java中实现list或set转map的方法

java中实现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://www.jb51.net/article/104114.htm