Is there a way to print a functional interface?

Question

Suppose I have an interface:

public interface Function {
    double function (double input);
}

Now, suppose I have created an instance of this interface somewhere in my main class,

Function f = (x) -> x;

How can I go about printing this function, in plain text? So, something a bit like this:

int f (double x) {return x}

Running a .toString on this Function prints something like Main$1@6d06d69c. How can I go about getting the java representation of this interface?

Answer 1

Remember that the text of a function (otherwise known as "code") only exists when you write it. You compile this to bytecode which is then run on the Java Virtual Machine. At runtime, the original code which you wrote no longer exists and cannot be easily retrieved.

Answer 2

Unfortunately, the answer (as of Java 9) is that there isn't a simple way to get the toString() method to give you a human-meaningful value for an arbitrary instance of a functional interface.

Here are a couple of alternatives that are applicable for some use-cases:

• Instead of using a lambda, implement the interface using a class, and include an appropriate override for the toString() method.

• Populate a Map<Function, String> with meaningful names for all of your Function instances.

It would be theoretically possible to build a library that can retrieve the ".class" file for (say) a lambda, analyse it, work out what the bytecodes do, and then produce an appropriate summary. But it would be difficult project.

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It would be nice if there was a simple, clean solution to this. Maybe "someone" could suggest it as an RFE for a future version of Java.