完全的地方,望不吝赐教

注：标题的<>被替换成了《》，标题带有<>会因为Bug被吞。

背景

在平时开发过程中难免会碰到有些时候需要将一个List转成Map又或者是将Map转成List，我们可以采用粗暴的方式来进行转换，但这样一是不够优雅，二是数据过多的时候性能不是很高。

Lambda如果平时经常使用就可以知道这种语法糖有多方便使用，可以让代码变得十分简洁，而本篇便是用来说明使用Stream流的Lambda语法来优雅的进行List和Map的互转操作，并分析其性能差异。

一些简单的便不做分析了，比如map()、peek()、filter()等方法，主要着重分析collect()的toMap()方法。

Stream流的Lambda语法应用实例

1、定义要操作的UserDto

先定义一个UserDto以便直接后续使用，其代码如下：

public class UserDto {
    private String name;
    private String sex;
    private String job;
    @Override
    public String toString() {
        return "{\"name\":\"" + name +
                "\",\"sex\":\"" + sex +
                "\",\"job\":\"" + job + "\"}";
    
    }
    // 后面其它的属性和getter、setter方法都忽略
}

2、List转成Map

List转成Map有很多方式，使用foreach的方式什么转换都可以完成，但在本篇将会分享几个遍历的操作。

List《UserDto》转成Map《String, UserDto》

首先分享一下最简单的转换：

由List<UserDto>转成以UserDto的name当成Key，而UserDto当成Value的Map操作，即Map<String, UserDto>形式。

代码如下：

public class MainTest {
    public static void main(String[] args) {
        UserDto userDto1 = new UserDto("test1", "man", "worker1");
        UserDto userDto2 = new UserDto("test2", "woman", "worker2");
        UserDto userDto3 = new UserDto("test3", "woman", "worker2");
        UserDto userDto4 = new UserDto("test4", "man", "worker3");
        List<UserDto> userList = Arrays
            .asList(userDto1, userDto2, userDto3, userDto4);
        System.out.println(userList);
        // 使用stream流将List转成Map
        Map<String, UserDto> userMap = userList.stream()
            .collect(Collectors.toMap(UserDto::getName, dto2 -> dto2));
        System.out.println(userMap);
    }
}
-------------打印结果-------------
[{"name":"test1","sex":"man","job":"worker1"}, {"name":"test2","sex":"woman","job":"worker2"}, 
{"name":"test3","sex":"woman","job":"worker2"}, {"name":"test4","sex":"man","job":"worker3"}]


{test4={"name":"test4","sex":"man","job":"worker3"},test2={"name":"test2","sex":"woman","job":"worker2"}, 
test3={"name":"test3","sex":"woman","job":"worker2"},test1={"name":"test1","sex":"man","job":"worker1"}}

List《UserDto》转成Map《String, Map《String, Object》》

接下来分享下将List<UserDto>转成Map<String, Map<String, Object>>形式的Lambda操作：

public class MainTest {
    public static void main(String[] args) {
        UserDto userDto1 = new UserDto("test1", "man", "worker1");
        UserDto userDto2 = new UserDto("test2", "woman", "worker2");
        UserDto userDto3 = new UserDto("test3", "woman", "worker2");
        UserDto userDto4 = new UserDto("test4", "man", "worker3");
        List<UserDto> userList = Arrays
            .asList(userDto1, userDto2, userDto3, userDto4);
        System.out.println(userList);
        // 使用stream流将List转成Map
        Map<String, Map<String, Object>> userMap = userList.stream()
        .collect(Collectors.toMap(UserDto::getName, dto2 ->
                new HashMap<String, Object>(){{
                    put("name", dto2.getName());
                    put("sex", dto2.getSex());
                    put("job", dto2.getJob());}}));
        System.out.println(userMap);
    }
}
-------------打印结果-------------
[{"name":"test1","sex":"man","job":"worker1"}, {"name":"test2","sex":"woman","job":"worker2"}, 
{"name":"test3","sex":"woman","job":"worker2"}, {"name":"test4","sex":"man","job":"worker3"}]


{test4={sex=man, name=test4, job=worker3}, test2={sex=woman, name=test2, job=worker2},
test3={sex=woman, name=test3, job=worker2}, test1={sex=man, name=test1, job=worker1}}

