Several different equals methods

Question

Hi,
I want to compare two different sets with eachother. The Set contains objects with a number of strings and Integers, and some of these need to be equal in order to find a match between the two different Sets. This differs from the "natural order" or the actual equal-value for the two objects, so I don�t want to implement Comparable and use the equals method directly in my Objects since I do not wat to mess with the "truth".
I tried to used Comparator, but the equals(Object obj) method only provides one of the two objects I need to compare, and the compare(Object obj1, Object obj2) is good, but should I use it to compare two objects from two DIFFERENT Collections. It seems the compare method is mostly used for sorting, and I don�t care about sorting at this point.
Set<MyObject> set1;
Set<MyObject> set2;
//compare all the objects in set 1 to all objects in set2 AND, if there is a match do:
myObjectFromSet1.setACertainValue(myObjectFromSet2.getACertainValue);The "ACertainValue" is of course not included in the comparison, since in set1, all the "ACertainValues" are empty, but I need to pick the value from its "equal" object from set2.

Answer 1

I compare them field by field in my object - strings
and Integers - (except for one field : position).
All must match to call them "equal". Then I want to
set that field (position) in the set that doesn�'t
have that info yet and pick the value from the other
set (where there is ONE corresponding Object
available only). After this the two objects from each
set are "really equal".
There might be objects available without a match, but
those I will handle seperately.
Thanks for taking you time, it is really appriciated.No problem. I think I understand what you mean, here's yet another little demo:import java.util.*;
public class Main {
    public static void main(String[] args) {
        MyObject[] array1 = {new MyObject(1, "A", new Integer(11)),
                             new MyObject(2, "B", new Integer(22)),
                             new MyObject(3, "C", new Integer(33))};
        Set<MyObject> set1 = new HashSet<MyObject>(Arrays.asList(array1));
        MyObject[] array2 = {new MyObject(1, "A"),
                             new MyObject(2, "B"),
                             new MyObject(3, "C")};
        Set<MyObject> set2 = new HashSet<MyObject>(Arrays.asList(array2));
        System.out.println("Before\nset1 : "+set1);
        System.out.println("set2 : "+set2);
        List<MyObject> list1 = new ArrayList<MyObject>(set1);
        for(MyObject mo2 : set2) {
            int index = list1.indexOf(mo2);
            if(index != -1) {
                MyObject mo1 = list1.get(index);
                if(mo2.position == null) {
                    mo2.position = mo1.position;
                } else {
                    mo1.position = mo2.position;
        System.out.println("\nAfter\nset1 : "+set1);
        System.out.println("set2 : "+set2);
class MyObject {
    protected Integer id, position;
    protected String str;
    public MyObject(int id, String str) {
        this(id, str, null);
    public MyObject(int id, String str, Integer position) {
        this.id = id;
        this.str = str;
        this.position = position;
    public boolean equals(Object o) {
        MyObject that = (MyObject)o;
        return this.id == that.id && this.str.equals(that.str);
    public int hashCode() {
        return (31*this.id) ^ (37*this.str.hashCode());
    public String toString() {
        return "{id="+this.id+", str="+this.str+", position="+this.position+"}";
}Produces the following output:Before
set1 : [{id=3, str=C, position=33}, {id=2, str=B, position=22}, {id=1, str=A, position=11}]
set2 : [{id=3, str=C, position=null}, {id=2, str=B, position=null}, {id=1, str=A, position=null}]
After
set1 : [{id=3, str=C, position=33}, {id=2, str=B, position=22}, {id=1, str=A, position=11}]
set2 : [{id=3, str=C, position=33}, {id=2, str=B, position=22}, {id=1, str=A, position=11}]