Throw exception by condition in stream

Question

How to filter stream expression to throw exception?

I have function to check sum of list elements, if sum is more than 1 -> throw exception

Double sum = list.stream().map(element::getNumber).reduce(0.0,Double::sum);
if(sum>1){
   throw new Exception("message);
}

Is it possible to add if condition to stream expression to get rid of the additional if condition?

Lino · Accepted Answer · 2021-04-29T11:32:55.943

9

You could make use of the single argument Stream.reduce() and Optional.orElseThrow()

list.stream()
    .map(element::getNumber)
    .reduce(Double::sum)
    .filter(sum -> sum <= 1)
    .orElseThrow(() -> new Exception("message"));

edited Apr 29 '21 at 11:32

answered Apr 29 '21 at 10:38

Lino

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A nice one. I wondered if there is a solution using `mapToDouble`... but nope. – Nikolas Charalambidis May 07 '21 at 12:13

score 2 · Answer 2 · answered Apr 29 '21 at 11:33

You can do it inside of the reduce operation, the downside is that you can't use a checked exception natively, but you can find workarounds to still throw a checked exception inside of a lambda

Double sum = list.stream()
                 .map(element::getNumber)
                 .reduce(0.0, (first, second) -> {
                     double s = first + second;
                     if (s > 1) throw new RuntimeException("");
                     return s;
                 });

Throw exception by condition in stream

2 Answers2