Using HashMap Value to remove object from TreeSet

Question

I have a HashMap where key is a character and value is some user-defined object. I am adding same objects in the TreeSet. The number of entries in HashMap and TreeSet are equal.

Later on I want to retrieve a object from HashMap using user supplied character input. Upon retrieving object from HashMap, I want to delete the same object from the TreeSet. However, the TreeSet.remove() doesn't identify the object.

import java.util.TreeSet;
import java.util.HashMap;

public class Trial {

char c;
int count;  
HashMap<Character, Trial> map;
TreeSet <Trial> set;

public Trial(char c, int count)
{
    this.c = c;
    this.count = count;
    map = new HashMap<Character, Trial>();
    set = new TreeSet<Trial>(new New_Comparison());     
}

public static void main(String[] args) {
    // TODO Auto-generated method stub

    Trial root = new Trial('0', 0);
    int i;
    for(i = 0; i < 26; i++) //add all characters a-z with count 0-25 resp. in root
    {
        char ch = (char)('a' + i);
        Trial t = new Trial(ch, i);
        root.map.put(ch, t);
        root.set.add(t);            
    }

    System.out.println(root.set.size()); //size = 26
    Trial t = root.map.get('c');
    if(t == null)
        return;
    root.set.remove(t);     
    System.out.println(root.set.size());        //size remains the same, expecting 25

}

}

Comparator class:

import java.util.Comparator;

public class New_Comparison implements Comparator <Trial>{

public int compare(Trial n1, Trial n2) {
    if(n1.c <= n2.c)
        return 1;
    return -1;
}


}

Output : 26 26

Please help. If the object is either String or Integer, TreeSet.remove() works perfectly. But doesn't work for user-defined objects.

score 9 · Accepted Answer · answered Dec 12 '14 at 06:20

9

The compare method of the New_Comparison comparator you use to create the TreeSet never returns 0, so it means that, as far as the TreeSet is concerned, no two Trial elements are equal.

That's why set.remove(t) doesn't remove anything.

Change it to :

public int compare(Trial n1, Trial n2) {
    if(n1.c < n2.c)
        return 1;
    else if (n1.c > n2.c)
        return -1;
    return 0;
}

answered Dec 12 '14 at 06:20

Eran

387,369
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So hashcode is not compared? Compare method is invoked? – Curious Dec 12 '14 at 06:23
@Curious hashCode + equals are used in HashSet and HashMap. TreeMap and TreeSet use the Comparator (or the natural ordering of the class if you don't supply a Comparator) – Eran Dec 12 '14 at 06:24

score 1 · Answer 2 · answered Dec 12 '14 at 06:23

1

Implement your compare method as if two values are equal your compare method will fail.

public int compare(Trial n1, Trial n2) {
   return  n1.c.compareTo(n2.c);
}

answered Dec 12 '14 at 06:23

Shoaib Chikate

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score 0 · Answer 3 · answered Dec 12 '14 at 06:26

0

Comparator is wrong, try this

public int compare(Trial n1, Trial n2) {
    return n1.c - n2.c;
}

answered Dec 12 '14 at 06:26

Evgeniy Dorofeev

133,369
30
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score -1 · Answer 4 · answered Dec 12 '14 at 06:28

-1

One approach that i would be use, without implementing the comparator is contains() method of TreeSet. It's one way, but how to do the logic is your choice.

if(root.set.contains(t)){
    root.set.remove(t); 
}

answered Dec 12 '14 at 06:28

drgPP

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Using HashMap Value to remove object from TreeSet

4 Answers4