[filter()](<https://docs.oracle.com/javase/8/docs/api/java/util/Optional.html#filter-java.util.function.Predicate->) is used to indicate that you would like the value only if it matches your predicate.

Think of it like if (!somePredicate(x)) { x = null; }.

Code examples:

String value = null;
Optional.ofNullable(value) // nothing
        .filter(x -> x.equals("cool string"))// this is never run since value is null
        .isPresent(); // false

String value = "cool string";
Optional.ofNullable(value) // something
        .filter(x -> x.equals("cool string"))// this is run and passes
        .isPresent(); // true

String value = "hot string";
Optional.ofNullable(value) // something
        .filter(x -> x.equals("cool string"))// this is run and fails
        .isPresent(); // false