Java Method Overloading Called with an Interface

Question

I'm sure this is elementary, but I am stumped. The example is grossly over-simplified but boils down to the following. I have some overloaded methods in a class like so:

public void build(MyImplOneOfAnInterface item); public void build(MyImplTwoOfAnInterface item);

I then have another method that does the following:

public void buildIt(MyInterface item) { build(item); }

When I attempt to compile, I get the following error:

cannot find symbol

This is because the JVM cannot determine the implementation of the interface at compile time so that it knows which overloaded method to call.

How can this be decided at runtime? It seems like the JVM ought to be able to figure this out.

PS: I don't want to define a method that takes the interface as the argument and then does a bunch of if/else statements using instanceof operators.

"cannot find symbol" sounds like a compiler error (not a runtime error from the JVM) saying that a variable or class you've specified in the code cannot be found. — coobird
– coobird, Commented Jul 18, 2011 at 15:51

Oliver Charlesworth · Accepted Answer · 2011-07-18 15:51:55Z

7

You could use double dispatch. Have a MyInterface.build, which is then overloaded in your implementation classes to call the relevant method. e.g.:

interface MyInterface { void build(Thingy t); } class MyImplOneOfAnInterface implements MyInterface { void build(Thingy t) { t.build(this); } } ... void buildIt(MyInterface item) { item.build(this); }

answered Jul 18, 2011 at 15:51

Oliver Charlesworth

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8 Comments

Kit Ho Over a year ago

what about using an extend? i.e., class MyImplOneOfAnInterface extends My Interface. sounds it would be better?

Mark Peters Over a year ago

This is simulated double dispatch, and is an application of the Visitor pattern. The fact is, Java can't do true double dispatch.

Oliver Charlesworth Over a year ago

@Kit Ho: I don't understand what you mean. I'm assuming MyImplOneOfAnInterface is a concrete class.

Mark Peters Over a year ago

@Kit: You can't extend an interface, and if you mean implementation inheritance via a (abstract) base class then it wouldn't work...you wouldn't be left with double dispatch but rather with the same compile error.

Kit Ho Over a year ago

@Mark Peters: interface can be extended. see this article [link] artima.com/objectsandjava/webuscript/… I think using inheritance architecture would be better.

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Bozho · Accepted Answer · 2011-07-18 15:52:00Z

Place the build() logic within the implementation. . So instead of build(item) call item.build().

Or take a look at the visitor pattern, where you can do item.build(builder) instead of builder.build(item)

SJuan76 · Accepted Answer · 2011-07-18 15:54:07Z

You are looking the inheritance at the wrong side.

MyImplOne... and MyImplTwo are both implementations of your interface. But if you call build(item), it will expect to find

public void build(MyInterface item):

else, it may find that you pass an object that implements your interface but is neither from class MyImplOne or MyImplTwo.

Like as defining a method

public void print(Vector vector);

and hoping that the compiler can resolve

print(new Object());

Ian Bishop · Accepted Answer · 2011-07-18 15:51:49Z

It is being passed into void buildIt(MyInterface item) as a MyInterface type. When you try to call build(item) it looks for a method with the signature void build(MyInterface item). As you can see, this method does not exist - this is why you get a symbol not found error.

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