List《UserDto》转Map《String, String》

接下来分享下将List<UserDto>转成Map<String, String>形式的Lambda操作，当然Map中String泛型也可以是Integer、Long甚至其它的类型，按照下面的方式进行相应的替换就行了：

public class MainTest {
    public static void main(String[] args) {
        UserDto userDto1 = new UserDto("test1", "man", "worker1");
        UserDto userDto2 = new UserDto("test2", "woman", "worker2");
        UserDto userDto3 = new UserDto("test3", "woman", "worker2");
        UserDto userDto4 = new UserDto("test4", "man", "worker3");
        List<UserDto> userList = Arrays
            .asList(userDto1, userDto2, userDto3, userDto4);
        System.out.println(userList);
        // 使用stream流将List转成Map
        Map<String, String> userMap = userList.stream()
        .collect(Collectors.toMap(UserDto::getName, UserDto::getSex));
        System.out.println(userMap);
    }
}
-------------打印结果-------------
[{"name":"test1","sex":"man","job":"worker1"}, {"name":"test2","sex":"woman","job":"worker2"}, 
{"name":"test3","sex":"woman","job":"worker2"}, {"name":"test4","sex":"man","job":"worker3"}]


{test4=man, test2=woman, test3=woman, test1=man}

List《Map《String, Object》》转Map《String, UserDto》

由List<Map<String, Object>>转Map<String, UserDto>操作中，Map中的String也可以是Long、Integer或者其它的类型：

public class MainTest {
    public static void main(String[] args) {
        Map<String, Object> userMap1 = new HashMap<String, Object>(8) {{
            put("name", "test1");put("sex", "man");put("job", "worker1");
        }};
        Map<String, Object> userMap2 = new HashMap<String, Object>(8) {{
            put("name", "test2");put("sex", "woman");put("job", "worker2");
        }};
        Map<String, Object> userMap3 = new HashMap<String, Object>(8) {{
            put("name", "test3");put("sex", "woman");put("job", "worker2");
        }};
        Map<String, Object> userMap4 = new HashMap<String, Object>(8) {{
            put("name", "test4");put("sex", "man");put("job", "worker3");
        }};
        List<Map<String, Object>> userList = Arrays.asList(userMap1, userMap2, userMap3, userMap4);
        System.out.println(userList);
        // 使用stream流将List转成Map
        Map<String, UserDto> userMap = userList.stream()
                .collect(Collectors.toMap(map1 -> (String) map1.get("name"), map2 ->
                        new UserDto((String) map2.get("name"), (String) map2.get("sex"), (String) map2.get("job"))));
        System.out.println(userMap);
    }
}
-------------打印结果-------------
[{name=test1, job=worker1, sex=man}, {name=test2, job=worker2, sex=woman}, 
{name=test3, job=worker2, sex=woman}, {name=test4, job=worker3, sex=man}]


{test4={"name":"test4","sex":"man","job":"worker3"}, test2={"name":"test2","sex":"woman","job":"worker2"}, 
test3={"name":"test3","sex":"woman","job":"worker2"}, test1={"name":"test1","sex":"man","job":"worker1"}}

List《Map《String, Object》》转Map《String, String》

由List<Map<String, Object>>转Map<String, String>操作中，Map中的String也可以是Long、Integer或者其它的类型：

public class MainTest {
    public static void main(String[] args) {
        Map<String, Object> userMap1 = new HashMap<String, Object>(8) {{
            put("name", "test1");put("sex", "man");put("job", "worker1");
        }};
        Map<String, Object> userMap2 = new HashMap<String, Object>(8) {{
            put("name", "test2");put("sex", "woman");put("job", "worker2");
        }};
        Map<String, Object> userMap3 = new HashMap<String, Object>(8) {{
            put("name", "test3");put("sex", "woman");put("job", "worker2");
        }};
        Map<String, Object> userMap4 = new HashMap<String, Object>(8) {{
            put("name", "test4");put("sex", "man");put("job", "worker3");
        }};
        List<Map<String, Object>> userList = Arrays.asList(userMap1, userMap2, userMap3, userMap4);
        System.out.println(userList);
        // 使用stream流将List转成Map
        Map<String, String> userMap = userList.stream()
                .collect(Collectors.toMap(map1 -> (String) map1.get("name"), 
                    map2 -> (String) map2.get("sex")));
        System.out.println(userMap);
    }
}
-------------打印结果-------------
[{name=test1, job=worker1, sex=man}, {name=test2, job=worker2, sex=woman}, {name=test3, job=worker2, sex=woman}, {name=test4, job=worker3, sex=man}]

