Java method override/interface problem

Question

I have

interface FooI
class FooA implements FooI
class FooB implements FooI
class FooC implements FooI

I wrote a class "Handler" that has the following methods

static double handle(FooA f)
static double handle(FooB f)
static double handle(FooI f)

and I have a function like the following:

void caller(FooI f)
{
    Handler.handle(f);
}

with f just being known as a class implementing FooI. However f is an instance of FooA

Instead of calling the method for FooA, the method for FooI is called.

When I use f.getClass().getName() I get the correct class name (FooA).

I am confused because I expected the best-suited method to get called, namely the one for FooA.

I wanted to use the FooI method as a fallback to handle classes that could be implemented later, and I also want to handle f without having one big function where I am doing instanceof checks for all currently known classes implementing my interface.

What would be a good approach to do this?

Answer 1

Overload resolution is done at compile time rather than execution time in Java. The signature was picked based on the compile-time type of your variable. Only overrides are determined at execution time.

A common solution for this in languages like Java is to use double dispatch and the visitor pattern.

Personally I'm not a huge fan of this. I prefer to try to design my way around it. That's not always possible, of course, but it's worth at least attempting.