compare more field of a object in compareTo

Question

I'd compare more than one only field of a object using the compareTo method. Is it possible?

for istance:

public int compareTo(Object o) {
    return field.compareTo(o.field);
}

I create this method to sort a collection. Obviously my object has to implement Comparable interface.

I'm guessing if is possible to compare not only one field in the same method compareTo.

possible duplicate of [Best way to compare objects by multiple fields?](http://stackoverflow.com/questions/369512/best-way-to-compare-objects-by-multiple-fields) — blank, Dec 13 '11 at 15:13

Mark Peters · Answer 1 · 2011-12-13T15:18:23.357

3

Yes, it's possible. If the result of comparing the first field returns zero, then return the result of comparing the second field.

public int compareTo(SomeClass o) {
    int result = field1.compareTo(o.field1);
    if ( result == 0 ) {
        result = field2.compareTo(o.field2);
    }
    return result;
}

This gets cumbersome fairly quickly, which is why Guava provides a ComparisonChain. Example use:

public int compareTo(SomeClass o) {
    return ComparisonChain.start()
         .compare(field1, o.field1)
         .compare(field2, o.field2)
         .result();
}

edited Dec 13 '11 at 15:18

answered Dec 13 '11 at 15:10

Mark Peters

80,126
17
159
190

and also pretty nested really quick, why @aix's Solution should be preferred – Simon Woker Dec 13 '11 at 15:12
2

@Simon: If you have more than two, you shouldn't be using either my first solution or aix's; use a helper like I listed. – Mark Peters Dec 13 '11 at 15:14

NPE · Answer 2 · 2011-12-13T15:18:26.730

3

Yes, it's possible, for example like so:

public int compareTo(MyClass o){
  int ret = field1.compareTo(o.field1);
  if (ret != 0) return ret;
  ret = field2.compareTo(o.field2);
  if (ret != 0) return ret;
  ...
  return fieldN.compareTo(o.fieldN);
}

edited Dec 13 '11 at 15:18

answered Dec 13 '11 at 15:10

NPE

486,780
108
951
1,012

With generics, it wouldn't be `Object` unless for some very odd reason (like backward compatibility) you have implemented `Comparable – Tom Hawtin - tackline Dec 13 '11 at 15:16
Reusing that variables is a bit nasty. – Tom Hawtin - tackline Dec 13 '11 at 15:21

score 0 · Answer 3 · answered Dec 13 '11 at 15:11

You can certainly factor in other fields when comparing, but there's typically some order of precedence, just like sorting alphabetically only looks at the second letter if the first is the same:

public int compareTo(Object o){

   int comparison = field.compareTo(o.field);
   if (comparison != 0)
      return comparison;
   comparison = field2.compareTo(o.field2);
   if (comparison != 0)
      return comparison;
   //etc...
}

score 0 · Answer 4 · edited Dec 13 '11 at 15:53

0

public int compareTo(Object o){
  int res = field.compareTo(o.field);
  if(res==0)
    res=field1.compareTo(o.field1);
  return res;
}

should work

edited Dec 13 '11 at 15:53

Kris

14,426
7
55
65

answered Dec 13 '11 at 15:14

reederz

983
1
12
29

score 0 · Answer 5 · answered Dec 13 '11 at 15:14

sure you can. however you must define the rule of comparison .

e.g.

you have

objectA{a=1;b=2;c=3}
objectB{a=20;b=1;c=6}

in your compareTo(Object o) method, you could compare this.fields with o.fields. you can even compare this.a to o.c if you really need. point is you have to define the rule, in which case objectA < objectB. etc..

score 0 · Answer 6 · answered Dec 13 '11 at 15:14

0

Sure. Here's a relatively concise way of doing it.

public int compareTo(MyClass other) {
    return
        a!=other.a ? Integer.compare(a, other.a) :
        b!=other.b ? Integer.compare(b, other.b) :
                     Integer.compare(c, other.c);
}

(Integer.compare is from Java SE 7, but the implementation isn't difficult. Assumes int fields a, b, c, but is essentially the same for any field types you can compare.)

answered Dec 13 '11 at 15:14

Tom Hawtin - tackline

145,806
30
211
305

Concise, but you're doing most comparisons twice. That might be needlessly expensive for non-primitives. – Mark Peters Dec 13 '11 at 15:19
The equality comparison is likely to be substantially faster in many cases. Substitute `(x – Tom Hawtin - tackline Dec 13 '11 at 15:22

Azodious · Answer 7 · 2011-12-13T15:25:33.193

-1

You can do it any of following ways:

public int compareTo(Object o)
{    
    return (field.compareTo(o.field)==1 && field2.compareTo(o.field2)==0)? 0 : 1; 
}

OR

public int compareTo(Object o)
{
    // add various if-else blocks
    // OR
    // call a separate method    
}

edited Dec 13 '11 at 15:25

answered Dec 13 '11 at 15:12

Azodious

13,752
1
36
71

3

You are returning a boolean instead of an int. And it's just very wrong. – Tom Hawtin - tackline Dec 13 '11 at 15:16
1

It still doesn't make any sense. – Tom Hawtin - tackline Dec 13 '11 at 15:30
If you are pointing towards checking for `1` and `0` respectively, i just wanted to show OP that it's possible. logic for comparision he can have whatever he wants. – Azodious Dec 13 '11 at 16:17

compare more field of a object in compareTo

7 Answers7