Method References

As we all know by now, we can use Method References, like String::isEmpty, in Java 8 to reference a method that is being used when we, for example, stream over elements. Take a look at this code snippet:

    Stream.of("A", "", "B").filter(String::isEmpty).count();

which will produce the result 1 (because there is just one empty element in the stream). But, if we want to filter out non-empty strings, we need to write .filter(s -> !s.isEmpty()) which is a Lambda. Clearly, there is an annoying asymmetry here. We can use a method reference, but not its negation. We can write predicate.negate() but we cannot write String::isEmpty.negate() or !String::isEmpty.

Why is that? It's because a Method Reference is not a Lambda or a Functional Interface. However, a Method Reference can be resolved to one or several Functional Interfaces using Java's type inference. Our example String::isEmpty can, in fact, be resolved to at least:

• Predicate<String>
• Function<String, Boolean>

So, we need to somehow resolve all the potential ambiguities and decide which Functional Interface we want to turn the Method Reference into. Read this post and find out how to partially fix this issue. I have used code presented here in the open-source project Speedment that makes databases look like Java 8 Streams. Feel free to try Speedment out.

Speedment  also contains predicate builders that allow you to use functions like Entity.NAME::isEmpty and Entity.NAME::isNotEmpty directly.

Resolving Method References

The problem can be partially fixed by introducing some "plumbing" in the form of static methods that takes a Method Reference and returns it as a view of a specific Functional Interface. Consider this short static method:

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

Now, if we import that method statically we can, in fact, use a Method Reference more easily as shown in this short example:

    Stream.of("A", "", "B").filter(as(String::isEmpty).negate()).count();

The code will return 2 which is the number of non-empty elements in the stream. This is a step forward in terms of Method Reference usage. Another benefit is that this solution allows us to compose our predicates more easily like this:

.filter(as(String::isEmpty).negate().and("A"::equals))

Resolving All Method References

But there is still a problem we have to resolve. We can not simply start creating a lot of static as() functions, because a Method Reference might be resolvable to several type of potential as() methods in the same way as listed in the beginning of this post. So, a better approach is to append the Functional Interface type name to each static method, allowing us to programmatically select a particular Method Reference to Functional Interface conversion method. Here is a utility class that allows Method References to be converted to any matching Functional Interface that resides in the standard Java package java.util.function.

Pull down the latest version directly from GitHub here

import java.util.function.*;

/**
 *
 * @author Per Minborg
 */
public class FunctionCastUtil {

    public static <T, U> BiConsumer<T, U> asBiConsumer(BiConsumer<T, U> biConsumer) {
        return biConsumer;
    }

    public static <T, U, R> BiFunction<T, U, R> asBiFunction(BiFunction<T, U, R> biFunction) {
        return biFunction;
    }

    public static <T> BinaryOperator<T> asBinaryOperator(BinaryOperator<T> binaryOperator) {
        return binaryOperator;
    }

    public static <T, U> BiPredicate<T, U> asBiPredicate(BiPredicate<T, U> biPredicate) {
        return biPredicate;
    }

    public static BooleanSupplier asBooleanSupplier(BooleanSupplier booleanSupplier) {
        return booleanSupplier;
    }

    public static <T> Consumer<T> asConsumer(Consumer<T> consumer) {
        return consumer;
    }

    public static DoubleBinaryOperator asDoubleBinaryOperator(DoubleBinaryOperator doubleBinaryOperator) {
        return doubleBinaryOperator;
    }

    public static DoubleConsumer asDoubleConsumer(DoubleConsumer doubleConsumer) {
        return doubleConsumer;
    }

    public static <R> DoubleFunction<R> asDoubleFunction(DoubleFunction<R> doubleFunction) {
        return doubleFunction;
    }

    public static DoublePredicate asDoublePredicate(DoublePredicate doublePredicate) {
        return doublePredicate;
    }

    public static DoubleToIntFunction asDoubleToIntFunction(DoubleToIntFunction doubleToIntFunctiontem) {
        return doubleToIntFunctiontem;
    }

    public static DoubleToLongFunction asDoubleToLongFunction(DoubleToLongFunction doubleToLongFunction) {
        return doubleToLongFunction;
    }

    public static DoubleUnaryOperator asDoubleUnaryOperator(DoubleUnaryOperator doubleUnaryOperator) {
        return doubleUnaryOperator;
    }

    public static <T, R> Function<T, R> asFunction(Function<T, R> function) {
        return function;
    }

    public static IntBinaryOperator asIntBinaryOperator(IntBinaryOperator intBinaryOperator) {
        return intBinaryOperator;
    }

    public static IntConsumer asIntConsumer(IntConsumer intConsumer) {
        return intConsumer;
    }

