Java8 將一個List<T>轉為Map<String,T>的操作

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將 List 轉為 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);
 }
}

程序運行輸出

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

將 List 轉為 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);
}

程序運行輸出

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

將 List 轉為 Map<String, T>

實現方式1

public class AnswerApp {
 public static void main(String[] args) throws Exception {
  List<User> users = new ArrayList<>();
  for (int i = 0; i < 3; i++) {
   // 改為此代碼, 轉map時會報錯 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));
 }
}

該方式如果 map 的 key(如上述例子的 User::getName 的值) 重復， 會拋錯java.lang.IllegalStateException: Duplicate key User

程序運行輸出

[{"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"}}

實現方式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 重復, 則根據 沖突方法 ·(key1, key2) -> key2· 判斷. 解釋: key1 key2 沖突時 取 key2 
  Map<String, User> map = users.stream().collect(Collectors.toMap(User::getName, Function.identity(), (key1, key2) -> key2));
  System.out.println(JSON.toJSONString(map));
 }
}

程序運行輸出

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

如果改為 (key1, key2) -> key1 則輸出 {“answer”:{“age”:24,”name”:”answer”}}

User 實體

@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;
 }
}

補充：java8中使用Lambda表達式將list中實體類的兩個字段轉Map

代碼：

List<Entity> list = new ArrayList<>();
Map<Integer, String> map = list.stream().collect(Collectors.toMap(Entity::getId, Entity::getType));

常用的lambda表達式：

**
 * List -> Map
 * 需要註意的是：
 * toMap 如果集合對象有重復的key，會報錯Duplicate key ....
 * apple1,apple12的id都為1。
 * 可以用 (k1,k2)->k1 來設置，如果有重復的key,則保留key1,舍棄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 對象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);
 
分組函數
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);

以上為個人經驗，希望能給大傢一個參考，也希望大傢多多支持WalkonNet。如有錯誤或未考慮完全的地方，望不吝賜教。

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