Converting ArrayList<Integer> into int[] won't work in Java

Question

I have been developing a program where I have to convert an ArrayList into an int[]. I do not get any syntax errors when I run the code. However, when I print the int[] to see if it does work, it prints out a random string which is "[I@2a139a55" How can I fix this?

I cannot use an int[] from the start. This program HAS to convert an ArrayList into int[].

 ArrayList<Integer> student_id = new ArrayList<Integer>(); student_id.add(6666); student_id.add(7888); int[] student_id_array = new int[student_id.size()]; for (int i=0, len = student_id.size(); i < len; i ++){ student_id_array[i] = student_id.get(i); } System.out.println(student_id_array);

You cannot print an array directly. You have to use System.out.println(java.util.Arrays.toString(student_id_array)); — marstran
– marstran, Commented Oct 22, 2016 at 22:45
Possible duplicate of How to convert List<Integer> to int[] in Java? — MordechayS
– MordechayS, Commented Oct 22, 2016 at 22:46

Aly · Accepted Answer · 2016-10-22 22:45:05Z

1

You are printing out the reference to the array. Use Arrays.toString(int[]).

answered Oct 22, 2016 at 22:45

Aly

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Comments

DimaSan · Accepted Answer · 2016-10-22 22:51:40Z

You can do the converting your ArrayList<Integer> to int[] with one line in Java 8:

int[] student_id_array = student_id.stream().mapToInt(id -> id).toArray();

And if you want to output array's values instead of representation of your array (like [I@2a139a55) use Arrays.toString() method:

System.out.println(Arrays.toString(student_id_array));

thetraveller · Accepted Answer · 2016-10-22 22:46:40Z

That because you are printing the memory address of that array.

try this :

 ArrayList<Integer> student_id = new ArrayList<Integer>(); student_id.add(6666); student_id.add(7888); int[] student_id_array = new int[student_id.size()]; for (int i=0, len = student_id.size(); i < len; i ++){ student_id_array[i] = student_id.get(i); } System.out.println(student_id_array[0]+" "+student_id_array[1]);

Output :

6666 7888

OR use for-loop if you populate the ArrayList later on.

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