The main goal of this project is to show Collectors.toMap functionality.

1. keyMapper, valueMapper; default: HashMap::new

   public static <T, K, U>
       Collector<T, ?, Map<K,U>> toMap(
                                   Function<? super T, ? extends K> keyMapper,
                                   Function<? super T, ? extends U> valueMapper)
   

2. keyMapper, valueMapper, mergeFunction; default: HashMap::new

   public static <T, K, U>
       Collector<T, ?, Map<K,U>> toMap(
                                   Function<? super T, ? extends K> keyMapper,
                                   Function<? super T, ? extends U> valueMapper,
                                   BinaryOperator<U> mergeFunction)
   

3. keyMapper, valueMapper, mergeFunction, mapFactory

   public static <T, K, U, M extends Map<K, U>>
       Collector<T, ?, M> toMap(Function<? super T, ? extends K> keyMapper,
                                Function<? super T, ? extends U> valueMapper,
                                BinaryOperator<U> mergeFunction,
                                Supplier<M> mapFactory)
   

In CollectorsToMapTest we show examples of above methods.

• keyMapper is null -> NullPointerException
• valueMapper is null -> NullPointerException
• keyMapper returns null -> OK
• valueMapper returns null -> NullPointerException
• same keys + 1. type -> IllegalStateException
• examples of merge function (takeFirst, takeLast, exception if same)
• valuable example of factoryMap with TreeMap

We have two more methods to create maps:

• toUnmodifiableMap

  public static <T, K, U>
  Collector<T, ?, Map<K,U>> 
  toUnmodifiableMap(Function<? super T, ? extends K> keyMapper,
                    Function<? super T, ? extends U> valueMapper)
  

  public static <T, K, U>
  Collector<T, ?, Map<K,U>> 
  toUnmodifiableMap(Function<? super T, ? extends K> keyMapper,
                    Function<? super T, ? extends U> valueMapper,
                    BinaryOperator<U> mergeFunction)    
  

• toConcurrentMap, default: ConcurrentHashMap::new

  public static <T, K, U>
  Collector<T, ?, ConcurrentMap<K,U>> 
  toConcurrentMap(Function<? super T, ? extends K> keyMapper,
                  Function<? super T, ? extends U> valueMapper)
  

  public static <T, K, U>
  Collector<T, ?, ConcurrentMap<K,U>>
  toConcurrentMap(Function<? super T, ? extends K> keyMapper,
                  Function<? super T, ? extends U> valueMapper,
                  BinaryOperator<U> mergeFunction)    
  

  public static <T, K, U, M extends ConcurrentMap<K, U>>
  Collector<T, ?, M> 
  toConcurrentMap(Function<? super T, ? extends K> keyMapper,
                  Function<? super T, ? extends U> valueMapper,
                  BinaryOperator<U> mergeFunction,
                  Supplier<M> mapFactory)