Question

I have a type in Java called Item which is defined as follows:

private Integer itemNo;
private String itemName;
private String itemDescription;
...

And I would like to be able to sort an arraylist of this type in descending order according to itemName.

From what I read, this can be done via:

Collections.sort(items, Collections.reverseOrder());

Where items is:

ArrayList<Item> items = new ArrayList<Item>();

But I find that the call to Collections.sort gives me a:

Item cannot be cast to java.lang.Comparable

Run time exception.

Can anyone advise on what I need to do?

Answer 1

Declare Item to be Comparable, and implement the comapreTo method to compare itemName in reverse order (ie compare “that to this“, rather than the normal “this to that“).

Like this:

public class Item implements Comparable<Item> {
    private Integer itemNo;
    private String itemName;
    private String itemDescription;

    public int compareTo(Item o) {
        return o.itemName.compareTo(itemName); // Note reverse of normal order
    }

    // rest of class
}