how to convert int[] to ArrayList

Question

I want to know how convert int[] to Integer[] in java.

Basically i am trying to convert int[] to ArrayList in java.

I found some examples but they are based on Integer object and not based on int[].

referred: Convert int[] into ArrayList

Thanks

Traverse each element in your `int[]` and add it to your `List`. — Luiggi Mendoza, Nov 19 '13 at 15:50
You cannot use generics like the `List` interface and the `ArrayList` class with primitive types as `int`. You have to use the boxed type `Integer`. — Robin Krahl, Nov 19 '13 at 15:51
is there any other way other than this. because if I use traverse ,this will be in loop which i am going to iterate over 10k times,not sure how it is going to impact — mahesh, Nov 19 '13 at 15:52
use [`Arrays.asList()`](http://docs.oracle.com/javase/7/docs/api/java/util/Arrays.html#asList(T...)) and then use it to initialize an `ArrayList` if you need additional functionality — vandale, Nov 19 '13 at 15:53
If you use the `Integer.valueOf(int)` method, the `Integer` instances are cached, so you do not create many objects, if that was your concern. — Robin Krahl, Nov 19 '13 at 15:53
Do you really need an ArrayList, or is any List implementation sufficient? — isnot2bad, Nov 19 '13 at 15:58

score 7 · Answer 1 · answered Nov 19 '13 at 15:53

7

Something like this:

int[] array = ...;
List<Integer> list = new ArrayList<Integer>(array.length);
for (int i : array) list.add(i);

Note the parameter to the constructor. This will set the initial capacity of the underlying array to what it will need, for best performance.

answered Nov 19 '13 at 15:53

Martijn Courteaux

67,591
47
198
287

What's wrong with just using Arrays.asList(myArray)? I know the underlying array will be the given array but why should we create a new array? – Alfredo Osorio Nov 19 '13 at 16:08
1

@Alfredo Osorio: Read this: http://stackoverflow.com/questions/1248763/arrays-aslist-of-an-array – Martijn Courteaux Nov 19 '13 at 16:14
Ok got it, you won't be able to modify the list. If it is just for read only operations you would be fine. +1 – Alfredo Osorio Nov 19 '13 at 16:17
@AlfredoOsorio: Nope, I think you didn't understand. It would create a `List` with one element, which is the array itself. – Martijn Courteaux Nov 19 '13 at 16:19
your code will give ArrayIndexOutOfBoundsException - Exception – Suganthan Madhavan Pillai Nov 19 '13 at 16:23
I do think that OP already knows this answer. As he has already mentioned about such an answer by linking it in his question. – Sage Nov 19 '13 at 16:25
@Suganthan: Why do you think that? – Martijn Courteaux Nov 19 '13 at 17:35
@ Martijn Courteaux just try iterate a predefined array, throwing exception. When adding the index it is fine, when try to access the element from particular index it will throw exception – Suganthan Madhavan Pillai Nov 19 '13 at 17:38
@ Martijn Courteaux if you don't mind you can try in the code that I posted above – Suganthan Madhavan Pillai Nov 19 '13 at 17:39
@Suganthan: I really don't know what you are thinking, but this works perfectly fine... http://ideone.com/GSJIUj – Martijn Courteaux Nov 19 '13 at 19:03
@Martijn Courteaux you are right, when accessing the index element we have to access like this list.add(array[i-1]),first I tried list.add(array[i]), so it causes the problem. – Suganthan Madhavan Pillai Nov 20 '13 at 04:16
@Suganthan: You should really read about the for each loop in Java. http://www.leepoint.net/notes-java/flow/loops/foreach.html – Martijn Courteaux Nov 20 '13 at 06:27

score 2 · Answer 2 · answered Nov 19 '13 at 15:54

You have to wrap your ints into an Integer object because you cannot create aList<int> list.

int[] array = new int[1, 2, 3, 4, 5, 6];
List<Integer> list = new ArrayList<Integer>(array.length);
for (int i = 0; i < array.length; i++) {
  list.add(Integer.valueOf(array[i]);
}

score 0 · Answer 3 · answered Nov 19 '13 at 15:53

0

for(int i : yourArray) {
    yourArrayList.add(i);
}

That should do it

answered Nov 19 '13 at 15:53

Voidpaw

910
1
5
18

score 0 · Answer 4 · edited Dec 13 '20 at 11:15

0

int[] old = something;
ArrayList<Integer> list = new ArrayList<Integer>();
for (int x : old){
    list.add(x);
}

edited Dec 13 '20 at 11:15

Mark Rotteveel

100,966
191
140
197

answered Nov 19 '13 at 15:53

JoeC

1,850
14
12

score 0 · Answer 5 · answered Nov 19 '13 at 15:55

0

You can use guava's newArrayList() for this.

answered Nov 19 '13 at 15:55

Alexander Bezrodniy

8,474
7
22
24

Which one? Neither of the overloaded methods accepts an int[]! – isnot2bad Nov 19 '13 at 16:05

score 0 · Answer 6 · answered Nov 19 '13 at 16:14

If you want to make a List<Integer> out of an int[], and performance is your concern (so all other provided answers here are still 'too slow' for you), you can write a custom List<Integer> implementation that is directly backed by the int[]:

public class IntegerList extends AbstractList<Integer> implements List<Integer> {
    private final int[] data;

    public IntegerList(int[] data) { this.data = data; }

    @Override
    public Integer get(int index) { return data[index]; }

    @Override
    public int size() { return data.length; }
}

// usage
List<Integer> myIntegerList = new IntegerList(myIntArray);

The drawbacks are:

The list is not modifiable
Changes of the underlying int[] will affect the list instance.
Creating Integer objects out of your ints will still happen, it is just deferred until get(int) is called.

score 0 · Answer 7 · answered Nov 19 '13 at 16:20

I would use org.apache.commons.lang.ArrayUtils from the Apache Commons Lang library to convert the int[] to an Integer[] and then use Arrays.asList() to back the Array in a List:

import java.util.Arrays;
import org.apache.commons.lang.ArrayUtils;

int[] array = ...;
List<Integer> list = Arrays.asList(ArrayUtils.toObject(array));

how to convert int[] to ArrayList

7 Answers7