Java 8 – Performance Benefits of Method Reference vs Lambda

javajava8lambdaperformancesyntax

Do method references skip the overhead of the lambda wrapper? Might they in the future?

According to the Java Tutorial on Method References:

Sometimes… a lambda expression does nothing but call an existing method.
In those cases, it's often clearer to refer to the existing method by
name. Method references enable you to do this; they are compact,
easy-to-read lambda expressions for methods that already have a name.

I prefer the lambda syntax to the method reference syntax for several reasons:

Lambdas are clearer

Despite Oracle's claims, I find the lambda syntax easier-to-read than the object method reference short-hand because the method reference syntax is ambiguous:

Bar::foo

Are you calling a static one-argument method on the class of x and passing it x?

x -> Bar.foo(x)

Or are you calling a zero-argument instance method on x?

x -> x.foo()

The method reference syntax could stand in for either one. It hides what your code is actually doing.

Lambdas are safer

If you reference Bar::foo as a class method and Bar later adds an instance method of the same name (or vice-versa), your code will no longer compile.

You can use lambdas consistently

You can wrap any function in a lambda – so you can use the same syntax consistently everywhere. The method reference syntax won't work on methods that take or return primitive arrays, throw checked exceptions, or have the same method name used as an instance and a static method (because the method reference syntax is ambiguous about which method would be called). They don't work when you have overloaded methods with the same number of arguments, but you shouldn't do that anyway (see Josh Bloch's item 41) so we can't hold that against method references.

Conclusion

If there's no performance penalty for doing so, I'm tempted to turn off the warning in my IDE and use the lambda syntax consistently without sprinkling the occasional method reference into my code.

P.S.

Neither here nor there, but in my dreams, object method references look more like this and apply invoke-dynamic against the method directly on the object without a lambda wrapper:

_.foo()

Best Answer

In many scenarios, I think lambda and method-reference is equivalent. But the lambda will wrap the invocation target by the declaring interface type.

For example

public class InvokeTest {

    private static void invoke(final Runnable r) {
        r.run();
    }

    private static void target() {
        new Exception().printStackTrace();
    }

    @Test
    public void lambda() throws Exception {
        invoke(() -> target());
    }

    @Test
    public void methodReference() throws Exception {
        invoke(InvokeTest::target);
    }
}

You will see the console output the stacktrace.

In lambda(), the method calling target() is lambda$lambda$0(InvokeTest.java:20), which has traceable line info. Obviously, that is the lambda you write, the compiler generates an anonymous method for you. And then, the caller of the of the lambda method is something like InvokeTest$$Lambda$2/1617791695.run(Unknown Source), that is the invokedynamic call in JVM, it means the call is linked to the generated method.

In methodReference(), the method calling target() is directly the InvokeTest$$Lambda$1/758529971.run(Unknown Source), it means the call is directly linked to the InvokeTest::target method.

Conclusion

Above all, compare to method-reference, using lambda expression will only cause one more method call to the generating method from lambda.

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