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()); }

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