Why elements are added incorrectly in List in Java?

Question

On my question user @deadshot give me answer.

I decided to little change the program.
I add one object - ArrayList<int[]> combinations, which will store array int[] with numbers (size this array is always 3).
I add to combinations arrays (array[]) using the method add().


package com.company;

import java.util.*;

public class Main {

    static int POINTS_ON_LINE = 3;

    public static void main(String[] args) {
        int[] points = new int[]{1, 2, 3, 4};

        System.out.println("no repetitions:");
        p1(points, POINTS_ON_LINE);
    }

    public static void p1(int[] arr, int pointsOnLine) {
        List<int[]> combinations = new ArrayList<>();

        int lengthArray = arr.length, i;
        int[] index = new int[pointsOnLine];
        int[] temp = new int[pointsOnLine];

        for (i = 0; i < pointsOnLine; i++) {
            index[i] = i;
        }

        if (pointsOnLine < lengthArray) {
            for (int j : index) {
                temp[j] = arr[j];
            }

            boolean flag;
            while (true) {
                System.out.println("Add array: " + Arrays.toString(temp));
                combinations.add(temp);

                flag = false;

                for (i = pointsOnLine - 1; i >= 0; i--) {
                    if (index[i] != i + lengthArray - pointsOnLine) {
                        flag = true;
                        break;
                    }
                }

                if (!flag) {
                    break;
                }

                index[i] += 1;

                for (int j = i + 1; j < pointsOnLine; j++) {
                    index[j] = index[j - 1] + 1;
                }

                for (i = 0; i < pointsOnLine; i++) {
                    temp[i] = arr[index[i]];
                }
            }
            System.out.println("End");
        }
        System.out.println("Result");
        for(int[] q : combinations) {
            System.out.println(Arrays.toString(q));
        }
    }
}

Why is the result like this:

no repetitions:
Add array: [1, 2, 3]
Add array: [1, 2, 4]
Add array: [1, 3, 4]
Add array: [2, 3, 4]
End

Result:
[2, 3, 4]
[2, 3, 4]
[2, 3, 4]
[2, 3, 4]

Why object combinations contain:

[2, 3, 4]
[2, 3, 4]
[2, 3, 4]
[2, 3, 4]

But object combinations should be contain this:

[1, 2, 3]  
[1, 2, 4]  
[1, 3, 4]  
[2, 3, 4]

@Nikolas, `ArrayList` doesn't store `HIS COPY` of the array? — West Side, Aug 23 '20 at 18:00
@Progman, yes, it looks like this. So, what I must do, so that the values don't change? — West Side, Aug 23 '20 at 18:05

beastlyCoder · Accepted Answer · 2020-08-23T18:11:34.733

0

Change this line:

combinations.add(temp);

To this:

combinations.add(Arrays.stream(temp).toArray());

This will get a copy of the array at it's current state, and add it to the List. This is using the stream api from java 8.

edited Aug 23 '20 at 18:11

answered Aug 23 '20 at 18:08

beastlyCoder

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I did so ```java combinations.add(Arrays.copyOf(temp, temp.length)); ```. But, thank you. – West Side Aug 23 '20 at 18:09

Why elements are added incorrectly in List in Java?

1 Answers1