Solve no final variable inside Java 8 Stream

问题

Is there is a way to convert the following code to Java 8 Stream.

    final List ret = new ArrayList(values.size());
    double tmp = startPrice;
    for (final Iterator it = values.iterator(); it.hasNext();) {
      final DiscountValue discountValue = ((DiscountValue) it.next()).apply(quantity, tmp, digits, currencyIsoCode);
      tmp -= discountValue.getAppliedValue();
      ret.add(discountValue);
    }

Java 8 streams complains about no final variable tmp ? Is there a way to solve such situations ?

Local variable tmp defined in an enclosing scope must be final or effectively final

回答1:

First, change the code to use generics and an enhanced for loop. Assuming values is then a List<DiscountValue>, this is what you get:

List<DiscountValue> ret = new ArrayList<>(values.size());
double tmp = startPrice;
for (DiscountValue value : values) {
    DiscountValue discountValue = value.apply(quantity, tmp, digits, currencyIsoCode);
    tmp -= discountValue.getAppliedValue();
    ret.add(discountValue);
}

I'd suggest staying with that, and not convert it to streams, but if you insist, you can use a one-element array as a value-holder.

Note that ret and tmp doesn't have to be declared final, as long as they are effectively-final.

List<DiscountValue> ret = new ArrayList<>(values.size());
double[] tmp = { startPrice };
values.stream().forEachOrdered(v -> {
    DiscountValue discountValue = v.apply(quantity, tmp[0], digits, currencyIsoCode);
    tmp[0] -= discountValue.getAppliedValue();
    ret.add(discountValue);
});

As you can see, you haven't gained anything by using streams. The code is actually worse, so ... don't.

来源：https://stackoverflow.com/questions/38374961/solve-no-final-variable-inside-java-8-stream