Why is it not legal to have the following two methods in the same class?
class Test{
void add(Set<Integer> ii){}
void add(Set<String> ss){}
}
I get the compilation error
Method add(Set) has the same erasure add(Set) as another method in type Test.
while I can work around it, I was wondering why javac doesn't like this.
I can see that in many cases, the logic of those two methods would be very similar and could be replaced by a single
public void add(Set<?> set){}
method, but this is not always the case.
This is extra annoying if you want to have two constructors that takes those arguments because then you can't just change the name of one of the constructors.
The problem is that Set<Integer> and Set<String> are actually treated as a Set from the JVM. Selecting a type for the Set (String or Integer in your case) is only syntactic sugar used by the compiler. The JVM can't distinguish between Set<String> and Set<Integer>.
I bumped into this when tried to write something like:
Continuable<T> callAsync(Callable<T> code) {....}
and
Continuable<Continuable<T>> callAsync(Callable<Continuable<T>> veryAsyncCode) {...}
They become for compiler the 2 definitions of
Continuable<> callAsync(Callable<> veryAsyncCode) {...}
The type erasure literally means erasing of type arguments information from generics. This is VERY annoying, but this is a limitation that will be with Java for while. For constructors case not much can be done, 2 new subclasses specialized with different parameters in constructor for example. Or use initialization methods instead... (virtual constructors?) with different names...
for similar operation methods renaming would help, like
class Test{
void addIntegers(Set<Integer> ii){}
void addStrings(Set<String> ss){}
}
Or with some more descriptive names, self-documenting for oyu cases, like addNames and addIndexes or such.
This is because Java Generics are implemented with Type Erasure.
Your methods would be translated, at compile time, to something like:
Method resolution occurs at compile time and doesn't consider type parameters. (see erickson's answer)
void add(Set ii);
void add(Set ss);
Both methods have the same signature without the type parameters, hence the error.
Define a single Method without type like void add(Set ii){}
You can mention the type while calling the method based on your choice. It will work for any type of set.
Java generics uses type erasure. The bit in the angle brackets (<Integer> and <String>) gets removed, so you'd end up with two methods that have an identical signature (the add(Set) you see in the error). That's not allowed because the runtime wouldn't know which to use for each case.
If Java ever gets reified generics, then you could do this, but that's probably unlikely now.
It could be possible that the compiler translates Set(Integer) to Set(Object) in java byte code. If this is the case, Set(Integer) would be used only at compile phase for syntax checking.
Source: Stackoverflow.com