Java8 将一个List转为Map的操作

Java8 将一个List转为Map的操作

将 List 转为 Map

public class AnswerApp {

public static void main(String[] args) throws Exception {

List names = Lists.newArrayList("Answer", "AnswerAIL", "AI");

Map map = names.stream().collect(Collectors.toMap(v -> v, v -> 1));

System.out.println(map);

}

}

程序运行输出

{Answer=1, AnswerAIL=1, AI=1}

将 List 转为 Map

public static void main(String[] args) throws Exception {

List users = new ArrayList<>();

for (int i = 0; i < 3; i++) {

users.add(new User("answer" + i, new Random().nextInt(100)));

}

System.out.println(jsON.toJSONString(users));

System.out.println();

Map map = users.stream().collect(Collectors.toMap(User::getName, User::getAge));

System.out.println(map);

}

程序运行输出

[{"age":78,"name":"answer0"},{"age":89,"name":"answer1"},{"age":72,"name":"answer2"}]

{answer2=72, answer1=89, answer0=78}

将 List 转为 Map

实现方式1

public class AnswerApp {

public static void main(String[] args) throws Exception {

List users = new ArrayList<>();

for (int i = 0; i < 3; i++) {

// 改为此代码, 转map时会报错 Duplicate key User

// users.add(new User("answer", new Random().nextInt(100)));

users.add(new User("answer" + i, new Random().nextInt(100)));

}

System.out.println(JSON.toJSONString(users));

System.out.println();

Map map = users.stream().collect(Collectors.toMap(User::getName, Function.identity()));

System.out.println(JSON.toJSONString(map));

}

}

该方式如果 map 的 key(如上述例子的 User::getName 的值) 重复， 会抛错java.lang.IllegalStateException: Duplicate key User

程序运行输出

[{"age":22,"name":"answer0"},{"age":79,"name":"answer1"},{"age":81,"name":"answer2"}]

{"answer2":{"age":81,"name":"answer2"},"answer1":{"age":79,"name":"answer1"},"answer0":{"age":22,"name&quothttp://;:"answer0"}}

实现方式2

public class AnswerApp {

public static void main(String[] args) throws Exception {

List users = new ArrayList<>();

for (int i = 0; i < 3; i++) {

users.add(new User("answer", new Random().nextInt(100)));

}

System.out.println(JSON.toJSONString(users));

System.out.println();

// 如果 key 重复, 则根据 冲突方法 ·(key1, key2) -> key2· 判断. 解释: key1 key2 冲突时 取 key2

Map map = users.stream().collect(Collectors.toMap(User::getName, Function.identity(), (key1, key2) -> key2));

System.out.println(JSON.toJSONString(map));

}

}

程序运行输出

[{"age":24,"name":"answer"},{"age":89,"name":"answer"},{"age":68,"name":"answer"}]

{"answer":{"age":68,"name":"answer"}}

如果改为 (key1, key2) -> key1 则输出 {"answer":{"age":24,"name":"answer"}}

User 实体

@Data

@NoArgsConstructor

@AllArgsConstructor

public class User {

private Long id;

private String name;

private Integer age;

public User(String name) {

this.name = name;

}

public User(String name, Integer age) {

this.name = name;

this.age = age;

}

}

补充：java8中使用Lambda表达式将list中实体类的两个字段转Map

代码：

List list = new ArrayList<>();

Map map = list.stream().collect(Collectors.toMap(Entity::getId, Entity::getType));

常用的lambda表达式：

**

* 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 appleMap = appleList.stream().collect(Collectors.toMap(Apple::getId, a -> a,(k1,k2)->k1));

安照某一字段去重

list = list.stream().filter(distinctByKey(p -> ((ModCreditColumn) p).getFieldCode())).collect(Collectors.toList());

List unitNetValue = listIncreaseDto.stream().map(IncreaseDto :: getUnitNetValue).collect(Collectors.toList());

//求和 对象List

BigDecimal allFullMarketPrice = entityList.stream().filter(value -> value.getFullMarketPrice()!= null).map(SceneAnalysisRespVo::getFullMarketPrice).reduce(BigDecimal.ZERO, BigDecimal::add);

List naturalDayList;

BigDecimal total = naturalDayList.stream().reduce(BigDecimal.ZERO, BigDecimal::add);

分组函数

Map> groupMap = total.getGroupList().stream().collect(Collectors.groupingBy(SceneAnalysisRespVo::getVmName));

//DV01之和

BigDecimal allDV01 = values.stream().filter(sceneAnalysisRespVo -> sceneAnalysisRespVo.getDv() != null).map(SceneAnalysisRespVo::getDv).reduce(BigDecimal.ZERO, BigDecimal::add);

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