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 

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： 

@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 工具類庫 

@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()); 
}