Is there a trick for creating comparator for a Class?

Question

I am testing Comparator. Here I have my Class Item which has two field int price and int comments.

I override the compareTo function simply by sorting the price in ascending order then by comments.

Then I create my own Comparator by overriding the compare(Object o1, Object o2) function. It simply return the compareTo function from Item Class.

But as result it just does not work when I use my Comparator with Arrays.sort(array, new MyComparator()) Where did I go wrong?

    class Item implements Comparable{
    private double price;
    private double comments;
    public Item(double price, double comments){
        this.comments = comments;
        this.price = price;
    }

    @Override
    public int compareTo(Object o) {
        if (o instanceof Item){
            Item item2  = (Item) o;  
            int priceCompare =  Double.compare(this.price, item2.price); 
            if(priceCompare != 0) {
                return priceCompare;
            }
            return Double.compare(this.comments, item2.comments);  
        }else{
                throw new RuntimeException("Wrong compare Class");
        }
    }

    @Override
    public String toString() {
        StringBuilder stringBuilder = new StringBuilder();
        stringBuilder.append("Items:{").append(" price: ").
                append(price).append(" comments: ").append(comments).append(" }");
        return stringBuilder.toString();
    }
}

    class MyComparator implements Comparator {

    @Override
    public int compare(Object o1, Object o2) {
        System.out.println("asdasd");
        if (o1 instanceof Item && o2 instanceof Item) {
            Item item1 = (Item) o1;
            Item item2 = (Item) o1;
            return item1.compareTo(item2);
        } else {
            throw new RuntimeException("Wrong input type");
        }
    }

}

    @Test
    public void test3Comparator() {
        Item[] items = new Item[]{new Item(65, 70),
                new Item(45, 7),new Item(98, 89),
                new Item(23, 56),new Item(78, 90)};
        System.out.println(Arrays.toString(items));
        Arrays.sort(items, new MyComparator());  
        System.out.println(Arrays.toString(items));   // Still shows the original order

    }

This line: `Item item2 = (Item) o1;` contains a typo: it should be `o2`. — khelwood, Apr 22 '20 at 11:07
Side note: [What is a raw type and why shouldn't we use it?](https://stackoverflow.com/questions/2770321/). — Slaw, Apr 22 '20 at 11:09

score 1 · Answer 1 · answered Apr 22 '20 at 11:08

problem here

if (o1 instanceof Item && o2 instanceof Item) {
            Item item1 = (Item) o1;
            Item item2 = (Item) o1; //HERE
            return item1.compareTo(item2);
        } else {
            throw new RuntimeException("Wrong input type");
        }

you should fix this line

Item item2 = (Item) o1;

by this

Item item2 = (Item) o2;

Moreover use generic version of interfaces to avoid writing all these verbose code cast to Item, instanceOf..

So Comparable<Item> and this Comparator<Item>

Is there a trick for creating comparator for a Class?

1 Answers1