java 三种将list转换为map的方法详解

java 三种将list转换为map的方法详解

 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());
}

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