Functional Interface in Java

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public static <A, B> A fold(BiFunction<A,B,A> folder, A s, List<B> bList){
    if (bList.isEmpty()) return s;
    B b = bList.get(0);
    return folder.apply(fold(folder, s, bList.subList(1, bList.size())), b);
}

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Integer sum = fold((Integer::sum), 0, List.of(10, 10,20));

public static Integer length(int state, int a) {
    return state + 1;
}
Integer length = fold((Application::length), 0, List.of(10, 10,20));

public static List<Integer> incrementByOne(List<Integer> state, int a) {
    //If we don't add (0) as starting index it will be reversed list
    state.add(0, a + 1);
    return state;
}
List<Integer> incrementByOne = fold((Application::incrementByOne), new ArrayList<>(), List.of(10, 10,20));