三种将list转换为map的方法

  在本文中，介绍三种将list转换为map的方法：

1） 传统方法

假设有某个类如下
  

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();
    }
}

    使用传统的方法：
   

@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） JAVA 8直接用流的方法：
   

@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） 使用guava 工具类库
   

@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());
}