Type erasure, overriding and generics
The signature of fooMethod(Class<?>)
is the same as the signature of fooMethod(Class)
after erasure, since the erasure of Class<?>
is simply Class
(JLS 4.6). Hence, fooMethod(Class)
is a subsignature of the fooMethod(Class<?>)
but not the opposite (JLS 8.4.2).
For overriding with instance methods you need the overriding method to be a subsignature of the overridden method (JLS 8.4.8.1). This is clearly not the case here.
Now that we have established the fact that your subclass method doesn't override the superclass method according to the JLS, let's look at the runtime implications when type erasure has occured. We now have two methods that look exactly the 'same' (same name, same parameter types) but do not override each other. If they don't override, they must be both available on the subtype as separate methods, but they have identical runtime signatures: conflict. So Java has to disallow it.
Overriding generic parameter types using raw parameter types is allowed because raw types exist just for this reason: they are a convenient mechanism with specific unsound type rules to accommodate interaction with legacy code. So the type system here will decide that the subclass method does override the superclass one, they are identical after type erasure and we can never have a conflict. As a consequence of this libraries can be generified independently of existing non-generic code.
Henrik Paul
Currently programming mainly in PHP in the spare time, but writing Java for a living. Started writing 5-liners of BASIC on a C64 when I was about 6, and hooked in programming ever since.
Updated on July 05, 2020Comments
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Henrik Paul almost 4 years
Can someone explain to me why
@Override public void fooMethod(Class<?> c)
doesn't override
public void fooMethod(Class c)
and gives me the following errors instead:
- Name clash: The method fooMethod(Class<?>) of type SubClass has the same erasure as fooMethod(Class) of type SuperClass but does not override it - The method fooMethod(Class<?>) of type SubClass must override a superclass method
?
Edit: "
java -version
" says Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0_16-b06-284). As for the code snippet, it's already above, pretty much; the above extends the one below. -
Staale over 15 yearsNo, it's related to generic types not beeing castable. Class<List> is not a subtype of Class<Collection> - they are entirely seperate. You must use super or extends to express the relation.
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Craig P. Motlin over 13 yearsYou're talking about the fact that generics are invariant in Java.
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flodin almost 13 yearsThis answers the question but leads to a followup question: what do you do about the warning that inevitably comes when you use
class
in the subclass?Class is a raw type. References to generic type Class<T> should be parameterized
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Nathan about 11 yearsAnnotate the parameter or method with @SuppressWarnings("rawtypes").
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David T. over 10 years@eljenso just to clarify, so if he had done
public void fooMethod(Class<SomethingSpecific> c)
it would be fine?