Mock Question on Generics

[M Srilatha]

Hi All,

The following mock question is from http://enigma.vm.bytemark.co.uk/webstart.html

public <T> List<T> meth(List<?> type)
{
System.out.println(type); // 1
return new ArrayList<String>(); // 2
}

This code doesnt compile because of 2 line.
And the error message is Type mismatch: cannot convert from ArrayList<String> to List<T>.

As per my knowledge, return type List<T> means the method can return a collection of type List or any subtype of List.

But the code here doesnt compile!

Could you please explain why the compiler error occurs?

[Thomas Thevis]

The problem is not the ArrayList subtype of List, but the generic parameter <T>. You cannot simply return a List of generic type String.

Regards,
Thomas

[Ankit Garg]

When you write List<T> it means that you can return List or its sub-type which are typed as T. But T is not necessarily String.

This is why returning return ArrayList<String> gives you an error as <T> might not always be <String>. Even if you write List<String>, then also it will give you an error...

[M Srilatha]

Thanks!
I got it.