    public static <R> IntFunction<R> asIntFunction(IntFunction<R> intFunction) {
        return intFunction;
    }

    public static IntPredicate asIntPredicate(IntPredicate intPredicate) {
        return intPredicate;
    }

    public static IntSupplier asIntSupplier(IntSupplier intSupplier) {
        return intSupplier;
    }

    public static IntToDoubleFunction asIntToDoubleFunction(IntToDoubleFunction intToDoubleFunction) {
        return intToDoubleFunction;
    }

    public static IntToLongFunction asIntToLongFunction(IntToLongFunction intToLongFunction) {
        return intToLongFunction;
    }

    public static IntUnaryOperator asIntUnaryOperator(IntUnaryOperator intUnaryOperator) {
        return intUnaryOperator;
    }

    public static LongBinaryOperator asLongBinaryOperator(LongBinaryOperator longBinaryOperator) {
        return longBinaryOperator;
    }

    public static LongConsumer asLongConsumer(LongConsumer longConsumer) {
        return longConsumer;
    }

    public static <R> LongFunction<R> asLongFunction(LongFunction<R> longFunction) {
        return longFunction;
    }

    public static LongPredicate asLongPredicate(LongPredicate longPredicate) {
        return longPredicate;
    }

    public static <T> LongSupplier asLongSupplier(LongSupplier longSupplier) {
        return longSupplier;
    }

    public static LongToDoubleFunction asLongToDoubleFunction(LongToDoubleFunction longToDoubleFunction) {
        return longToDoubleFunction;
    }

    public static LongToIntFunction asLongToIntFunction(LongToIntFunction longToIntFunction) {
        return longToIntFunction;
    }

    public static LongUnaryOperator asLongUnaryOperator(LongUnaryOperator longUnaryOperator) {
        return longUnaryOperator;
    }

    public static <T> ObjDoubleConsumer<T> asObjDoubleConsumer(ObjDoubleConsumer<T> objDoubleConsumer) {
        return objDoubleConsumer;
    }

    public static <T> ObjIntConsumer<T> asObjIntConsumer(ObjIntConsumer<T> objIntConsumer) {
        return objIntConsumer;
    }

    public static <T> ObjLongConsumer<T> asObjLongConsumer(ObjLongConsumer<T> objLongConsumer) {
        return objLongConsumer;
    }

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

    public static <T> Supplier<T> asSupplier(Supplier<T> supplier) {
        return supplier;
    }

    public static <T, U> ToDoubleBiFunction<T, U> asToDoubleBiFunction(ToDoubleBiFunction<T, U> toDoubleBiFunction) {
        return toDoubleBiFunction;
    }

    public static <T> ToDoubleFunction<T> asToDoubleFunction(ToDoubleFunction<T> toDoubleFunction) {
        return toDoubleFunction;
    }

    public static <T, U> ToIntBiFunction<T, U> asToIntBiFunction(ToIntBiFunction<T, U> toIntBiFunction) {
        return toIntBiFunction;
    }

    public static <T> ToIntFunction<T> asToIntFunction(ToIntFunction<T> ioIntFunction) {
        return ioIntFunction;
    }

    public static <T, U> ToLongBiFunction<T, U> asToLongBiFunction(ToLongBiFunction<T, U> toLongBiFunction) {
        return toLongBiFunction;
    }

    public static <T> ToLongFunction<T> asToLongFunction(ToLongFunction<T> toLongFunction) {
        return toLongFunction;
    }

    public static <T> UnaryOperator<T> asUnaryOperator(UnaryOperator<T> unaryOperator) {
        return unaryOperator;
    }

    private FunctionCastUtil() {
    }

}

So after we have imported the relevant methods statically, we can write:

    Stream.of("A", "", "B").filter(asPredicate(String::isEmpty).negate()).count();

An Even Better Solution

It would be even better if all the Functional Interfaces themselves contained a static method that could take a suitable Method Reference and turn it into a typed Functional Interface. For example, the standard Java Predicate Functional Interface would then look like this:

@FunctionalInterface
public interface Predicate<T> {

    boolean test(T t);

    default Predicate<T> and(Predicate<? super T> other) {...}

    default Predicate<T> negate() {...}

    default Predicate<T> or(Predicate<? super T> other) {...}

    static <T> Predicate<T> isEqual(Object targetRef) {...}

    // New proposed support method to return a 
    // Predicate view of a Functional Reference 
    public static <T> Predicate<T> of(Predicate<T> predicate) {
        return predicate;
    }
    
}

This would allow us to write:

    Stream.of("A", "", "B").filter(Predicate.of(String::isEmpty).negate()).count();

Which I personally think looks good!

Contact your nearest Open JDK developer and make the proposal!