Brian Lin

I just did a little more testing on TreeSet. Here is what I got.
import java.util.TreeSet; public class TreeSetTesting { public static void main(String[] args) { TreeSet s = new TreeSet(); MyNode a = new MyNode("a", 1); s.add(a); MyNode b = new MyNode("b", 1); System.out.println("Are they equal: " + (b==null ? a==null : b.equals(a))); System.out.println("Do they have the same hashcode: " + (a.hashCode() == b.hashCode())); System.out.println(a.compareTo(b)); //s.add(b); System.out.println("Is a equals to b: " + a.equals(b)); System.out.println("Is b in the TreeSet: " + s.contains(b)); s.add(b); System.out.println(s.size()); } static class MyNode implements Comparable{ String a; int i; MyNode (String a, int i) { this.a = a; this.i = i; } public boolean equals(Object o) { if (!(o instanceof MyNode)) return false; else return this.a.equals(((MyNode)o).a); } public int hashCode() { return this.a.hashCode(); } public int compareTo(Object o) { //this should be changed.... return 0; } } }
The output is:
Are they equal: false
Do they have the same hashcode: false
0
Is a equals to b: false
Is b in the TreeSet: true
1
How could this be happening? a and b have are not equal, and they have different hashCode. why is b in the TreeSet after I added a only? It seems to me that the contains() method only looks at compareTo method, and return true if compareTo returns 0.
in the above if I change the compareTo method and return -1 (it is bad, used just for demonstration purpose). then s.contains(b) in the last line of main method will return false.
Any idea?
Thanks in advance.
[ edited to break long line and to remove the evil tab character -ds ]
[ April 27, 2004: Message edited by: Dirk Schreckmann ]

Hi All,
I have the following code. Can anyone please tell me why the second println prints true while the first println prints false.
I looked up the API of Set, and it says

Returns true if this set contains the specified element. More formally, returns true if and only if this set contains an element e such that (o==null ? e==null : o.equals(e)).

package poly.algorithm; import java.util.TreeSet; public class TreeSetTesting { public static void main(String[] args) { TreeSet s = new TreeSet(); Node a = new Node("a"); s.add(a); Node b = new Node("b"); //print false System.out.println(a.equals(b)); //print true, why???? System.out.println(s.contains(b)); //print false!! System.out.println((b==null ? a==null : b.equals(a))); s.add(b); //print 1 System.out.println(s.size()); } }
Thanks in advance.