I have a method to validate no negative numbers in List of numbers:
private void validateNoNegatives(List<String> numbers) {
List<String> negatives = numbers.stream().filter(x->x.startsWith("-")).collect(Collectors.toList());
if (!negatives.isEmpty()) {
throw new RuntimeException("negative values found " + negatives);
}
}
Is it possible to use a method reference instead of x->x.startsWith("-")? I thought about String::startsWith("-") but is not working.
No, you can't use a method reference because you need to provide an argument, and because the startsWith method doesn't accept the value you're trying to predicate. You could write your own method, as:
private static boolean startsWithDash(String text) {
return text.startsWith("-");
}
... then use:
.filter(MyType::startsWithDash)
Or as a non-static method, you could have:
public class StartsWithPredicate {
private final String prefix;
public StartsWithPredicate(String prefix) {
this.prefix = prefix;
}
public boolean matches(String text) {
return text.startsWith(text);
}
}
Then use:
// Possibly as a static final field...
StartsWithPredicate predicate = new StartsWithPredicate("-");
// Then...
List<String> negatives = numbers.stream().filter(predicate::matches)...
But then you might as well make StartsWithPredicate implement Predicate<String> and just pass the predicate itself in :)
noneMatch. For example,numbers.stream().noneMatch(x -> x.startsWith("-"));