Java 8 Streams: Map the same object multiple times based on different properties

Question

I was presented with an interesting problem by a colleague of mine and I was unable to find a neat and pretty Java 8 solution. The problem is to stream through a list of POJOs and then collect them in a map based on multiple properties - the mapping causes the POJO to occur multiple times

Imagine the following POJO:

private static class Customer {     public String first;     public String last;      public Customer(String first, String last) {         this.first = first;         this.last = last;     }      public String toString() {         return "Customer(" + first + " " + last + ")";     } } 

Set it up as a List<Customer>:

// The list of customers List<Customer> customers = Arrays.asList(         new Customer("Johnny", "Puma"),         new Customer("Super", "Mac")); 

Alternative 1: Use a Map outside of the "stream" (or rather outside forEach).

// Alt 1: not pretty since the resulting map is "outside" of // the stream. If parallel streams are used it must be // ConcurrentHashMap Map<String, Customer> res1 = new HashMap<>(); customers.stream().forEach(c -> {     res1.put(c.first, c);     res1.put(c.last, c); }); 

Alternative 2: Create map entries and stream them, then flatMap them. IMO it is a bit too verbose and not so easy to read.

// Alt 2: A bit verbose and "new AbstractMap.SimpleEntry" feels as // a "hard" dependency to AbstractMap Map<String, Customer> res2 =         customers.stream()                 .map(p -> {                     Map.Entry<String, Customer> firstEntry = new AbstractMap.SimpleEntry<>(p.first, p);                     Map.Entry<String, Customer> lastEntry = new AbstractMap.SimpleEntry<>(p.last, p);                     return Stream.of(firstEntry, lastEntry);                 })                 .flatMap(Function.identity())                 .collect(Collectors.toMap(                         Map.Entry::getKey, Map.Entry::getValue)); 

Alternative 3: This is another one that I came up with the "prettiest" code so far but it uses the three-arg version of reduce and the third parameter is a bit dodgy as found in this question: Purpose of third argument to 'reduce' function in Java 8 functional programming. Furthermore, reduce does not seem like a good fit for this problem since it is mutating and parallel streams may not work with the approach below.

// Alt 3: using reduce. Not so pretty Map<String, Customer> res3 = customers.stream().reduce(         new HashMap<>(),         (m, p) -> {             m.put(p.first, p);             m.put(p.last, p);             return m;         }, (m1, m2) -> m2 /* <- NOT USED UNLESS PARALLEL */); 

If the above code is printed like this:

System.out.println(res1); System.out.println(res2); System.out.println(res3); 

The result would be:

{Super=Customer(Super Mac), Johnny=Customer(Johnny Puma), Mac=Customer(Super Mac), Puma=Customer(Johnny Puma)}
{Super=Customer(Super Mac), Johnny=Customer(Johnny Puma), Mac=Customer(Super Mac), Puma=Customer(Johnny Puma)}
{Super=Customer(Super Mac), Johnny=Customer(Johnny Puma), Mac=Customer(Super Mac), Puma=Customer(Johnny Puma)}

So, now to my question: How should I, in a Java 8 orderly fashion, stream through the List<Customer> and then somehow collect it as a Map<String, Customer> where you split the whole thing as two keys (first AND last) i.e. the Customer is mapped twice. I do not want to use any 3rd party libraries, I do not want to use a map outside of the stream as in alt 1. Are there any other nice alternatives?

The full code can be found on hastebin for simple copy-paste to get the whole thing running.

Answer 1

I think your alternatives 2 and 3 can be re-written to be more clear:

Alternative 2:

Map<String, Customer> res2 = customers.stream()     .flatMap(         c -> Stream.of(c.first, c.last)         .map(k -> new AbstractMap.SimpleImmutableEntry<>(k, c))     ).collect(toMap(Map.Entry::getKey, Map.Entry::getValue)); 

Alternative 3: Your code abuses reduce by mutating the HashMap. To do mutable reduction, use collect:

Map<String, Customer> res3 = customers.stream()     .collect(         HashMap::new,          (m,c) -> {m.put(c.first, c); m.put(c.last, c);},          HashMap::putAll     ); 

Note that these are not identical. Alternative 2 will throw an exception if there are duplicate keys while Alternative 3 will silently overwrite the entries.

If overwriting entries in case of duplicate keys is what you want, I would personally prefer Alternative 3. It is immediately clear to me what it does. It most closely resembles the iterative solution. I would expect it to be more performant as Alternative 2 has to do a bunch of allocations per customer with all that flatmapping.

However, Alternative 2 has a huge advantage over Alternative 3 by separating the production of entries from their aggregation. This gives you a great deal of flexibility. For example, if you want to change Alternative 2 to overwrite entries on duplicate keys instead of throwing an exception, you would simply add (a,b) -> b to toMap(...). If you decide you want to collect matching entries into a list, all you would have to do is replace toMap(...) with groupingBy(...), etc.