"find differences between data sets X and Y" helper

Diff.java

public class Diff<Type> {
  public interface Finder<Type> {
    /** Return true to denote a "represents the same thing" match (do not compare contents) */
    boolean find(Type left, Type right);
  }
  public interface Equality<Type> {
    boolean equals(Type left, Type right);
  }

  private final Finder<Type> finder;
  private final Equality<Type> detector;

  private final List<Type> added = new ArrayList<>();
  private final List<Type> removed = new ArrayList<>();
  private final List<Pair<Type, Type>> changed = new ArrayList<>();

  public Diff(Finder<Type> finder, Equality<Type> detector) {
    this.finder = finder;
    this.detector = detector;
  }

  public void populate(List<Type> left, List<Type> right) {
    // TODO: ok, so not strictly correct.
    ASSERT(isEmpty(), "Must not have populated already");

    List<Type> rightCopy = new ArrayList<>(right);
    for (Type l : left) {
      Type found = null;

      for (Iterator<Type> i = rightCopy.iterator(); i.hasNext(); ) {
        Type r = i.next();
        if (finder.find(l, r)) {
          found = r;
          i.remove();
          break;
        }
      }

      if (found == null) {
        removed.add(l);
      } else if (!detector.equals(l, found)) {
        changed.add(new Pair<>(l, found));
      }
    }

    for (Type r : rightCopy) {
      added.add(r);
    }
  }

  public List<Type> getAdded() {
    return added;
  }

  public List<Type> getRemoved() {
    return removed;
  }

  public List<Pair<Type, Type>> getChanged() {
    return changed;
  }

  public boolean isEmpty() {
    return added.isEmpty() && removed.isEmpty() && changed.isEmpty();
  }
}

// use:
Diff<Thing> diff =
    new Diff<>(
      // how to find equivalent things
      (a,b) -> a.getId().equals(b.getId(),
      // how to check for updates.
      (a,b) -> a.latitude == b.latitude &&
               a.longitude == b.longitude &&
               a.speed = b.speed);
diff.populate(oldData, newData);
diff.getAdded() // returns all new ID things
diff.getRemoved() // returns all things that have disappeared
diff.getChanged() // returns Pair<old, new> so you can figure out how to handle each one