Tag: Collectors

Once in a while I come across the task where I have a list of Items that I want to filter and afterwards store in a map. Usually the key is a property of the Item: anItem.name -> anItem

In the usual Java way this looked like the following:

Map<String, Stuff> map = new HashMap<>();
for (Stuff s : list) {
    if (!s.name.equals("a")){
        map.put(s.name, s);
    }
}

Nothing really special, but it somehow doesn’t look too nice. Yesterday I thought: in Scala I would emit tuples and call .toMap. Isn’t that also possible with Java 8 Streams? And indeed it is:

Map<String, Item> map = l.stream()
    .filter(s -> !s.name.equals("a"))
    .collect(toMap(s -> s.name, s -> s)); // toMap is a static import of Collectors.toMap(...)

This looks compact and readable!

If you don’t like s -> s, just use identity() function of the Function class. Actually I do not like static imports very much as as they make the code less readable, but in this case I would decide for static imports.

Map<String, Item> map = l.stream()
    .filter(s -> !s.name.equals("a"))
    .collect(toMap(s -> s.name, identity())); // toMap and identity are imported statically