java-design-patterns/collection-pipeline/src/main/java/com/iluwatar/collectionpipeline/ImperativeProgramming.java

/**
 * The MIT License
 * Copyright (c) 2014 Ilkka Seppälä
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */
package com.iluwatar.collectionpipeline;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

/**
 * Imperative-style programming to iterate over the list and get the names of
 * cars made later than the year 2000. We then sort the models in ascending
 * order by year.
 * 
 * <p>As you can see, there's a lot of looping in this code. First, the
 * getModelsAfter2000UsingFor method takes a list of cars as its parameter. It
 * extracts or filters out cars made after the year 2000, putting them into a
 * new list named carsSortedByYear. Next, it sorts that list in ascending order
 * by year-of-make. Finally, it loops through the list carsSortedByYear to get
 * the model names and returns them in a list.
 * 
 * <p>This short example demonstrates what I call the effect of statements. While
 * functions and methods in general can be used as expressions, the {@link Collections}
 * sort method doesn't return a result. Because it is used as a statement, it
 * mutates the list given as argument. Both of the for loops also mutate lists
 * as they iterate. Being statements, that's just how these elements work. As a
 * result, the code contains unnecessary garbage variables
 */
public class ImperativeProgramming {
  private ImperativeProgramming() {
  }

  /**
   * Method to return the car models built after year 2000 using for loops.
   * @param cars {@link List} of {@link Car} to iterate over
   * @return {@link List} of {@link String} of car models built after year 2000
   */
  public static List<String> getModelsAfter2000(List<Car> cars) {
    List<Car> carsSortedByYear = new ArrayList<>();

    for (Car car : cars) {
      if (car.getYear() > 2000) {
        carsSortedByYear.add(car);
      }
    }

    Collections.sort(carsSortedByYear, new Comparator<Car>() {
      public int compare(Car car1, Car car2) {
        return new Integer(car1.getYear()).compareTo(car2.getYear());
      }
    });

    List<String> models = new ArrayList<>();
    for (Car car : carsSortedByYear) {
      models.add(car.getModel());
    }

    return models;
  }
  
  /**
   * Method to group cars by category using for loops
   * 
   * @param cars {@link List} of {@link Car} to be used for grouping
   * @return {@link Map} of {@link String} and {@link List} of {@link Car} with 
   * category as key and cars belonging to that category as value
   */
  public static Map<String, List<Car>> getGroupingOfCarsByCategory(List<Car> cars) {
    Map<String, List<Car>> groupingByCategory = new HashMap<>();
    for (Car car: cars) {
      if (groupingByCategory.containsKey(car.getCategory())) {
        groupingByCategory.get(car.getCategory()).add(car);
      } else {
        List<Car> categoryCars = new ArrayList<>();
        categoryCars.add(car);
        groupingByCategory.put(car.getCategory(), categoryCars);
      }
    }
    return groupingByCategory;
  }
  
  /**
   * Method to get all Sedan cars belonging to a group of persons sorted by year of manufacture using for loops 
   * 
   * @param persons {@link List} of {@link Person} to be used
   * @return {@link List} of {@link Car} to belonging to the group
   */
  public static List<Car> getSedanCarsOwnedSortedByDate(List<Person> persons) {
    List<Car> cars = new ArrayList<>();
    for (Person person: persons) {
      cars.addAll(person.getCars());
    }
  
    List<Car> sedanCars = new ArrayList<>();
    for (Car car: cars) {
      if ("Sedan".equals(car.getCategory())) {
        sedanCars.add(car);
      }
    }
  
    sedanCars.sort(new Comparator<Car>() {
      @Override
      public int compare(Car o1, Car o2) {
        return o1.getYear() - o2.getYear();
      }
    });
  
    return sedanCars;
  }
}