java 3 methods for converting list to map

In this article, there are three ways to convert list to map:

1. traditional methods

Let’s say I have a class that looks like this

class Movie {

  private Integer rank;
  private String description;

  public Movie(Integer rank, String description) {
    super();
    this.rank = rank;
    this.description = description;
  }

  public Integer getRank() {
    return rank;
  }

  public String getDescription() {
    return description;
  }

  @Override
  public String toString() {
    return Objects.toStringHelper(this)
        .add("rank", rank)
        .add("description", description)
        .toString();
  }
}

Using traditional methods:

@Test
public void convert_list_to_map_with_java () {

  List<Movie> movies = new ArrayList<Movie>();
  movies.add(new Movie(1, "The Shawshank Redemption"));
  movies.add(new Movie(2, "The Godfather"));

  Map<Integer, Movie> mappedMovies = new HashMap<Integer, Movie>();
  for (Movie movie : movies) {
    mappedMovies.put(movie.getRank(), movie);
  }

  logger.info(mappedMovies);

  assertTrue(mappedMovies.size() == 2);
  assertEquals("The Shawshank Redemption", mappedMovies.get(1).getDescription());
}

2. direct streaming of JAVA 8:

@Test
public void convert_list_to_map_with_java8_lambda () {

  List<Movie> movies = new ArrayList<Movie>();
  movies.add(new Movie(1, "The Shawshank Redemption"));
  movies.add(new Movie(2, "The Godfather"));

  Map<Integer, Movie> mappedMovies = movies.stream().collect(
      Collectors.toMap(Movie::getRank, (p) -> p));

  logger.info(mappedMovies);

  assertTrue(mappedMovies.size() == 2);
  assertEquals("The Shawshank Redemption", mappedMovies.get(1).getDescription());
}

3. use the guava tool library

@Test
public void convert_list_to_map_with_guava () {


  List<Movie> movies = Lists.newArrayList();
  movies.add(new Movie(1, "The Shawshank Redemption"));
  movies.add(new Movie(2, "The Godfather"));


  Map<Integer,Movie> mappedMovies = Maps.uniqueIndex(movies, new Function <Movie,Integer> () {
     public Integer apply(Movie from) {
      return from.getRank();
  }});

  logger.info(mappedMovies);

  assertTrue(mappedMovies.size() == 2);
  assertEquals("The Shawshank Redemption", mappedMovies.get(1).getDescription());
}

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