Transform and filter a Java Map with streams

Question

I have a Java Map that I\'d like to transform and filter. As a trivial example, suppose I want to convert all values to Integers then remove the odd entries.

Map         


        

Answer 1

You could use the Stream.collect(supplier, accumulator, combiner) method to transform the entries and conditionally accumulate them:

Map even = input.entrySet().stream().collect(
    HashMap::new,
    (m, e) -> Optional.ofNullable(e)
            .map(Map.Entry::getValue)
            .map(Integer::valueOf)
            .filter(i -> i % 2 == 0)
            .ifPresent(i -> m.put(e.getKey(), i)),
    Map::putAll);

System.out.println(even); // {a=1234, c=3456}

Here, inside the accumulator, I'm using Optional methods to apply both the transformation and the predicate, and, if the optional value is still present, I'm adding it to the map being collected.