Output variable K has incompatible bounds

I was trying to write a solution to the problem described here as Stuart Marks pointed out the use of a helper class. I am stuck in the code due to this error:

Test.java:27: error: incompatible types: inference variable K has incompatible bounds                                                                                                                          
         .collect(Collectors.groupingBy(p -> p.getT2()));                                                                                                                                                      
                 ^                                                                                                                                                                                             
    equality constraints: Tuple                                                                                                                                                                                
    lower bounds: Integer                                                                                                                                                                                      
  where K,T are type-variables:                                                                                                                                                                                
    K extends Object declared in method <T,K>groupingBy(Function<? super T,? extends K>)                                                                                                                       
    T extends Object declared in method <T,K>groupingBy(Function<? super T,? extends K>) 

      

        
        
        
      

    

My current code:

import java.util.stream.IntStream;
import java.util.stream.Collectors;
import java.util.Map;

public class Test{

    private static final class Tuple {
        private final Integer t1;
        private final Integer t2;

        private Tuple(final Integer t1, final Integer t2) {
            this.t1 = t1;
            this.t2 = t2;
        }

        public Integer getT1() { return t1; }
        public Integer getT2() { return t2; }
    }

     public static void main(String []args){
        System.out.println("Hello World");

        Map<Tuple, Integer> results =
        IntStream.range(1, 10).boxed()
         .map(p -> new Tuple(p, p % 2))                     // Expensive computation
         .filter(p -> p.getT2() != 0)
         .collect(Collectors.groupingBy(p -> p.getT2())); 

        results.forEach((k, v) -> System.out.println(k + "=" + v));
     }

}

      

        
        
        
      

    

I'm completely new to Java 8, the solution might not be right, but I don't know what the error means or how to resolve it. The Tuple class was generic as well, but since I thought this was causing the problem, I removed the generic types, but the same error remained.