Set issue.

[L. Wei]

import java.util.*;
 
class Ele {
  Integer x;
  Ele(int a) {x=a;}
  void setX(int b) {x=b;}
  int getX() {return x;}
  public int hashCode() {
    return x;
  }
  public boolean equals(Object o) {
    return this.x==((Ele)o).x;
  }
}
 
public class Test {
  public static void main(String[] args) {
    LinkedHashSet<Ele> hs=new LinkedHashSet<Ele>();
    Ele t3=new Ele(50);
    Ele t4=new Ele(60);
    hs.add(t3);
    hs.add(t4);
    t3.setX(60);
 
    System.out.println("==="+t3.equals(t4));
    Iterator<Ele> i=hs.iterator();
    while(i.hasNext()) {
      System.out.print(i.next().getX()+" ");
    }
  }
}

I got the return as

===true
60 60

Is it supposed to be caught by Java coz Set is not allow to have duplicated elements. The add() method adds the object reference into the Set. I can still use the reference of the object to change the object. If I change the class Test to

public class Test {
  public static void main(String[] args) {
    LinkedHashSet<Ele> hs=new LinkedHashSet<Ele>();
    Ele t3=new Ele(60);
    Ele t4=new Ele(60);
    hs.add(t3);
    hs.add(t4);
 
    System.out.println("==="+t3.equals(t4));
    Iterator<Ele> i=hs.iterator();
    while(i.hasNext()) {
      System.out.print(i.next().getX()+" ");
    }
  }
}

I will get the return as

===true
60

Please help me to clarify it. Does java have a way to avoid it? Thanks a lot!!!

[ramesh maredu]

This is what java API says about Set.

"Great care must be exercised if mutable objects are used as set elements. The behavior of a set is not specified if the value of an object is changed in a manner that affects equals comparisons while the object is an element in the set".

[Sumukh Deshpande]

hi ramesh!!
Will you please explain your comment in a greater detail?

"Great care must be exercised if mutable objects are used as set elements. The behavior of a set is not specified if the value of an object is changed in a manner that affects equals comparisons while the object is an element in the set"

[ramesh maredu]

Java API says if we have any mutable object in Set, and if we modify that object in such a way that equals method is effected then Set behavior is not specified

Here Ele class object is mutable, initially the object of this class t3 is added to Set with value of x as 50,then its value is changed to 60, and the property x is used in equlas method so modification to x value will effect its behavior.

[L. Wei]

I see. thanks a lot

[Sumukh Deshpande]

Thanks a lot Ramesh for explanation!

[Yaroslav Ashurin]

Hey,

Thanks to Roota for clear explanation of the Set problem; I've asked same question in the Beginners forum here: https://coderanch.com/t/410931/java/java/Duplicates-Set , and now I got the picture.