java中list或set转map的方法

在开发中我们有时需要将list或set转换为map（比如对象属性中的唯一键作为map的key，对象作为map的value），一般的想法就是new一个map，然后把list或set中的值一个个push到map中。

类似下面的代码：

List<String> stringList = Lists.newArrayList("t1", "t2", "t3");
Map<String, String> map = Maps.newHashMapWithExpectedSize(stringList.size());
for (String str : stringList) {
map.put(str, str);
}

是否还有更优雅的写法呢？答案是有的。
guava提供了集合（实现了Iterables接口或Iterator接口）转map的方法，方法定义如下：

/**
* Returns an immutable map for which the {@link Map#values} are the given
* elements in the given order, and each key is the product of invoking a
* supplied function on its corresponding value.
*
* @param values the values to use when constructing the {@code Map}
* @param keyFunction the function used to produce the key for each value
* @return a map mapping the result of evaluating the function {@code
* keyFunction} on each value in the input collection to that value
* @throws IllegalArgumentException if {@code keyFunction} produces the same
* key for more than one value in the input collection
* @throws NullPointerException if any elements of {@code values} is null, or
* if {@code keyFunction} produces {@code null} for any value
*/
public static <K, V> ImmutableMap<K, V> uniqueIndex(
Iterable<V> values, Function<? super V, K> keyFunction) {
return uniqueIndex(values.iterator(), keyFunction);
}

/**
* Returns an immutable map for which the {@link Map#values} are the given
* elements in the given order, and each key is the product of invoking a
* supplied function on its corresponding value.
*
* @param values the values to use when constructing the {@code Map}
* @param keyFunction the function used to produce the key for each value
* @return a map mapping the result of evaluating the function {@code
* keyFunction} on each value in the input collection to that value
* @throws IllegalArgumentException if {@code keyFunction} produces the same
* key for more than one value in the input collection
* @throws NullPointerException if any elements of {@code values} is null, or
* if {@code keyFunction} produces {@code null} for any value
* @since 10.0
*/
public static <K, V> ImmutableMap<K, V> uniqueIndex(
Iterator<V> values, Function<? super V, K> keyFunction) {
checkNotNull(keyFunction);
ImmutableMap.Builder<K, V> builder = ImmutableMap.builder();
while (values.hasNext()) {
V value = values.next();
builder.put(keyFunction.apply(value), value);
}
return builder.build();
}这样我们就可以很方便的进行转换了，如下：
List<String> stringList = Lists.newArrayList("t1", "t2", "t3");
Map<String, String> map = Maps.uniqueIndex(stringList, new Function<String, String>() {
@Override
public String apply(String input) {
return input;
}
});需要注意的是，如接口注释所说，如果Function返回的结果产生了重复的key，将会抛出异常。
java8也提供了转换的方法，这里直接照搬别人博客的代码：

@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());
} 参考：http://jackyrong.iteye.com/blog/2158009