{test4=man, test2=woman, test3=woman, test1=man}

List《Map《String, Object》》转Map《String, Map《String, Object》》

由List<Map<String, Object>>转Map<String, Map<String, Object>>操作中，Map中的String也可以是Long、Integer或者其它的类型：

public class MainTest {
    public static void main(String[] args) {
        Map<String, Object> userMap1 = new HashMap<String, Object>(8) {{
            put("name", "test1");put("sex", "man");put("job", "worker1");
        }};
        Map<String, Object> userMap2 = new HashMap<String, Object>(8) {{
            put("name", "test2");put("sex", "woman");put("job", "worker2");
        }};
        Map<String, Object> userMap3 = new HashMap<String, Object>(8) {{
            put("name", "test3");put("sex", "woman");put("job", "worker2");
        }};
        Map<String, Object> userMap4 = new HashMap<String, Object>(8) {{
            put("name", "test4");put("sex", "man");put("job", "worker3");
        }};
        List<Map<String, Object>> userList = Arrays.asList(userMap1, userMap2, userMap3, userMap4);
        System.out.println(userList);
        // 使用stream流将List转成Map
        Map<String, Map<String, Object>> userMap = userList.stream()
        .collect(Collectors.toMap(map1 -> (String) map1.get("name"), map2 -> map2));
        System.out.println(userMap);
    }
}
-------------打印结果-------------
[{name=test1, job=worker1, sex=man}, {name=test2, job=worker2, sex=woman}, 
{name=test3, job=worker2, sex=woman}, {name=test4, job=worker3, sex=man}]


{test4={name=test4, job=worker3, sex=man}, test2={name=test2, job=worker2, sex=woman},
test3={name=test3, job=worker2, sex=woman}, test1={name=test1, job=worker1, sex=man}}

List《Map》转Map《String, List《Map》》

由List转Map>，根据Map中的某个值进行分组：

public class MainTest {
    public static void main(String[] args) {
        Map<String, Object> userMap1 = new HashMap<String, Object>(8) {{
            put("name", "test1");put("sex", "man");put("job", "worker1");
        }};
        Map<String, Object> userMap2 = new HashMap<String, Object>(8) {{
            put("name", "test2");put("sex", "woman");put("job", "worker2");
        }};
        Map<String, Object> userMap3 = new HashMap<String, Object>(8) {{
            put("name", "test3");put("sex", "woman");put("job", "worker2");
        }};
        Map<String, Object> userMap4 = new HashMap<String, Object>(8) {{
            put("name", "test4");put("sex", "man");put("job", "worker3");
        }};
        List<Map<String, Object>> userList = Arrays.asList(userMap1, userMap2, userMap3, userMap4);
        System.out.println(userList);
        // 使用stream流将List转成Map
        Map<String, List<Map<String, Object>>> userMap = userList.stream()
                .collect(Collectors.groupingBy(map1 -> (String) map1.get("sex")));
        System.out.println(userMap);
    }
}
-------------打印结果-------------
[{name=test1, job=worker1, sex=man}, {name=test2, job=worker2, sex=woman}, {name=test3, job=worker2, sex=woman}, {name=test4, job=worker3, sex=man}]
{woman=[{name=test2, job=worker2, sex=woman}, {name=test3, job=worker2, sex=woman}], man=[{name=test1, job=worker1, sex=man}, {name=test4, job=worker3, sex=man}]}

性能说明

需要注意的是Stream使用流处理也是有使用限制的，比如初始化时间和性能限制：

因此使用Stream时适用于低数据量的情况，当数据量级在1W以下是Stream流和foreach都可以使用，性能差别不大；到了10W-100W时应该使用foreach性能更快；而到了1000W量级的情况下就该使用Stream或者其它的大数据解析框架了。

使用上述方式的20次平均运行时间表（仅代表本机I5-8400 2.8Ghz-2.81Ghz的规格CPU运行效率）：

运行方式

数据量级

初始化时间(ms)

初始化后的

平均运行效率(ms)

foreach

1W

38

1

stream流

1W

1

1

foreach

10W

28

6

stream流

10W

54

12

foreach

100W

139

111

stream流

100W

1300

181

foreach

1000W

2500

3500

stream流

1000W

1130

6000

总结

以上为个人经验，希望能给大家一个参考，也希望大家多多支持脚本之家。

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