Possible Duplicate:
How to create ArrayList (ArrayList<T>) from array (T[]) in Java
How to implement this method:
List<Integer> toList(int[] integers) {
???
//return Arrays.asList(integers); doesn't work
}
-
Is there a reason for the second part of this question, before I edit it out?DJClayworth– DJClayworth2011-07-08 16:20:03 +00:00Commented Jul 8, 2011 at 16:20
-
@DJClayworth: I think, no. I've already flagged the question and explaned that I met some difficulties during submitting this question.Roman– Roman2011-07-08 16:21:08 +00:00Commented Jul 8, 2011 at 16:21
-
2@Merlyn: Not a duplicate. Primitive arrays are different from Object arraysAlexander Pogrebnyak– Alexander Pogrebnyak2011-07-08 16:32:18 +00:00Commented Jul 8, 2011 at 16:32
-
@Alexander: Wow. That's gross :) I can't rescind my vote, so I'll just delete my comment.Merlyn Morgan-Graham– Merlyn Morgan-Graham2011-07-08 16:36:46 +00:00Commented Jul 8, 2011 at 16:36
-
Instead of being a duplicate of the given question, this is a duplicate of stackoverflow.com/q/1073919/1108305.M. Justin– M. Justin2023-06-14 04:40:51 +00:00Commented Jun 14, 2023 at 4:40
Add a comment
|
6 Answers
There's probably a built-in method to do it somewhere* (as you note, Arrays.asList won't work as it expects an Integer[] rather than an int[]).
I don't know the Java libraries well enough to tell you where that is. But writing your own is quite simple:
public static List<Integer> createList(int[] array) {
List<Integer> list = new ArrayList<Integer>(array.length);
for (int i = 0; i < array.length; ++i) {
list.add(array[i]);
}
return list;
}
Obviously one downside of this is that you can't do it generically. You'll have to write a separate createList method for each autoboxed primitive type you want.
*And if there isn't, I really wonder why not.
1 Comment
cHao
For efficiency's sake, i might recommend
new ArrayList<Integer>(array.length). Should keep Java from having to resize the collection to add elements.Use commons-lang3 org.apache.commons.lang3.ArrayUtils.toObject(<yout int array>) and then java.util.Arrays.asList(<>)
ArrayUtils.toObject() will copy the array, and Array.asList() will simply create list that is backed by new array.
int[] a = {1, 2, 3};
List<Integer> aI = Arrays.asList(ArrayUtils.toObject(a));
EDIT: This wont work if you want to add() new elements (resize) though the list interface, if you want to be able to add new elements, you can use new ArrayList(), but this will create one more copy.
2 Comments
Alexander Pacha
You can do it directly with
List<Integer> xValues = Arrays.asList(new Integer[] { 150, 160, 175, 165, 165 });
Dávid Horváth
@AlexanderPacha Definitely, it works with an
Integer[] literal, but not with an existing int[]. It can not be placed in the method skeleton from the OP.List<Integer> asList(final int[] integers) {
return new AbstractList<Integer>() {
public Integer get(int index) {
return integers[index];
}
public int size() {
return integers.length;
}
};
}
answered Jul 8, 2011 at 16:27
Christoffer Hammarström
28k4 gold badges52 silver badges59 bronze badges
13 Comments
cHao
There are a couple of big issues: (1) the list is backed by the array -- that is, if the array elements change, the list elements will too. (2) For larger ints, you'll get back a different Integer each time -- that is, you can not have any expectation of reference equality. Either of those might or might not be an issue, but they're semi-surprising behavior. Before i add (3), can Java actually do closures like that?
Alexander Pogrebnyak
@cHao: 3. Yes, Java can do closures like that!
Joey
Technically this should then be named as List, at least when assuming that Java follows the same naming convention as .NET in this regard (as providing a differently-typed view on the same collection while to actually doing a conversion).
Senthess
These are not closures. They are anonymous classes.
Bart Kiers
@Christoffer, ah, yes, then there's no problem: I thought
add(...) was implemented in AbstractList. My bad, thanks for the info! |
List<Integer> toList(int[] integers) {
// Initialize result's size to length of the incoming array
// this way it will not require reallocations
ArrayList<Integer> result = new ArrayList<Integer>( integers.length );
for ( int cur: integers )
{
result.add( Integer.valueOf( cur ) );
}
return result;
}
answered Jul 8, 2011 at 16:26
Alexander Pogrebnyak
45.8k10 gold badges111 silver badges123 bronze badges
Comments
I do not think there is a quick way to do it unfortunately. I believe you will have to iterate the array and add it one by one.
Comments
import java.util.Arrays;
import java.util.ArrayList;
import java.util.List;
public class Listing {
public static void main(String[] args) {
int[] integers = {1,2,3,4};
java.util.List<Integer> list = new ArrayList<Integer>();
for (int i=0; i< integers.length; i++)
{
list.add(integers[i]);
}
System.out.println(list);
}
}
Tested and working as expected!
