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
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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:
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@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:
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@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
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@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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