Local variables referenced from a lambda expression must be final

Question

I´m trying to create a getValue() function from a pojo that use summatory of details class values in this sense:

@Transient
public BigDecimal getValue() {
   BigDecimal sum = new BigDecimal(0);

   details.stream().forEach((detail) -> {
      sum = sum.add(detail.getValue());
   });

   return sum;
}

but I don't know why this the line sum = sum.add(detail.getValue()); provoke this error:

local variables referenced from a lambda expression must be final or effectively final

Can you say me what's I'am doing wrong. Thanks.

Answer 1

You cannot modify variables from inside a lambda. That's just not a thing you're allowed to do.

What you can do here is write this method as

return details.stream()
    .map(Detail::getValue)
    .reduce(BigDecimal.ZERO, BigDecimal::add);