Binnur Kurt's Blog: Exercises to study Java Stream API

In this blog post, we will study Stream API of Java 9 with several exercise questions. In these exercises, we will use two different domains:

IMDB Movies
World Countries

Before we dive into the exercise questions, let's take a look at these domains:

IMDB Movies: We have three domain classes: Movie, Director and Genre. Each movie has a title, a year and a unique id assigned by Internet Movie DataBase (IMDB). Each movie has been directed by at least one director, and each director has many movies. Each movie belongs to at least one genre.

package com.example.domain;

import java.util.ArrayList;
import java.util.List;

public class Movie {

private int id;
   private String title;
   private int year;
   private String imdb;
   private List<Genre> genres;
   private List<Director> directors;

   {
      genres = new ArrayList<>();
      directors = new ArrayList<>();
   }

   public Movie() {
   }

   public Movie(int id, String title, int year, String imdb) {
      this.id = id;
      this.title = title;
      this.year = year;
      this.imdb = imdb;
   }

   // getters and setters

   @Override

   public String toString() {
      return "Movie [title=" + title + ", year=" + year + "]";
   }

package com.example.domain;

import java.util.ArrayList;
import java.util.List;

public class Director {
   private int id;
   private String name;
   private String imdb;
   private List<Movie> movies= new ArrayList<>();

   public Director() {
   }

   public Director(int id, String name, String imdb) {
      this.id = id;
      this.name = name;
      this.imdb = imdb;
   }

   // getters and setters

   @Override

   public String toString() {
      return "Director [id=" + id + ", name=" + name + ", imdb=" + imdb + "]";
   }
}

package com.example.domain;

public class Genre {
   private int id;
   private String name;

   public Genre() {
   }

   public Genre(int id, String name) {
      this.id = id;
      this.name = name;
   }

   // getters and setters
   @Override

   public String toString() {
      return "Genre [id=" + id + ", name=" + name + "]";
   }

}

World Countries: There are two domain classes: Country and City. Each city belongs to a country defined by the attribute, countryCode. Each country has a unique code and has many cities.

package com.example.domain;

import java.util.ArrayList;
import java.util.List;

public class Country {
   private String code;
   private String name;
   private String continent;
   private double surfaceArea;
   private int population;
   private double gnp;
   private int capital;
   private List<City> cities;
   {
      cities = new ArrayList<>();
   }

   public Country() {
   }

   public Country(String code, String name, String continent, int population,
         double surfaceArea, double gnp, int capital) {
      this.code = code;
      this.name = name;
      this.continent = continent;
      this.surfaceArea = surfaceArea;
      this.population = population;
      this.capital = capital;
      this.gnp = gnp;
   }

   // getters and setters

   @Override

   public String toString() {
      return "Country [ name=" + name + ", population=" + population + "]";
   }

}

package com.example.domain;


public class City {
   private int id;
   private String name;
   private int population;
   private String countryCode;

   public City() {
   }

   public City(int id, String name, String countryCode, int population) {
      this.id = id;
      this.name = name;
      this.population = population;
      this.countryCode = countryCode;
   }

   // getters and setters

   @Override

   public String toString() {
      return "City [id=" + id + ", name=" + name + ", population=" + population + ", countryCode=" + countryCode + "]";
   };

}

EXERCISE #1

Find the highest populated city of each country:

package com.example.exercise;

import java.util.Comparator;
import java.util.List;
import java.util.Optional;
import java.util.stream.Collectors;

import com.example.dao.CountryDao;
import com.example.dao.InMemoryWorldDao;
import com.example.domain.City;

public class Exercise1 {

   public static void main(String[] args) {
      CountryDao countryDao= InMemoryWorldDao.getInstance();
      List<City> highPopulatedCitiesOfCountries = countryDao.findAllCountries()
                .stream()
                .map( country -> country.getCities().stream().max(Comparator.comparing(City::getPopulation)))
                .filter(Optional::isPresent)
                .map(Optional::get)
                  .collect(Collectors.toList());
      highPopulatedCitiesOfCountries.forEach(System.out::println);
   }

}