Array gets overwritten for no apparent reason

Question

Problem

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I have written a loop in which I fill an array with Sum objects. Everything works fine, but as soon as the loop gets to the next iteration it overwrites the first index of the array.

What have I tried

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I tried to see if maybe my problem resides in a different piece of code (such as my Sum class). But could not find anything that would disturb the loop.
I tried to find other variables with the same name (even in other methods, since I was desperate) and see if I maybe changed my iterator somewhere else. I couldn't find anything related to that.
I tried looking around on the internet and SO to find something related to accidentally overwriting arrays but couldn't find anything either.

Code

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 public Task(Object[] parameters) { this.number_of_sums = Integer.parseInt((String)parameters[0]); this.variables_per_sum = Integer.parseInt((String)parameters[1]); this.sum_parameters = new Object[this.variables_per_sum]; this.sums = new Sum[this.number_of_sums]; int z = 0; for(int i = 0; i < this.number_of_sums; i++) { int x = 0; for(int j = (2 + z); j < ((this.variables_per_sum + 2) + z); j++) { this.sum_parameters[x] = parameters[j]; x++; } this.sums[i] = new Sum(this.sum_parameters); System.out.println("Index 0: "+sums[0]); //1st iteration: 1 + 1 //2nd iteration: 2 - 1 System.out.println("Index 1: "+sums[1]); //1st iteration: null //2nd iteration: 2 - 1 z += this.variables_per_sum; } } 

Expectations

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I'm expecting the output of 1 + 1 and 2 - 1. I am however getting the following: 2 - 1 and 2 - 1 when I'm done.

If anyone spots anything I'm doing wrong or would like to see more information or code on my side please say so. Thanks in advance.

Answer 1

I'm going to assume the Sum class doesn't store its sum, but instead computes it from the array it was constructed with whenever it's needed.

It looks like all the Sum objects will share the same array -- you're passing the same reference every time you construct a Sum . Furthermore, every time you loop over j you overwrite the contents of that array.

So when everything is done, all the sums are the same.

You should be able to get around this by giving each Sum a different sum_parameters :

public Task(Object[] parameters)
{
    this.number_of_sums = Integer.parseInt((String)parameters[0]);
    this.variables_per_sum = Integer.parseInt((String)parameters[1]);
    this.sums = new Sum[this.number_of_sums];
    int z = 0;

    for(int i = 0; i < this.number_of_sums; i++)
    {
        Object[] sum_parameters = new Object[this.variables_per_sum];
        int x = 0;
        for(int j = (2 + z); j < ((this.variables_per_sum + 2) + z); j++)
        {
            sum_parameters[x] = parameters[j];
            x++;
        }

        this.sums[i] = new Sum(sum_parameters);

        System.out.println("Index 0: "+sums[0]); //1st iteration: 1 + 1 //2nd iteration: 2 - 1
        System.out.println("Index 1: "+sums[1]); //1st iteration: null //2nd iteration: 2 - 1

        z += this.variables_per_sum;
    }
}

Answer 2

Each one of your Sum objects is constructed with this.sum_parameters as a parameter:

this.sums[i] = new Sum(this.sum_parameters);

When sum_parameters is modified in each iteration of the outer loop, it changes internally in the objects constructed around references to it .

You should make an internal copy of sum_parameters in each Sum object.