Java stream: use optional filter() operations on chaining

Chain the predicates according to the conditions using Predicate::and returning a new Predicate.

Predicate<FooBar> predicate = c -> whatever();

if (condition1) { predicate = predicate.and(c -> whatever1()); }
if (condition2) { predicate = predicate.and(c -> whatever2()); }

List<FooBar> dest = list.stream()
    .filter(predicate)
    .collect(Collectors.toList());

---

Upon an update requesting a single expression. You need a source of mapped conditions to predicates anyway. With the data structure Map<Supplier<Boolean>, Predicate<Integer>>, where a key is a Supplier of a condition deciding whether a value (Predicate<FooBar>) shall be used.

Reduce the entries of a map to a new Predicate<FooBar> using chaining these Predicates with Predicate::and, for which their Supplier<Boolean> returns true (the condition is valid).

Having a Map of the conditions:

Map<Supplier<Boolean>, Predicate<FooBar>> map = new HashMap<>();
map.put(() -> needsFilter1, c -> whatever1());
map.put(() -> needsFilter2, c -> whatever2());
...

Here is a single Stream statement:

List<Integer> dest = list
        .stream()
        .filter(map.entrySet()                            // filter with a predicate ...
                .stream()
                .filter(e -> e.getKey().get())            // .. where a condition is 'true'
                .map(Entry::getValue)                     // .. get Predicates
                .reduce(i -> true, (l, r) -> l.and(r)))   // .. reduce them using AND
        .collect(Collectors.toList());               

I am a bit late with my solution, anyway I'll leave it here.

I had an idea of writing a builder to construct a complex Predicate but ended up with a class FilterCondition and a method FilterCondition.combine.

Stream.of("123", "1", "12345", "", "12", "", "2")
    .filter(FilterCondition.<String>combine(
                FilterCondition.of(() -> true, s -> s.contains("3")),
                FilterCondition.of(() -> true, s -> s.contains("2")),
                FilterCondition.of(() -> false, s -> s.isEmpty())
            ).toPredicate())
    .collect(Collectors.toList());

With the static import of FilterCondition.of and FilterCondition.combine, it would look even better.

Stream.of("123", "1", "12345", "", "12", "", "2")
    .filter(combine(
                of(() -> true, s -> s.contains("3")),
                of(() -> true, s -> s.contains("2")),
                of(() -> false, String::isEmpty)
            ).toPredicate())
    .collect(Collectors.toList());

FilterCondition<T> is basically a Predicate<T> with an extra condition for checking whether the predicate should be applied.

FilterCondition.combine takes some FilterConditions and makes up a combined one.

class FilterCondition<T> {
    private final Supplier<Boolean> filterEnabled;
    private final Predicate<T> predicate;

    private FilterCondition(Supplier<Boolean> filterEnabled, Predicate<T> predicate) {
        this.filterEnabled = filterEnabled;
        this.predicate = predicate;
    }

    public static <T> FilterCondition<T> of(Supplier<Boolean> filterEnabled, Predicate<T> predicate) {
        return new FilterCondition<>(filterEnabled, predicate);
    }

    @SafeVarargs
    public static <T> FilterCondition<T> combine(FilterCondition<T>... conditions) {
        return new FilterCondition<>(
                () -> true,
                Arrays.stream(conditions).filter(i -> i.filterEnabled.get()).map(i -> i.predicate).reduce(Predicate::and).orElse(t -> true)
        );
    }

    public Predicate<T> toPredicate() {
        return predicate;
    }

}