Any nice way to make a chain of immutable objects loop around?

Question

This question is an extension of this question.

I have a class similar to the following.

 class HighlightableStructure {
      private final HighlightableStructure NEXT;  

      HighlightableStructure(HighlightableStructure next) {
           NEXT = next;
      }    
 }

where a HighlightableStructure points to the next structure to highlight.

Sometimes, these HighlightableStructures loop around and refer to a previous HighlightableStructure, but not the first in the chain. Something like h_1 -> h_2 ->h_3 -> ... -> h_n -> h_2, where h_i is an instance of HighlightableStructure.

Is there anyway I could construct something like this without reflection or losing immutability?

Answer 1

One possible solution:

class HSequenceBuilder {
    private long id = 0;
    private final Map<Integer, H> map = new HashMap<Integer, H>();

    static H create(int next) {
        H h = new H(id, next, Collections.unmodifiableMap(map));
        map.put(id, h);
        id++;
        return h;
    }

    static H create() {
        create(id + 1);
    }

}

class H {
    private final id;
    private final Map<Integer, H> map;
    private final int next;

    H(int id, int next, Map<Integer, H> map) {
        this.id = id;
        this.next = next;
        this.map = map;
    }

    int getId() { return id; }
}
HSequenceBuilder builer = new HSequenceBuilder();
H h1 = builder.create(); // 1.
H h2 = builder.create(); // 2.
H h3 = builder.create(h2.getId()); // 3.