Issue with Java 8 Lambda for effective final while incrementing counts

Question

I want to use Java 8 Lambda expression in following scenario but I am getting Local variable fooCount defined in an enclosing scope must be final or effectively final.

Answer 1

To get both numbers, matching and non-matching elements, you can use

Map result = map.keySet().stream()
    .collect(Collectors.partitioningBy(k -> k.contains("FOO"), Collectors.counting()));
long fooCount = result.get(true);
long barCount = result.get(false);

But since your source is a Map, which knows its total size, and want to calculate a percentage, for which barCount is not needed, this specific task can be solved as

private int calculateFooPercentage() {
    return (int)(map.keySet().stream().filter(k -> k.contains("FOO")).count()
                 *100/map.size());
}

Both variants are thread safe, i.e. changing stream() to parallelStream() will perform the operation in parallel, however, it’s unlikely that this operation will benefit from parallel processing. You would need humongous key strings or maps to get a benefit…