Question

I tried to write generic function that remove the duplicate elements from array.

public static <E extends Comparable<E>> ArrayList<E> removeDuplicate(E[] arr) {
    //do quicksort
    Arrays.sort(arr);
    ArrayList<E> list = new ArrayList<E>();
    int i;
    for(i=0; i<arr.length-1; i++) {
        if(arr[i].compareTo(arr[i+1]) != 0) { //if not duplicate, add to the list
            list.add(arr[i]);
        }
    }
    list.add(arr[i]); //add last element
    return list;
}

As you can see you can’t pass primitive type like int[] array since I am comparing elements by compareTo() method that defined in Comparable interface.

I noticed the first line (method declaration):

public static <E extends Comparable<E>> ArrayList<E> removeDuplicate(E[] arr) {

How come it says “extends Comparable” ?

Comparable is an interface so why is it not “implement Comparable”? This is first time I wrote generic function so I’m bit confused about such detail. (any wondering would prevent me from understanding..)

EDIT: Found this article related to this topic.

http://www.tutorialspoint.com/java/java_generics.htm

Answer 1

If You want to use the thing that implements You just write is as generic parameter

class Bar extends  Foo<String> { /* Code */}

The wildcard that You are talking about are three

1. “? extends Type”: Denotes a family of subtypes of type Type. This
   is the most useful wildcard
2. “? super Type”: Denotes a family of supertypes of type Type
3. “?”: Denotes the set of all types or any

You method should look like

public static <T extends Comparable<? super T>> Collection<T> sort(T[] list) {

        Collection<T> list = new ArrayList<T>();

         //do quicksort
        Arrays.sort(arr);

        Collection<T> list = new ArrayList<T>();
        int i;
        for(i=0; i<arr.length-1; i++) {
            if(arr[i].compareTo(arr[i+1]) != 0) { //if not duplicate, add to the list
                list.add(arr[i]);
            }
        }
        list.add(arr[i]); //add last element
//btw how do You know that last is not duplicate 
        return list;

}

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