package com.insightfullogic.java8.answers.chapter3;
import java.util.ArrayList;
import java.util.List;
import java.util.function.Predicate;
import java.util.stream.Stream;
/**
* Advanced Exercises Question 1
*/
public class FilterUsingReduce {
public static <I> List<I> filter(Stream<I> stream, Predicate<I> predicate) {
List<I> initial = new ArrayList<>();
return stream.reduce(initial,
(List<I> acc, I x) -> {
if (predicate.test(x)) {
// We are copying data from acc to new list instance. It is very inefficient,
// but contract of Stream.reduce method requires that accumulator function does
// not mutate its arguments.
// Stream.collect method could be used to implement more efficient mutable reduction,
// but this exercise asks to use reduce method explicitly.
List<I> newAcc = new ArrayList<>(acc);
newAcc.add(x);
return newAcc;
} else {
return acc;
}
},
FilterUsingReduce::combineLists);
}
private static <I> List<I> combineLists(List<I> left, List<I> right) {
// We are copying left to new list to avoid mutating it.
List<I> newLeft = new ArrayList<>(left);
newLeft.addAll(right);
return newLeft;
}
}
