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I have a question regarding creating an array of variables; if I create an array of variables of the integer type, and I change the value of those variables outside of the array will the values inside the array also get update? I have tried this out in eclipse and it doesn't seem to work (see code below)

int num1 = 1;
int num2 = 2;
int num3 = 3;
int [] nums = {num1, num2, num3};
num1++;
System.out.println(num1);
System.out.println(nums[0]);

What I am hoping for is for the two outputs (num1 and nums[0]) to be the same, but what I am experiencing so far is that nums[0] stays the same and doesn't get updated, any tips on how to get this value updated (without needing to go in and update it as well) is most appreciated

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  • You can just write nums[0]++; to increment a number within the array. Commented Apr 18, 2014 at 5:58

2 Answers 2

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Java's int type is a value type and not a reference type, so when you create the array, the value of the old int is copied into the array. Therefore, changing the original (outside the array) will not change the independent copy inside the array.

To achieve the behavior you want, you can use Erwin's solution, or you will have to create your own wrapper class to achieve an extra value of indirection, which is unfortunately rather verbose.

class IntWrapper {
    public int value;
    public IntWrapper(int i) {
       value = i;
    }
}

class Main{
public static void main(String[] args) { 
   IntWrapper num1 = new IntWrapper(1);
   IntWrapper num2 = new IntWrapper(2);
   IntWrapper num3 = new IntWrapper(3);

   IntWrapper[] nums = {num1, num2, num3};
   num1.value++; 
   System.out.println(num1.value);
   System.out.println(nums[0].value);
}
}
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4 Comments

thanks this helps, I used this method to create an integer wrapper, but now if I have several separate instances of the class and wanted to add different values to different instances (i.e add 1 to num1, subtract 1 from num2) it does it to all three, any idea how I can solve this?
@cvarza, The instances of this class that are created independently using the new keyword should be completely independent of each other. If you make a copy with IntWrapper a = b, that will make a reference to the same object so changes in one will be propagated to another. If that is not clear enough, please ask a new question referencing this question and show some code that describes the problem. If you add a comment here linking to the new question, I will look at it.
@cvarza, If you want to make an independent copy with the same value under the current implementation, you could do IntWrapper a = new IntWrapper(b.value);
thx for your help on this one merlin, I managed to find a solution on managing separate instances (the link to my other question is stackoverflow.com/questions/23177900/… if there is additional information you can add), thanks.
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In Java, primitive types are passed by value. When you put num1 into array nums, a copy was made. If you later change num1, it has no effect on the value in the array. Object references are passed by value as well, but objects themselves are not - they are passed by passing a reference to them. So if you create an array of objects that represent integers, and those objects can be modified, then you could do what you want.

The standard API has one class that represents an integer, called java.lang.Integer. However, it represents an immutable integer so it's no use for you. In the standard API, there is another class called java.lang.concurrent.atomic.AtomicInteger. Objects of this class can be modified.

You can use it like this:

AtomicInteger num1 = new AtomicInteger(1);
AtomicInteger num2 = new AtomicInteger(2);
AtomicInteger num3 = new AtomicInteger(3);
AtomicInteger[] nums = {num1, num2, num3};
num1.incrementAndGet();
System.out.println(num1);
System.out.println(nums[0]);
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