Question

I have an ArrayList of Objects(POJOs) that have an Id and another field. I have implemented the equals()/hashcode() override in the POJO for the Id field. When I compare two objects using the equals() method of the Object class, it works perfectly fine. However when I add these objects to an arraylist and implement the

Collections.sort(arrListOfObjects);

it gives me a classCastexception. I looked up and found that I need to implement a Comparator. This comparator also does something to equals/hashcode override. If that is so then why does the above code not work?(I know that there is no comparator, but my question is, is it not possible to implement a sort based on the hashcode of the object?)

Answer 1

As the message says, your object needs to implement the Comparable interface to be sortable. Alternatively, you can provide a comparator to your sort() method. For example, assuming your objects are Strings and you want to sort based on hashcodes, you could do this:

public static void main(String[] args) {
    List<String> list = Arrays.asList("string", "sdkj");
    for (String s : list) {
        System.out.println(s + "=" + s.hashCode());
    }
    Collections.sort(list, new Comparator<String>() {

        @Override
        public int compare(String o1, String o2) {
            return o1.hashCode() - o2.hashCode();
        }
    });
    System.out.println("After Sorting");
    for (String s : list) {
        System.out.println(s + "=" + s.hashCode());
    }
}