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