Reordering a collection according to a related list of ids

Question

I have a Collection (unordered) of objects with an id property, and an (ordered) List of ids. The id list is not sorted. I\'d like to crea

Answer 1

Create a class that implements Comparable. In that class, do the sorting according to your ordered list of ids. Then define a TreeSet based on the Comparable class. A greatly simplified example is shown below.

e.g.

public class MyObject implements Comparable {
  private Integer id;

  // a map of IDs to how they are ordered.
  private static Map idOrder = null;

  public MyObject(Integer id) {
      setId(id);

      if (idOrder == null) {
           idOrder = new HashMap();
           idOrder.put(17, 1);
           idOrder.put(27, 2);
           idOrder.put(12, 3);
           idOrder.put(14, 4);
      }
  }

  public int getId() {
      return (this.id);
  }

  public void setId(int id) {
      this.id = id;
  }

  public int compareTo(MyObject anotherThing) {
    return (idOrder.get(this.getId()).compareTo(idOrder.get(anotherThing.getId()))); 
  }
}

Then, define and populate your set like so:

private Set mySet = new TreeSet;
mySet.add(new MyObject(12));
mySet.add(new MyObject(17));

When you do a mySet.add(), it will automatically sort according to your MySort class. If you iterate over the resulting TreeSet, the "17" entry will come before the "12" entry.