Passing a void method with Callable using Java generics and Java 8 lambda

Question

I'm attempting to write a generic performance logging function that I can pass any method to be timed into and it will log the execution time to my database. I have this working in most cases, but I get compiler errors when passing a method that returns void:

no instance(s) of type variable(s) T exist so that void conforms to T

Here is my class:

public class Performance {
    public static <T> T measureExecTime(Callable<T> c, String gid, String name) {
        T call = null;
        try {
            if (Constants.DEBUG) {
                Calendar start = Calendar.getInstance();
                call = c.call();
                Calendar end = Calendar.getInstance();
                long diff = end.getTimeInMillis() - start.getTimeInMillis();
                // log diff to database
            } else {
                call = c.call();
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
        return call;
    }
}

And here is how I am calling it:

Performance.measureExecTime(() -> myMethod(myMethodParam1, myMethodParam2), "gid", "database_qualifier");

Is it possible for this functionality to behave on void methods, or am I out of luck?

Answer 1

Callable must have a return value. In this case you can simply return null, but you'll need a block lambda:

Performance.measureExecTime(() -> {
    myMethod(myMethodParam1, myMethodParam2);
    return null;
}, "gid", "database_qualifier");

If you want, you can create a helper method to make things a little cleaner:

static Callable<?> wrap(Runnable r) {
    return () -> {
        r.run();
        return null;
    };
}
// ...
Performance.measureExecTime(wrap(() -> myMethod(myMethodParam1, myMethodParam2)), "gid", "database_qualifier");