Inconsistent comparison

gistfile1.txt

package changeme;

import java.util.Comparator;
import java.util.TreeSet;

public class SortDemo {

    public static void main(String[] args) {
        Comparator<Element> comparator = new Comparator<Element>() {
            @Override
            public int compare(Element o1, Element o2) {
                if (o1.getIntField() < o2.getIntField()) {
                    return -1;
                } else if (o1.getIntField() > o2.getIntField()) {
                    return 1;
                }
                // Inconsistent comparison: doesn't return 0 for equal objects
                return 1;
            }
        };

        TreeSet<Element> set = new TreeSet<>(comparator);
        Element element1 = new Element(1);
        Element element2 = new Element(1);
        Element element3 = new Element(-1);
        Element element4 = new Element(-1);
        Element element5 = new Element(0);
        Element element6 = new Element(2);
        Element element7 = new Element(-2);

        set.add(element1);
        set.add(element2);
        set.add(element3);
        set.add(element4);
        set.add(element5);
        set.add(element6);
        set.add(element7);

        System.out.println(set.higher(element5)); // expect 1, got 1
        System.out.println(set.lower(element5)); // expect -1, got 0

        System.out.println(set.tailSet(element1)); // expect 1 2, got 2

    }

    static class Element {
        private Integer intField;

        public Element(Integer intField) {
            this.intField = intField;
        }

        public Integer getIntField() {
            return intField;
        }

        public void setIntField(Integer intField) {
            this.intField = intField;
        }

        @Override
        public String toString() {
            return "Element{" +
                    "intField=" + intField +
                    '}';
        }
    }
}