java implements list or set to map

java implements list or set to map

In development, we sometimes need to convert list or set to map (for example, the only key in the object property is key of map, and the object is value of map). The idea of 1 is new1 map, and then the value of list or set is 1 push to map.

Similar to the following code:

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

Is there a more elegant way to write it? The answer is yes.

guava provides a method for transferring the collection (which implements the Iterables interface or Iterator interface) to map. The method is defined as follows:

/** 

* 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(); 
} 

In this way, we can easily carry out the conversion, as follows:

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; 
  } 
}); 

It is important to note that, as the interface annotation says, if the result returned by Function produces a duplicate key, an exception will be thrown.

java8 also provides a conversion method, where you can directly copy the code of someone else's blog:

@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());  
}  

Reference: https: / / www. ofstack. com article / 104114. htm