Instance variables are the same in each object initialization

Question

I'm making a genetic algorithm that deals with evolving an array of chars into "Hello World". The problem is whenever I initialize a Chromosome object and call the generateChromosome method, all the chromosomes of "test" population remains the same?

public class Chromosome{
   private static int defaultLength = 11;
   private static char []genes = new char[defaultLength]; <--- this remains the same for each object :/


   //Generates a random char chromosome
   public void generateChromosome(){
      char []newGene =  new char[defaultLength];
      for(int x = 0; x<size(); x++){
         char gene = (char)(32+Math.round(96*Math.random()));
         newGene[x] = gene;

      }
      genes = newGene;

   }

   //Returns a specific gene in the chromosome
   public char getGene(int index){
     return genes[index];
   }

   public char[] getChromosome(){
      return genes;
   }

   public void setGene(char value, int index){

   genes[index] = value;

   }

   public static  void setDefaultLength(int amount){
      defaultLength = amount;
   }

   public static int getDefaultLength(){
      return defaultLength;
   }

   public int size(){
      return genes.length;
   }

   @Override
   public String toString(){
      String geneString = "";
      for(int x= 0; x<genes.length; x++){
         geneString += genes[x];
      }
      return geneString;
   }
}

score 3 · Answer 1 · answered Dec 29 '14 at 04:24

3

That's because your variables are static, i.e. class variables. They will be the same in each instantiation of your class.

If you want an instance variable, remove the static keyword.

answered Dec 29 '14 at 04:24

ktm5124

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score 2 · Accepted Answer · answered Dec 29 '14 at 04:22

2

static means one per class (not one per instance). Remove the static

private char[] genes = new char[defaultLength];

and your genes becomes an instance field.

answered Dec 29 '14 at 04:22

Elliott Frisch

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0.o when I did that and gave it a test run on main class, this appeared for all chromosomes. 0000 0000 0000 0000 0000 0000 0000 – monsterbasher Dec 29 '14 at 04:26
Sounds like you didn't run `generateChromosome()`. – Elliott Frisch Dec 29 '14 at 04:27
nope I did, strange because making it a class variable does work even if it's value is limited to the class. – monsterbasher Dec 29 '14 at 04:29
public class GA{ public static void main(String[] args){ Population test = new Population(10); FitnessCalc.setSolution("Hello World"); for(int x = 0; x – monsterbasher Dec 29 '14 at 04:32
You aren't calling `generateChromosome()` unless it's in `Population`. And please [**edit**](http://stackoverflow.com/posts/27683357/edit) your question. – Elliott Frisch Dec 29 '14 at 04:33
generateChromosome is called when you initialize a population. – monsterbasher Dec 29 '14 at 04:35
If you get all `0`(s) out then it isn't called correctly. I can't tell you what is wrong without seeing it. – Elliott Frisch Dec 29 '14 at 04:36
He is re - assigning the reference 'gene' so the values will be default – TheLostMind Dec 29 '14 at 04:39
@TheLostMind Appears to `newGene` (which is populated). At least in the posted code. Who knows what's going on the in the `Population` constructor. – Elliott Frisch Dec 29 '14 at 04:40
Ok so I removed newGene and reassigned the value to genes, looks like it worked! Thanks for help! – monsterbasher Dec 29 '14 at 04:42

score 2 · Answer 3 · answered Dec 29 '14 at 04:24

static members belong to the class - they are shared across all its instances. You should define genes as an instance member by removing the static keyword:

public class Chromosome{
   private static int defaultLength = 11; // should probably be final, BTW
   private char[] genes = new char[defaultLength]; // not static!

score 1 · Answer 4 · answered Dec 29 '14 at 04:25

1

Since you are using static keyword in genes it is not an instance variable. Therefore though you assigned the created array to it, there will be only one instance through out the application.

You cam remove the static keyword and uses instance gene variable.

answered Dec 29 '14 at 04:25

Thusitha Thilina Dayaratne

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score 1 · Answer 5 · edited May 23 '17 at 12:14

1

Make genes as instance variable by removing static keyword. Because,

1) static means one per class

2) instance means one per instance

For more information on static, see https://stackoverflow.com/a/413904/2127125

edited May 23 '17 at 12:14

Community

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answered Dec 29 '14 at 05:05

TSKSwamy

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Instance variables are the same in each object initialization

5 Answers5