Object References Java

Question

I have this problem of object references which is driving me crazy. Is it normal that when I get an Integer[] array from an object and modify some elements in this variable, it gets modified in the object as well without using any set method? For example below I want to modify timetable variable locally but not in the bestSoFar object. How can I do this? and what is really happening here?

for (IntTuple examRel: examsRel) 
{   
     int[] examsTogether = examRel.getData();
     double maxFitness = 0.0;
     Integer[] timetable = bestSoFar.getChromosome();

     for (int i=0; i < noOfTimeslots; i++)
     { 
          for (int j=0; j < examsTogether.length; j++)
          {
               timetable[examsTogether[j]] = i;
          }

          BestChromosome thisChromosome = evaluateChromosome(new BestChromosome(timetable));
          double thisFitness = thisChromosome.getFitness();

          if (thisFitness > maxFitness)
          {
              maxFitness = thisFitness;
              bestSoFar = thisChromosome;
          }
       }
    }
    return bestSoFar;
}

Evidently getChromosome() is returning a reference to the underlying storage, rather than creating a copy first. — Oliver Charlesworth
– Oliver Charlesworth, Commented May 3, 2014 at 17:18
Yes, it is normal, because you get reference to Integer[] array. If you need other behavior, you should consider cloning array before returning it. — Alexey Malev
– Alexey Malev, Commented May 3, 2014 at 17:19
A Array is a normal object. you have a reference on it and you can change its member. You must create a copy of it. You can use Arrays.copyOf (docs.oracle.com/javase/7/docs/api/java/util/…) for it — pL4Gu33
– pL4Gu33, Commented May 3, 2014 at 17:19
so I need to copy the array into another Integer[] array for example? — Bernice
– Bernice, Commented May 3, 2014 at 17:19

Mureinik · Accepted Answer · 2014-05-03 17:20:28Z

5

An array in Java is an Object, so when you modify its elements, they change for all the references pointing to that array. If you want a local copy, you should clone it. E.g.:

Integer[] origTimetable = bestSoFar.getChromosome();
Integer[] timetable = Arrays.copyOf (origTimetable, origTimeable.length);

answered May 3, 2014 at 17:20

Mureinik

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4 Comments

Bernice Over a year ago

thanks for your answer. So if I have a copy of this timetable and modify that instead, the timetable in chromosome wouldnt be modified. But would this be avoided if the array was a primitive int[] ?

Mureinik Over a year ago

@Bernice no, using primitive ints wouldn't help - the object reference here is the array itself - your code wasn't modifying the existing elements, it was switching them with different ones. The exact same problem would occur with an array of primitive ints.

Bernice Over a year ago

ok and same goes for the object of BestChromosome. where I am doing bestSoFat = thisChromosome I would be making them equal by having the same reference for both objects right?

Mureinik Over a year ago

@Bernice yup. Both variables would be pointing to the same reference.

JB Nizet · Accepted Answer · 2014-05-03 17:21:10Z

1

Yes, it's normal. The method is returning a reference to the array that is contained in your object. If you change what the array contains, the array is thus modified.

You'll have to make a defensive copy of the array before returning it.

You could avoid these copies by using a List<Integer> instead, and return an unmodifiable view of this list to prevent its modification. It would then be up to the caller to create a copy of the list if it needs to modify it:

return Collections.unmodifiableList(list);

answered May 3, 2014 at 17:21

JB Nizet

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