ArrayList sorting using collection

Question

I know that we use comparable function for sorting ArrayList of objects to sort according to a certain property of objects. But i always wonder when we call sort directly on ArrayList of objects for example: Collections.sort(arraylist); It gives error that you cant sort but actually when you print a object for example:

Instances in1=new Instances();
ArrayList  al=new ArrayList();
al.add(in1);
System.out.println(al.get(0));

You get output:

Instances@45a8123b

which is the id of a object.

So, if we call sorting on an ArrayList then it can sort objects according to id. So, why we get errors while sorting ArrayList of objects ?

sorting based on this id, actually it is hashcode, does not make any sense, why would people implement it? — Iłya Bursov
– Iłya Bursov, Commented Apr 25, 2015 at 0:16
@Lashane u are right i was just asking that by default u can have that option.For example if u want tp print student name according to the date of entry then u can print object. Its a vague example but something like this. — Arjun Chaudhary
– Arjun Chaudhary, Commented Apr 25, 2015 at 0:32
@ArjunChaudhary With Java 8 you can do things like that really easily. E.g. students.sort(Comparator.comparing(Student::getDate));. — Paul Boddington
– Paul Boddington, Commented Apr 25, 2015 at 0:37

Misha · Accepted Answer · 2015-04-25 00:23:32Z

3

If you want your objects to be sortable without supplying a comparator, define a natural ordering by implementing Comparable interface:

class Instances implements Comparable<Instances> {

    ....

    @Override
    public int compareTo(Instances other) {
        return Integer.compare(
                System.identityHashCode(this), 
                System.identityHashCode(other)
        );
    }
}

And now ArrayList<Instances> can be sorted by identity hashcode (however ridiculous it is to actually do that).

This way you actually have to make a deliberate effort for this to happen. Collections.sort is right to not assume a behavior that's likely wrong. If you didn't implement Comparable and didn't supply a Comparator, it is much better to error out than to just silently sort your list in semi-random order.

answered Apr 25, 2015 at 0:23

Misha

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