I'm relatively new to programming and I have been wondering for past two days how to make a Predicate that is made from a custom list of other Predicates. So I've came up with some kind of solution. Below is a code snippet that should give you an idea. Because I have written it based on solely reading various pieces of documentations I have two questions: 1/ is it a good solution? 2/ is there some other, recommended solution for this problem?
public class Tester {
private static ArrayList<Predicate<String>> testerList;
//some Predicates of type String here...
public static void addPredicate(Predicate<String> newPredicate) {
if (testerList == null)
{testerList = new ArrayList<Predicate<String>>();}
testerList.add(newPredicate);
}
public static Predicate<String> customTesters () {
return s -> testerList.stream().allMatch(t -> t.test(s));
}
}
You could have a static method that receives many predicates and returns the predicate you want:
public static <T> Predicate<T> and(Predicate<T>... predicates) {
// TODO Handle case when argument is null or empty or has only one element
return s -> Arrays.stream(predicates).allMatch(t -> t.test(s));
}
A variant:
public static <T> Predicate<T> and(Predicate<T>... predicates) {
// TODO Handle case when argument is null or empty or has only one element
return Arrays.stream(predicates).reduce(t -> true, Predicate::and);
}
Here I'm using Stream.reduce
, which takes the identity and an operator as arguments. Stream.reduce
applies the Predicate::and
operator to all elements of the stream to produce a result predicate, and uses the identity to operate on the first element of the stream. This is why I have used t -> true
as the identity, otherwise the result predicate might end up evaluating to false
.
Usage:
Predicate<String> predicate = and(s -> s.startsWith("a"), s -> s.length() > 4);