Create list of object from another using Java 8 Streams

Question

I m trying to understand Java 8 streams  I have two classes   public class UserMeal       protected final LocalDateTime dateTime       protected final String description       protected final int calories       public UserMeal LocalDateTime dateTime  String description  int calories            this dateTime   dateTime          this description   description          this calories   calories             public LocalDateTime getDateTime             return dateTime             public String getDescription             return description             public int getCalories             return calories            and   public class UserMealWithExceed       protected final LocalDateTime dateTime       protected final String description       protected final int calories       protected final boolean exceed       public UserMealWithExceed LocalDateTime dateTime  String description  int calories  boolean exceed            this dateTime   dateTime          this description   description          this calories   calories          this exceed   exceed            The exceed field should indicate whether the sum of calories for the entire day  This field is the same for all entries for that day   I try to get object from List lt UserMeal gt  mealList  group by the day  calculate calories for a period of time  and create List lt UserMealWithExceed gt    public static List lt UserMealWithExceed gt   getFilteredMealsWithExceeded List lt UserMeal gt  mealList  LocalTime startTime  LocalTime endTime  int caloriesPerDay         return mealList stream                filter userMeal - gt  userMeal getDateTime   toLocalTime   isAfter startTime  amp  amp userMeal getDateTime   toLocalTime   isBefore endTime                collect Collectors groupingBy userMeal - gt  userMeal getDateTime   getDayOfMonth                             Collectors summingInt userMeal - gt  userMeal getCalories                   forEach               but I don t understand how to create new object in forEach and return collection   How I see in pseudocode    foreach       if  sumCalories gt caloriesPerDay       return new UserMealWithExceed userMeal getdateTime  usermeal getDescription  usermeal getCalories  true        else      return new UserMealWithExceed userMeal getdateTime  usermeal getDescription  usermeal getCalories  false           foreach

User · Answer

I prefer to solve this in the classic way  creating a new array of my desired data type  List lt MyNewType gt  newArray   new ArrayList lt  gt     myOldArray forEach info - gt  newArray add objectMapper convertValue info  MyNewType class

User · Answer

If you want to iterate over a list and create a new list with  transformed  objects  you should use the map   function of stream   collect    In the following example I find all people with the last name  l1  and each person I m  mapping  to a new Employee instance   public class Test        public static void main String   args            List lt Person gt  persons   Arrays asList                  new Person  e1    l1                    new Person  e2    l1                    new Person  e3    l2                    new Person  e4    l2                       List lt Employee gt  employees   persons stream                    filter p - gt  p getLastName   equals  l1                     map p - gt  new Employee p getName    p getLastName    1000                    collect Collectors toList              System out println employees             class Person        private String name      private String lastName       public Person String name  String lastName            this name   name          this lastName   lastName                Getter  amp  Setter    class Employee extends Person        private double salary       public Employee String name  String lastName  double salary            super name  lastName           this salary   salary                Getter  amp  Setter

User · Answer

What you are possibly looking for is map    You can  convert  the objects in a stream to another by mapping this way         map userMeal - gt  new UserMealExceed

User · Answer

An addition to the solution by  Rafael Teles  The syntactic sugar Collectors mapping does the same in one step         List lt Employee gt  employees   persons stream      filter p - gt  p getLastName   equals  l1       collect      Collectors mapping        p - gt  new Employee p getName    p getLastName    1000         Collectors toList        Detailed example can be found here

[java] Create list of object from another using Java 8 Streams

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