Convert list to map

This example will show how to convert a list to a map using java, java 8 and guava. For the snippets below, we have created a Movie object. Typically when this this action occurs, the keys of the map are produced based on a specified object method and the values are the elements itself. In a related example we demonstrate how to transform a list to a map in groovy.

Setup

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();
    }
}

Straight up Java

When converting a list to a map using the core jdk, a traditional approach can be taken iterating over each element using a for each loop and calling the Map.put with a specified key and value.

@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());
}

Output

mappedMovies = {
1=Movie{rank=1, description=The Shawshank Redemption}, 
2=Movie{rank=2, description=The Godfather}
}

Java 8

Converting a list to a map in Java 8 can be performed by first converting the list to a stream and calling the collect terminal operation. In this instance, we will call Collectors.toMap which will return a collector that accumulates elements into a map. The keys are the generated from the Movie::getRank and the values are the Movie object.

@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());
}

Output

mappedMovies = {
1=Movie{rank=1, description=The Shawshank Redemption}, 
2=Movie{rank=2, description=The Godfather}
}

Google Guava

Guava Maps utility class contains Maps.uniqueIndex will create a map from a list of values. Each key is produced by calling a supplied function and the value is the value itself. Below, the function will pass in an object Movie and return an Integer by calling the Movie.getRank to create the Map.key.

@Test
public void convert_list_to_map_with_guava () {

    // create a list
    List<Movie> movies = Lists.newArrayList();
    movies.add(new Movie(1, "The Shawshank Redemption"));
    movies.add(new Movie(2, "The Godfather"));
    
    // convert list to map 
    Map<Integer,Movie> mappedMovies = Maps.uniqueIndex(movies, new Function <Movie,Integer> () {
          public Integer apply(Movie from) {
            return from.getRank(); // or something else
    }});
    
    logger.info(mappedMovies);
    
    assertTrue(mappedMovies.size() == 2);
    assertEquals("The Shawshank Redemption", mappedMovies.get(1).getDescription());
}

Output

mappedMovies = {
1=Movie{rank=1, description=The Shawshank Redemption}, 
2=Movie{rank=2, description=The Godfather}
}