Question

I’m using Java Comparator for sorting in my code. How I can only sort by certain values, e.g., the values range from 1 to 1000, (default is 1000), but I only want ascending sort to show 1,2,3,4,5,1000…, and descending sort to show 5,4,3,2,1,1000….
How do I do this?
Thanks,

David

Answer 1

The comparators below treat the value 1000 as always “greater” than everything else; 1000 will always sort last. Any other values are sorted either ascending or descending.

class AscendingComp implements Comparator<Integer>
{
    @Override
    public int compare(Integer o1, Integer o2)
    {
        if (o1.equals(o2))   return 0;
        if (o1.equals(1000)) return 1;
        if (o2.equals(1000)) return -1;
        return o1.compareTo(o2);
    }
}
class DescendingComp implements Comparator<Integer>
{
    @Override
    public int compare(Integer o1, Integer o2)
    {
        if (o1.equals(o2))   return 0;
        if (o1.equals(1000)) return 1;
        if (o2.equals(1000)) return -1;
        return o2.compareTo(o1);
    }
}

Edit:
This is not a general solution, but answers the specific question. In a general solution you must pay attention to the contract of equals() and compare(), especially being sure not to return inconsistent comparison results. For example, if you’re not careful and accidentally return, say +1 for compare(a,b) and compare(b,a) then the sort could get into an infinite loop.