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

Simply, don’t use forEach. A Stream supports more operations than this one. — Holger
– Holger, Commented Feb 29, 2016 at 13:26

Rafael Teles · Accepted Answer · 2018-12-13 14:50:29Z

180

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
}

edited Dec 13, 2018 at 14:50

answered Feb 26, 2016 at 12:07

Rafael Teles

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7 Comments

trojanc Over a year ago

Works perfect! I used similar pattern to convert a list of database entities to DTO's

bonapart3 Over a year ago

What about the case were Employee only has a no-arg constructor?

Rafael Teles Over a year ago

@csmith49 you can instantiate the object and call the set methods inside the map function. You can have a better look on lambda expressions on tutorials.jenkov.com/java/lambda-expressions.html and baeldung.com/java-8-lambda-expressions-tips

bonapart3 Over a year ago

Yeah thanks, I didn't see your message until now but worked it out myself yesterday, good to know this is the correct way to do it: List<Employee> employeeList = persons.stream() .filter(p -> p.getLastName().equals("l1")) .map(p -> { Employee e = new Employee(); e.setName(p.getName()); e.setSalary(p.getSalary()); return e; }) .collect(Collectors.toList());

Rinsen S Over a year ago

Hi thanks for the answer, but how to return multiple objects, eg: .map(p -> { return p + new Employee(p.getName(), p.getLastName(), 1000) }). As I need to return new object with p.

|

Robert Bräutigam · Accepted Answer · 2016-02-26 12:09:47Z

20

What you are possibly looking for is map(). You can "convert" the objects in a stream to another by mapping this way:

...
 .map(userMeal -> new UserMealExceed(...))
...

answered Feb 26, 2016 at 12:09

Robert Bräutigam

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Comments

Jónás Balázs · Accepted Answer · 2018-12-03 07:05:39Z

6

An addition to the solution by @Rafael Teles. The syntactic sugar Collectors.mapping does the same in one step:

//...
List<Employee> employees = persons.stream()
  .filter(p -> p.getLastName().equals("l1"))
  .collect(
    Collectors.mapping(
      p -> new Employee(p.getName(), p.getLastName(), 1000),
      Collectors.toList()));

Detailed example can be found here

answered Dec 3, 2018 at 7:05

Jónás Balázs

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