Question

I have a loop in which I calculate a value and add it it a list. So, I do something like that:

x = getValue()
values.add(x)
while (true) {
   x = getValue();
   values.add(x)
}

I found out that this approach does not work since I add the same instance to the list. In more details, in every cycle of the loop I re-assign a new value to the x and doing so I change values of all elements that were already added to the list (so in the end I get a list of identical elements).

To solve this problem I did the following:

x = getValue();
Integer[] valueToAdd = new Integer[n];
for (int i=0; i<n; i++) {
   valueToAdd[i] = x[i];
}
while (true) {
   x = getValue();
   y = new Integer[n];
   for (int i=0; i<n; i++) {
      valueToAdd[i] = x[i];
   }
   values.add(valueToAdd)
}

In this way I wanted to create a new instance every time want to add a value to the list. But it does not work since I get a duplicate local variable error.

It is also strange to me that I do not have this error if I declare the same variable many times in the loop. The problem appears only if I first declare a new variable outside the loop and then also in the loop.

Is there a way in Java to re-use the same name for different instances?

ADDED
I need to clarify some issues. I did not show all the code. I have the break command in the loop (when a new value cannot be generate, I exit the loop). x and value have Integer[] type.

ADDED 2
Since it was mentioned that the problem can be in the getValue() I need to in more details here. Actually I do not have getValue() in my code (I used getValue() here to make my example shorter). In my code I had:

   Integer[] x = new x[n];
    while (true) {
       for (int i=0; i<n; i++) {
          x[i] = y[i];
       }
       values.add(x)
    }

And it did not work since in my values list I had identical elements (and I know that in the loop on every cycle x had a new value).

ADDED 3

Why all elements of my list seems to be the same?

Answer 1

Your problem is not what you think it is. For example take a look at this simple program:

String x = null;
List<String> l = new ArrayList<String>();
for (int i = 0; i < 10; i ++) {
    x = String.valueOf(i);
    l.add(x);
}

System.out.println(l);

It prints the numbers from 0 to 9. This is because java is pass-by-value (check here). You are not passing the reference to x, you are passing the value of x (in the add method).

So the problem lies in the getValue() method, which returns the same object.

Update: Now the question makes more sense. You are working with the same object x everytime, and just changing its state. In order to put different values just move the declaration inside the loop:

while (true) {
   Integer[] x = new x[n];
   ...
}

If you need it outside the loop, well, simply use another variable there. It does not have to be named x. Since you won’t be using it inside the loop anyway.