Java Generic - subtypes check?

Question

How do I check generic subtypes passed as parameter?

For example:

public class A<T> { public T get();}
public class AString extends A<String> {}
public class ADate extends A<Date> {}

public void checkparam(List<? extends A<?>> params) {
///Want to check if params is of type List<AString> or List<ADate> ?
}

Is it possible? What part I am not understanding?

Answer 1

It is not possible because generic type information like that is erased and not available at runtime. At runtime, all that is known is that the parameter to checkparam is a List.

Answer 2

I don't believe you can do it directly on the list. You could do a check on the first element:

if (!params.isEmpty() && params.get(0) instanceof AString) {
   // it's List<AString>
}
...

Not pretty and won't work on empty lists, but should work otherwise.

Answer 3

This is made clear by trying to compile the following code:

import java.util.Date;
import java.util.List;

public class A<T> { 
    private T value;

    public T get() {
        return value;
    }

    public void checkparam(List<AString> list) {

    }

    public void checkparam(List<ADate> list) {

    }
}
class AString extends A<String> {}
class ADate extends A<Date> {}

which produces the following output from javac:

$ javac A.java 
A.java:11: name clash: checkparam(java.util.List<AString>) and checkparam(java.util.List<ADate>) have the same erasure
    public void checkparam(List<AString> list) {
                ^
A.java:15: name clash: checkparam(java.util.List<ADate>) and checkparam(java.util.List<AString>) have the same erasure
    public void checkparam(List<ADate> list) {
                ^
2 errors