Question

I did selection sort with integers and it was working, when I tried to modify the program to work with generics the compiler is complaining and I don’t know how to fix it. If anyone can point some tips and constructive comments I would be grateful. Here is the code.

public class SelelctionSort 
{
    public static void main(String[] args) 
    {
        int[] list = {34, 17, 23, 35, 45, 9, 1};
        System.out.println("Original Array: ");
        printArray(list);

        selectionSort(list);
        System.out.println("\nSelection sort:");
        printArray(list);

    }

    //selection sort
    public static <E extends Comparable<E>> void selectionSort(E[] list)
    {
        for(int i=0; i<list.length -1; i++)
        {
            int iSmallest = i;

            for(int j=i+1; j<list.length; j++)
            {
                if(list[iSmallest].compareTo((list[j])) > 0  )
                {
                    iSmallest = j;
                }
            }
            E iSwap = list[iSmallest];
            list[iSmallest] = list[i];
            list[i] = iSwap;

        }
    }

    public static <E> void printArray(E[] list)
    {

        for(int i=0; i<list.length; i++)
        {
            System.out.print(list[i] + ", ");
        }
    }
}

The following is what javac spits out.

SelelctionSort.java:7: error: method printArray in class SelelctionSort cannot be applied to given types;
        printArray(list);
        ^
  required: E[]
  found: int[]
  reason: inferred type does not conform to declared bound(s)
    inferred: int
    bound(s): Object
  where E is a type-variable:
    E extends Object declared in method <E>printArray(E[])
SelelctionSort.java:9: error: method selectionSort in class SelelctionSort cannot be applied to given types;
        selectionSort(list);
        ^
  required: E[]
  found: int[]
  reason: inferred type does not conform to declared bound(s)
    inferred: int
    bound(s): Comparable<int>
  where E is a type-variable:
    E extends Comparable<E> declared in method <E>selectionSort(E[])
SelelctionSort.java:11: error: method printArray in class SelelctionSort cannot be applied to given types;
        printArray(list);
        ^
  required: E[]
  found: int[]
  reason: inferred type does not conform to declared bound(s)
    inferred: int
    bound(s): Object
  where E is a type-variable:
    E extends Object declared in method <E>printArray(E[])

Answer 1

int[] list = {34, 17, 23, 35, 45, 9, 1};
...
selectionSort(list);

You are trying to call selectionSort() which signature is selectionSort(E[]), but int does not extend Comparable (It is a primitive, not even an object) – and thus the types are not matching.

You can try to create an Integer[] and pass it. Integer is an object and it extends Comparable<Integer>.

The alternative is to overload selectionSort() to accept both generic type for objects and to overload it for each needed primitive. This is the solution java uses for its Arrays.sort() method.

The same holds for printArray()