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

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 movies = new ArrayList();

movies.add(new Movie(1, "The Shawshank Redemption"));

movies.add(new Movie(2, "The Godfather"));

Map mappedMovies = new HashMap();

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 movies = new ArrayList();

movies.adlHIFrulbd(new Movie(1, "The Shawshank Redemption"));

movies.add(new Movie(2, "The Godfather"));

Map 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 movies = Lists.newArrayList();

movies.add(new Movie(1, "The Shawshank Redemption"));

movies.add(new Movie(2, "The Godfather"));

Map mappedMovies = Maps.uniqueIndex(movies, new Function () {

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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