Java8 将一个List＜T＞转为Map＜String,T＞的操作

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);

  }

}

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

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);

}

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

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

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));

  }

}

[{ "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" }}

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));

  }

}

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

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

@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;

  }

}

List<Entity> list = new ArrayList<>();

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

**

  * 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);