docs: Collection pipeline explanation (#2875)

* collection pipeline docs + refactoring

* restore imperative programming code

collection-pipeline/src/main/java/com/iluwatar/collectionpipeline/FunctionalProgramming.java

@@ -22,6 +22,7 @@
  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
  * THE SOFTWARE.
  */
+
 package com.iluwatar.collectionpipeline;
 
 import java.util.Comparator;
@@ -54,9 +55,8 @@ public class FunctionalProgramming {
    * @return {@link List} of {@link String} representing models built after year 2000
    */
   public static List<String> getModelsAfter2000(List<Car> cars) {
-    return cars.stream().filter(car -> car.getYear() > 2000)
-        .sorted(Comparator.comparing(Car::getYear))
-        .map(Car::getModel).toList();
+    return cars.stream().filter(car -> car.year() > 2000).sorted(Comparator.comparing(Car::year))
+        .map(Car::model).toList();
   }
 
   /**
@@ -66,7 +66,7 @@ public class FunctionalProgramming {
    * @return {@link Map} with category as key and cars belonging to that category as value
    */
   public static Map<Category, List<Car>> getGroupingOfCarsByCategory(List<Car> cars) {
-    return cars.stream().collect(Collectors.groupingBy(Car::getCategory));
+    return cars.stream().collect(Collectors.groupingBy(Car::category));
   }
 
   /**
@@ -76,8 +76,8 @@ public class FunctionalProgramming {
    * @return {@link List} of {@link Car} to belonging to the group
    */
   public static List<Car> getSedanCarsOwnedSortedByDate(List<Person> persons) {
-    return persons.stream().map(Person::getCars).flatMap(List::stream)
-        .filter(car -> Category.SEDAN.equals(car.getCategory()))
-        .sorted(Comparator.comparing(Car::getYear)).toList();
+    return persons.stream().map(Person::cars).flatMap(List::stream)
+        .filter(car -> Category.SEDAN.equals(car.category()))
+        .sorted(Comparator.comparing(Car::year)).toList();
   }
 }