How to assign values of a counting index to an ArrayList? NOT the object of the counting index

Question

for my project, I need an ArrayList, that saves a value (if the if-statement turns true) of the counting index in a do-while-loop . After the loop, the list should print out the wanted and various values. However, all values are equal to the last value of the counting index.

During searching for a solution (without success), I fount out, that the ArrayList should only be used to store objects. That could somehow explain, that the ArrayList has saved the object, not the wanted values. Nevertheless, I need a dynamic list for my loop with nested loops, which is processed more than 500 times, and multiple if-statements, therefore the ArrayList.

How to fix this problem? Any alternatives?

An easy script to describe my problem:

import java.util.ArrayList;

public class test {

public static void main(String[] args) {
    // TODO Auto-generated method stub
    int i1=0;
    int i2=0;
    ArrayList<int[]> iArray = new ArrayList<int[]>();
    int[] x = new int[2];

    do{
        i1++;
        do{ 

            i2++;

            x[0] = i1;
            x[1] = i2;

            iArray.add(x);

        }while(i2<15);
    }while(i1<15);

    for(int[] i : iArray){
        System.out.println("line 1: " + i[0] + " line 2: " + i[1]);
    }

}

}

console output:

    line 1: 15 line 2: 29
    line 1: 15 line 2: 29
    line 1: 15 line 2: 29
    line 1: 15 line 2: 29
    line 1: 15 line 2: 29
    line 1: 15 line 2: 29
    line 1: 15 line 2: 29
    line 1: 15 line 2: 29
    line 1: 15 line 2: 29
    line 1: 15 line 2: 29
    line 1: 15 line 2: 29
    line 1: 15 line 2: 29
    line 1: 15 line 2: 29
    line 1: 15 line 2: 29
    line 1: 15 line 2: 29
    line 1: 15 line 2: 29
    line 1: 15 line 2: 29
    line 1: 15 line 2: 29
    line 1: 15 line 2: 29
    line 1: 15 line 2: 29
    line 1: 15 line 2: 29
    line 1: 15 line 2: 29
    line 1: 15 line 2: 29
    line 1: 15 line 2: 29
    line 1: 15 line 2: 29
    line 1: 15 line 2: 29
    line 1: 15 line 2: 29
    line 1: 15 line 2: 29
    line 1: 15 line 2: 29

Thanks in advance.

Answer 1

You only create one array, and then you add that same array multiple times. Delete your early declaration, and declare (and initialize) it in your loop. Something like,

int[] x = {i1, i2};
// x[0] = i1;
// x[1] = i2;
iArray.add(x);

Which you could do without x on one line like,

iArray.add(new int[] {i1, i2});

Answer 2

You are reusing the same array. You create it once with int[] x = new int[2]; and then you insert the reference to the same object into iArray when you do iArray.add(x);

You can solve your issue by moving the creation of the array just before you assign its value. That way you'll always have a new instance to work on and to add to iArray .

Answer 3

You may be interested in a much simpler way of achieving this with Java 8 streams:

IntStream.range(0, 15).boxed().flatMap(i1 ->
    IntStream.range(0, 15).map(i2 -> new int[]{i1, i2}))
    .collect(Collectors.toList());