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import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Optional;
 
public class Main {
 
	public static void main(String[] args) throws Throwable {		
		List<Object> l = Arrays.<Object>asList(1, Optional.of("foo"), 2, Optional.of("bar"));
		List<Optional<?>> result = filterByClass(l, Optional.class);
	}	
 
	public static <T> List<T> filterByClass(List<?> list, Class<? extends T> clazz) {
	    List<T> filtered = new ArrayList<>();
	    for (Object o : list) {
	        if (clazz.isInstance(o)) {
	            filtered.add(clazz.cast(o));
	        }
	    }
	    return filtered;
	}
}
 

Main.java:11: error: incompatible types: cannot infer type-variable(s) T
		List<Optional<?>> result = filterByClass(l, Optional.class);
		                                        ^
    (argument mismatch; Class<Optional> cannot be converted to Class<? extends Optional<?>>)
  where T is a type-variable:
    T extends Object declared in method <T>filterByClass(List<?>,Class<? extends T>)
1 error

Standard output is empty