如何为BitSet类型的元素创建SortedSet（例如TreeSet）

Question

I have a number (power(2,k)) of BitSet objects and I want to store them in a SortedSet . I use the code:

Set <BitSet> S= new TreeSet<>();

However, I am getting this error: java.lang.ClassCastException: java.util.BitSet cannot be cast to java.lang.Comparable

How do I implement comparable interface? Or is there any other way to sort these elements of type BitSet ?

Answer 1

There are two ways to use a TreeSet .

1. Have it contain objects that implement Comparable
2. Have a custom Comparator object that compares the elements of your TreeSet .

Since you want to have your TreeSet contain BitSet s, and BitSet does not implement Comparable , you need to give your TreeSet a custom Comparator . How you implement that Comparator is up to you.

SortedSet<BitSet> s = new TreeSet<BitSet>(new CustomBitSetComparator());
s.add(bitSet1);
s.add(bitSet2);
//etc ...

The Comparator may look something like this

class CustomBitSetComparator implements Comparator<BitSet>{
    int compare(BitSet a, BitSet b) {
        if(a == b){
            return 0;
        } else if(a == null) {
            return -1;
        } else if(b == null) {
            return 1;
        } else if(a.equals(b)) {
            return 0;
        } else if(a.length() > b.length()) {
            return 1;
        } else if(b.lenght() > a.length()) {
            return -1;
        } else {
            for(int i = 0; i < a.length(); i++) {
               if(a.get(i) != b.get(i)) {
                   if(a.get(i)) {
                      return 1;
                   } else {
                      return -1;
                   }
                }
             }
             return 0;
         }
    }
}

Answer 2

I would convert them to BigIntegers (O(N)) and use a TreeSet. Otherwise you will have to write yourself a Comparator, which in the nature of things will run excruciatingly slowly, as you can see from other answers. I would also consider using PriorityQueue instead of a Set.

Answer 3

I'm not sure why you want to put BitSet in a treeSet, while a workaround is to create a wrapper class which implements Comparable interface.

Public class CustomComparableBitSet implements Comparable{
   private BitSet bs;

   public int compareTo(T o){
      //your comparison logic goes here.
   }
}

then in the client code, add instance of CustomComparableBitSet into treeset .