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<UserMeal> mealList, group by the day, calculate calories for a period of time, and create List<UserMealWithExceed>:

public static List<UserMealWithExceed>  getFilteredMealsWithExceeded(List<UserMeal> mealList, LocalTime startTime, LocalTime endTime, int caloriesPerDay) {      return mealList.stream()             .filter(userMeal -> userMeal.getDateTime().toLocalTime().isAfter(startTime)&&userMeal.getDateTime().toLocalTime().isBefore(endTime))             .collect(Collectors.groupingBy(userMeal -> userMeal.getDateTime().getDayOfMonth(),                          Collectors.summingInt(userMeal -> 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>caloriesPerDay)     {return new UserMealWithExceed(userMeal.getdateTime, usermeal.getDescription, usermeal.getCalories, true);}     else     {return new UserMealWithExceed(userMeal.getdateTime, usermeal.getDescription, usermeal.getCalories, false)     } )//foreach 

Answer 1

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<Person> persons = Arrays.asList(                 new Person("e1", "l1"),                 new Person("e2", "l1"),                 new Person("e3", "l2"),                 new Person("e4", "l2")         );          List<Employee> employees = persons.stream()                 .filter(p -> p.getLastName().equals("l1"))                 .map(p -> 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 & Setter }  class Employee extends Person {      private double salary;      public Employee(String name, String lastName, double salary) {         super(name, lastName);         this.salary = salary;     }      // Getter & Setter }