Sorting a List of Objects by specific value

Question

I want to sort a List of Objects (Remarks) by a value contained in another object (DefectClass).

public class Remark {
private int remarkId;
private DefectClass defectClass;
}

I have a List of Remarks and I'm using this code to sort them by the title of the defectClass :

Collections.sort(remarks, new Comparator<Remark>()
            {
                @Override
                public int compare(Remark arg0, Remark arg1) {
                return arg0.getDefectClass().compareTo(arg1.getDefectClass());
                }
            });

and this is the Compare to methode in my DefectClass model :

public class DefectClass implements Comparable<DefectClass> {
private String DefectClassTitle;
/* Getters And Setters */
    @Override
public int compareTo(DefectClass o) {
    if(o.getDefectClassTitle().equals(this.getDefectClassTitle()))
        return 0;
    else
        return 1;
}

by these codes I want to sort a List of remarks by the Title of the DefectClass of these Remarks ... but At the end I always get the same list with no sorting. What I'm doing wrong ?

use either one. 1). Collections.sort() with new Comparator 2.) Collections.sort() for the class that implements Comparable. — Braj
– Braj, Commented Jun 1, 2014 at 18:58
use return o.getDefectClassTitle().compareTo(this.getDefectClassTitle()). You're never returning -1 otherwise. — Jeroen Vannevel
– Jeroen Vannevel, Commented Jun 1, 2014 at 18:58
Check compareTo contract, you must return -1, 0 and 1. — lpiepiora
– lpiepiora, Commented Jun 1, 2014 at 18:59
You must create an order with your compareTo method. You must not check for equality but determine which one is greater or lower. — boumbh
– boumbh, Commented Jun 1, 2014 at 19:00
Use first option if you want to sort on different properties of the class. — Braj
– Braj, Commented Jun 1, 2014 at 19:01

Norbert Radyk · Accepted Answer · 2014-06-01 19:05:51Z

4

The implementation of the DefectClass.compareTo() method is incorrect, as it should compare the 2 objects and return whether the first one is lower (-1), equal(0) or greater (1) to the second one (and not only indicate whether they're equal as in your case). Assuming getDefectClassTitle() is returning String I'd implement the method as:

public int compareTo(DefectClass o) {
    return o.getDefectClassTitle().compareTo(this.getDefectClassTitle());
}

edited Jun 1, 2014 at 19:05

answered Jun 1, 2014 at 19:00

Norbert Radyk

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Comments

sinujohn · Accepted Answer · 2014-06-01 19:11:30Z

1

You need not even implement Comparable, as you are providing your own Comparator.
Use it as follows:

Collections.sort(remarks, (r1, r2) -> r1.getDefectClass().getDefectClassTitle().compareTo(r2.getDefectClass().getDefectClassTitle()));

answered Jun 1, 2014 at 19:11

sinujohn

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