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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