Repositioning an object in a TreeSet?

[Yuriy Zilbergleyt]

Hi everybody,

I'm assuming that when an object is placed into a TreeSet that uses a custom Comparator, it's position is determined by Comparator.compare(). What if the object property that compare() uses changes after the object is already inserted into the set?

For example I may be keeping a TreeSet of Document objects sorted by their lastAccessedDate, and the lastAccessedDate of an object changes. Is there a way of making the TreeSet aware of this, or should I just remove the Document from the TreeSet before changing the date and reinsert it afterwards?

Thank you,

Yuriy

[marc weber]

Order appears to be determined at insertion. I don't know of a way to "re-order" the TreeSet if elements change.

import java.util.*;
 
class MyObject {
  int value;
  MyObject(int i) {
    value = i;
  }
  public boolean equals(Object ob) {
    MyObject m = (MyObject)ob;
    if(m.value == value) return true;
    else return false;
  }
  public String toString() {
    return "ObValue" + value;
  }
}
 
class MyComp implements Comparator {
  public int compare(Object o1, Object o2) {
    MyObject comp1 = (MyObject)o1;
    MyObject comp2 = (MyObject)o2;
    return (comp1.value - comp2.value);
  }
}
 
public class TreeTest {
  public static void main(String[] args) {
    TreeSet ts = new TreeSet(new MyComp());
    ts.add(new MyObject(4));
    ts.add(new MyObject(2));
    ts.add(new MyObject(7));
    MyObject changer = new MyObject(5);
    ts.add(changer);
    System.out.println(ts);
    changer.value = 3;
    System.out.println(ts);
  }
}

[ July 27, 2005: Message edited by: marc weber ]

[Stan James]

You could get into something tricky that listens for changes (see Observer and Observable), removes the object and reinserts it. Or you could create a new TreeSet from your existing TreeSet to re-sort everything before you read from it again.