Object in ArrayList is changing

Question

I made a test program because I am trying to get back into Java after working in PL/SQL. I created a class Numbers that contains an integer, a getter and a setter. I have another class Test that is creating an instance of Numbers, and also adds that instance to a List. I created a for loop that loops two times and sets the value of the integer in Numbers equal to i. I then add that instance of Numbers to the List numbersList. I then do a print screen of the value that was added to the List. I do a total of 3 prints, one print the first time through the loop that prints the first position in the List, then I print two times during the second time through the loop,the first position in the List again, and the second position in the List. I was expecting to get 0,0,1 as the result. I am getting instead 0,1,1 as the result and I cannot figure out why. I am not changing anything in the first position in the List (numbersList[0]) during the second time through the loop, all I am doing is adding an instance of Numbers into the second position in the list (numbersList[1]).

import java.util.ArrayList;
import java.util.List;

public class Tests {
    static int x;

    public static void main(String[] args) {
        List<Numbers> numbersList = new ArrayList<Numbers>();
        Numbers numbers = new Numbers();
        Numbers numbers2 = new Numbers();
        for (int i = 0; i < 2; i++) {
            if (i == 0) {
                numbers.setVariable(i);
                numbersList.add(numbers);
                System.out.println(numbersList.get(0).getVariable());
            }

            if (i > 0) {
                numbers2.setVariable(i);
                numbersList.add(numbers2);
                System.out.println(numbersList.get(0).getVariable());
                System.out.println(numbersList.get(1).getVariable());
            }
        }
    }
}

public class Numbers {
    public static int a = 5;

    public static void setVariable(int b) {
        a = b;
    }

    public static int getVariable() {
        return a;
    }
}

Jean Logeart · Accepted Answer · 2014-10-14 02:02:59Z

4

public static int a = 5 means that all instances of Numbers share the same variable because of the static keyword.

Therefore, when you do numbers2.setVariable(i);, the variable is also changed for numbers. Hence the 0,1,1

If you want instance variables remove the static keywords from Numbers.

answered Oct 14, 2014 at 2:02

Jean Logeart

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Elliott Frisch · Accepted Answer · 2014-10-14 02:04:07Z

1

Your class Numbers has no instance fields (everything is static, or class level).

It should look something like (and overriding toString() is a good idea),

public class Numbers {
   public int a = 5;

   public void setVariable(int b){
         a = b;
   }
   public int getVariable(){
        return a;
   }
   @Override
   public String toString() {
        return String.valueOf(a);
   }
}

By overriding toString() you can more easily print instances of Numbers. For example,

System.out.println(numbersList);

answered Oct 14, 2014 at 2:04

Elliott Frisch

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