I have the following method:
public Comparator<T> getComparator() throws ReflectiveOperationException {
String className = "some.ClassName";
Class<Comparator<T>> c = Class.forName(className); // (1)
return (Comparator<T>) c. newInstance();
}
In the line (1) I get this error:
Type mismatch: cannot convert from Class <capture#1-of ?> to Class<Comparator<T>>
What's wrong in this code and how should I make an instance of Comparator<T>?
The best you can get so far is
public <T> Comparator<T> getComparator() throws ReflectiveOperationException {
Class<? extends Comparator> implementation
= Class.forName("some.ClassName").asSubclass(Comparator.class);
@SuppressWarnings("unchecked")
final Comparator<T> c = implementation.newInstance();
return c;
}
Note that there is still an unchecked operation which is unavoidable. The runtime type token Comparator.class produces a Class<Comparator> rather than Class<Comparator<T>> which reflects the type erasure and implies that you can use it via asSubclass to ensure that a Class indeed implements Comparator, but you can’t ensure that it implements Comparator<T> regarding any <T>. (Note that this method doesn’t even know what T is). Therefore there is still an unchecked conversion of Comparator to Comparator<T>.
Simply move the cast:
Class<Comparator<T>> c = (Class<Comparator<T>>) Class.forName(className); // (1)
return c.newInstance();
Class.forName() will return a Class<?> object and you will need to cast it to whatever you want and make sure the casting will work fine... Because compiler cannot make sure the string "some.ClassName" is the name of a class implementing Comparator<T>.
Try this solution
@SuppressWarnings("unchecked")
public Comparator<T> getComparator() throws Exception {
String className = "some.ClassName";
Class<?> c = Class.forName(className); // (1)
return (Comparator<T>) c. newInstance();
}
