Java8 将一个List＜T＞转为Map＜String,T＞的操作

将 List 转为 Map<String, T>

				?

									public class AnswerApp {

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

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

									  Map<String, Integer> 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<K, V>

				?

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

									 List<User> 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<String, Integer> 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<String, T>

实现方式1

				?

									public class AnswerApp {

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

									  List<User> 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<String, User> 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":"answer0"}}

实现方式2

				?

									public class AnswerApp {

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

									  List<User> 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<String, User> 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<Entity> list = new ArrayList<>();

									Map<Integer, String> 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<Integer, Apple> 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<Double> 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<BigDecimal> naturalDayList;

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

									分组函数

									Map<String, List<SceneAnalysisRespVo>> 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);