在本文中，介紹三種將list轉換為map的方法：
1） 傳統方法
假設有某個類如下
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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(); } } |
使用傳統的方法：
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@Testpublic 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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@Testpublic 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 工具類庫
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@Testpublic 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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