Вопрос по лямбдам в Java, функции map, filter

Пытаюсь переделать данные методы в stream, используя filter и map методы.

В учебнике предлагается переделать метод

printSightingsBy(int spotter)

в следующий:

    public void printSightingsBy(int spotter)
{
     sightings.stream()
      .filter(sighting -> sighting.getSpotter() == spotter)
      .map(sighting -> sighting.getDetails())
      .forEach (details -> System.out.println (details))
 }

Почему нельзя сделать так:

 public void printSightingsBy(int spotter)
    {
         sightings.stream()
          .filter(sighting -> sighting.getSpotter() == spotter)//получаем человека
          //получаем детали о животных, которые отслеживаются данным человеком
          .forEach (sighting -> System.out.println (sighting.getDetails()));
     }

Код класса следующий:

import java.util.ArrayList;
import java.util.Iterator;
/**
 * Monitor counts of different types of animal.
 * Sightings are recorded by spotters.
 * 
 * @author David J. Barnes and Michael Kölling
 * @version 2016.02.29 (imperative)
 */
public class AnimalMonitor 
{
    // Records of all the sightings of animals.
private ArrayList<Sighting> sightings;
/**
 * Create an AnimalMonitor.
 */
public AnimalMonitor()
{
    this.sightings = new ArrayList<>();
}
/**
 * Add the sightings recorded in the given filename to the current list.
 * @param filename A CSV file of Sighting records.
 */
public void addSightings(String filename)
{
    SightingReader reader = new SightingReader();
    sightings.addAll(reader.getSightings(filename));
}
/**
 * Print details of all the sightings.
 */
public void printList()
{
    //for(Sighting record : sightings) {
     //   System.out.println(record.getDetails());
    //}
    sightings.forEach(record -> System.out.println(record.getDetails()));
}
/**
 * Print the details of all the sightings of the given animal.
 * @param animal The type of animal.
 */
public void printSightingsOf(String animal)
{
    for(Sighting record : sightings) {
        if(animal.equals(record.getAnimal())) {
            System.out.println(record.getDetails());
        }
    }
}
/**
 * Print all the sightings by the given spotter.
 * @param spotter The ID of the spotter.
 */
public void printSightingsBy(int spotter)
{
    for(Sighting record : sightings) {
        if(record.getSpotter() == spotter) {
            System.out.println(record.getDetails());
        }
    }        
}
/**
 * Print a list of the types of animal considered to be endangered.
 * @param animalNames A list of animals names.
 * @param dangerThreshold Counts less-than or equal-to to this level
 *                        are considered to be dangerous.
 */
public void printEndangered(ArrayList<String> animalNames, 
                            int dangerThreshold)
{
    for(String animal : animalNames) {
        if(getCount(animal) <= dangerThreshold) {
            System.out.println(animal + " is endangered.");
        }
    }
}
/**
 * Return a count of the number of sightings of the given animal.
 * @param animal The type of animal.
 * @return The count of sightings of the given animal.
 */
public int getCount(String animal)
{
    int total = 0;
    for(Sighting sighting : sightings) {
        if(animal.equals(sighting.getAnimal())) {
            total = total + sighting.getCount();
        }
    }
    return total;
}
/**
 * Remove from the sightings list all of those records with
 * a count of zero.
 */
public void removeZeroCounts()
{
    Iterator<Sighting> it = sightings.iterator();
    while(it.hasNext()) {
        Sighting record = it.next();
        if(record.getCount() == 0) {
            it.remove();
        }
    }
}
/**
 * Return a list of all sightings of the given type of animal
 * in a particular area.
 * @param animal The type of animal.
 * @param area The ID of the area.
 * @return A list of sightings.
 */
public ArrayList<Sighting> getSightingsInArea(String animal, int area)
{
    ArrayList<Sighting> records = new ArrayList<>();
    for(Sighting record : sightings) {
        if(animal.equals(record.getAnimal())) {
            if(record.getArea() == area) {
                records.add(record);
            }
        }
    }
    return records;
}
/**
 * Return a list of all the sightings of the given animal.
 * @param animal The type of animal.
 * @return A list of all sightings of the given animal.
 */
public ArrayList<Sighting> getSightingsOf(String animal)
{
    ArrayList<Sighting> filtered = new ArrayList<>();
    for(Sighting record : sightings) {
        if(animal.equals(record.getAnimal())) {
            filtered.add(record);
        }
    }
    return filtered;
}
}

Код класса, определяющий информацию о нахождении животных:

public class Sighting
{

 // The animal spotted.
    private final String animal;
    // The ID of the spotter.
    private final int spotter;
    // How many were seen.
    private final int count;
    // The ID of the area in which they were seen.
    private final int area;
    // The reporting period.
    private final int period;
    /**
     * Create a record of a sighting of a particular type of animal.
     * @param animal The animal spotted.
     * @param spotter The ID of the spotter.
     * @param count How many were seen (>= 0).
     * @param area The ID of the area in which they were seen.
     * @param period The reporting period.
     */
public Sighting(String animal, int spotter, int count, int area, int period)
{
    this.animal = animal;
    this.spotter = spotter;
    this.count = count;
    this.area = area;
    this.period = period;
}
/**
 * Return the type of animal spotted.
 * @return The animal type.
 */
public String getAnimal() 
{
    return animal;
}
/**
 * Return the ID of the spotter.
 * @return The spotter's ID.
 */
public int getSpotter() 
{
    return spotter;
}
/**
 * Return how many were spotted.
 * @return The number seen.
 */
public int getCount() 
{
    return count;
}
/**
 * Return the ID of the area in which they were seen.
 * @return Where they were seen.
 */
public int getArea() 
{
    return area;
}
/**
 * Return the period in which they were seen.
 * @return When they were seen.
 */
public int getPeriod() 
{
    return period;
}
/**
 * Return a string containing details of the animal, the number seen,
 * where they were seen, who spotted them and when.
 * @return A string giving details of the sighting.
 */
public String getDetails() 
{
    return animal + 
           ", count = " + count + 
           ", area = " + area + 
           ", spotter = " + spotter + 
           ", period = " + period;
}
}