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java中實(shí)現(xiàn)list或set轉(zhuǎn)map的方法

更新時間：2017年01月24日 17:12:15 投稿：lqh

這篇文章主要介紹了java中實(shí)現(xiàn)list或set轉(zhuǎn)map的方法的相關(guān)資料,需要的朋友可以參考下

java中實(shí)現(xiàn)list或set轉(zhuǎn)map的方法

在開發(fā)中我們有時需要將list或set轉(zhuǎn)換為map（比如對象屬性中的唯一鍵作為map的key，對象作為map的value），一般的想法就是new一個map，然后把list或set中的值一個個push到map中。

類似下面的代碼：

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

是否還有更優(yōu)雅的寫法呢？答案是有的。

guava提供了集合（實(shí)現(xiàn)了Iterables接口或Iterator接口）轉(zhuǎn)map的方法，方法定義如下：


/** 

* 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(); 
}

這樣我們就可以很方便的進(jìn)行轉(zhuǎn)換了，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，如接口注釋所說，如果Function返回的結(jié)果產(chǎn)生了重復(fù)的key，將會拋出異常。

java8也提供了轉(zhuǎn)換的方法，這里直接照搬別人博客的代碼：

@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());  
}

參考：http://m.fzitv.net/article/104114.htm

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