Failure to choose a right overload for method called from a generic method

Question

I have a following code:

class A{} class B{} class StuffDoer { public void doStuff(A a) { System.out.println("A"); } public void doStuff(B b) { System.out.println("B"); } public void doStuff(Object o) { System.out.println("Object"); } } class Test { private final StuffDoer stuffDoer; public <T> void foo(T t) { stuffDoer.doStuff(t) } }

And following execution

Test test = new Test(); A a = new A(); B b = new B(); test.foo(a); test.foo(b);

prints "Object" twice, instead of expected "A" and "B" afterwards.

It doesn't work if i explicit pass a Class object either

class Test { private final StuffDoer stuffDoer; public <T> void foo(T t) { //doesnt work stuffDoer.doStuff(t.getClass().cast(t)) } public <T> void foo(T t, Class<T> tClass) { //doesnt work either stuffDoer.doStuff(tClass.cast(t)) } }

It only works if i explicity cast them to proper object in the foo method like this

class Test { private final StuffDoer stuffDoer; public <T> void foo(T t) { if ( t instanceof A ) stuffDoer.doStuff((A) t) // Prints "A" else if ( t instance of B ) stuffDoer.doStuff((B) t) // Prints "B" else stuffDoer.doStuff(t) // Prints "Object" } }

Why is that? How can i achive right method overloading from a generic type? Is it even possible in Java?

At runtime T gets erased to Object when you don't specify a bound. getClass() returns the runtime type. — Anthony Cathers
– Anthony Cathers, Commented Jan 25, 2022 at 10:09
Think about how you would have to call your method that has the tClass argument. test.foo(b, B.class); You're passing a class literal. — Anthony Cathers
– Anthony Cathers, Commented Jan 25, 2022 at 10:33
Does this answer your question? Overloading in Java and multiple dispatch Overloaded method selection based on the parameter's real type — dshelya
– dshelya, Commented Jan 26, 2022 at 13:09

dshelya · Accepted Answer · 2022-01-26 14:23:20Z

In Java only the method receiver is picked dynamically at runtime whereas the signature of a method is determined at compile time using the provided argument type(s), so that doStuff(Object o) method is being chosen as the only possible (commenting it out would result in a compile-time error).

There aren't many options around to "emulate" multiple dispatch in the way you want:

A chain of instanceof as you already mentioned
A Visitor/Strategy pattern
Dynamic method lookup via Reflection or MethodHandle

Pattern matching (since Java 17):

public <T> void foo(T t) { switch (t) { case A a -> stuffDoer.doStuff(a); case B b -> stuffDoer.doStuff(b); case null -> System.out.println("OOPS :("); default -> stuffDoer.doStuff(t); } }

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