Question

I’m something of a newbie with Java and I’m trying to learn how to use generics. Can anyone explain to me what’s wrong with this code?

import java.util.Collection;
import java.util.Iterator;

public class Generics {

    /**
     * @param args
     */
    public static void main(String[] args) {
        Integer a = new Integer(28);

        Integer[] b = {2, 4, 8, 16, 20, 28, 34, 57, 98, 139}; 
            //I'd prefer int[], but understand native types don't go with generics

        int c = which(a, b); // <--- error here, see below

        System.out.println("int: "+ c);
    }

    static <T extends Number> int which( T a, Collection<T> b) {
        int match = -1;
        int j = 0;
        for (Iterator<T> itr = b.iterator(); itr.hasNext();) {
            T t = (T) itr.next();
             if (a == t) {
                 match = j; 
                 break; 
             }
             j++;
        }
        return match;
    }
}

The Error: The method which(T, Collection<T>) in the type Generics is not applicable for the arguments (Integer, Integer[]).

Granted, I could just use int c = Arrays.binarySearch(b, a) in this particular case (sorted, comparable elements) instead of the custom method which, but this is a learning exercise.

Can anyone explain what I’m misunderstanding here?

Answer 1

An array is not a Collection

try

static <T extends Number> int which( T a, T[] b) {

And, as Yanflea points, this change implies that (other optimizations added)

int j = 0;
for(T t : b) {
  if (a.equals(t)) {
    return j;
  }
  j++;
}
return -1;