How to negate a method reference predicate

I'm planning to static import the following to allow for the method reference to be used inline:

public static <T> Predicate<T> not(Predicate<T> t) {
    return t.negate();


Stream<String> s = ...;
long nonEmptyStrings = s.filter(not(String::isEmpty)).count();

Update: Starting from Java-11, the JDK offers a similar solution built-in as well.

Predicate.not( … )

java-11 offers a new method Predicate#not

So you can negate the method reference:

Stream<String> s = ...;
long nonEmptyStrings = s.filter(Predicate.not(String::isEmpty)).count();

There is a way to compose a method reference that is the opposite of a current method reference. See @vlasec's answer below that shows how by explicitly casting the method reference to a Predicate and then converting it using the negate function. That is one way among a few other not too troublesome ways to do it.

The opposite of this:

Stream<String> s = ...;
int emptyStrings = s.filter(String::isEmpty).count();

is this:

Stream<String> s = ...;
int notEmptyStrings = s.filter(((Predicate<String>) String::isEmpty).negate()).count()

or this:

Stream<String> s = ...;
int notEmptyStrings = s.filter( it -> !it.isEmpty() ).count();

Personally, I prefer the later technique because I find it clearer to read it -> !it.isEmpty() than a long verbose explicit cast and then negate.

One could also make a predicate and reuse it:

Predicate<String> notEmpty = (String it) -> !it.isEmpty();

Stream<String> s = ...;
int notEmptyStrings = s.filter(notEmpty).count();

Or, if having a collection or array, just use a for-loop which is simple, has less overhead, and *might be **faster:

int notEmpty = 0;
for(String s : list) if(!s.isEmpty()) notEmpty++;

*If you want to know what is faster, then use JMH, and avoid hand benchmark code unless it avoids all JVM optimizations — see Java 8: performance of Streams vs Collections

**I am getting flak for suggesting that the for-loop technique is faster. It eliminates a stream creation, it eliminates using another method call (negative function for predicate), and it eliminates a temporary accumulator list/counter. So a few things that are saved by the last construct that might make it faster.

I do think it is simpler and nicer though, even if not faster. If the job calls for a hammer and a nail, don't bring in a chainsaw and glue! I know some of you take issue with that.

wish-list: I would like to see Java Stream functions evolve a bit now that Java users are more familiar with them. For example, the 'count' method in Stream could accept a Predicate so that this can be done directly like this:

Stream<String> s = ...;
int notEmptyStrings = s.count(it -> !it.isEmpty());


List<String> list = ...;
int notEmptyStrings = lists.count(it -> !it.isEmpty());