The best way to transform int[] to List<Integer> in Java? [duplicate]

This question already has answers here: 开发者_JAVA技巧 Closed 11 years ago.

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
}

---

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.}

---

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.

---

List<Integer> asList(final int[] integers) {
    return new AbstractList<Integer>() {
        public Integer get(int index) {
            return integers[index];
        }

        public int size() {
            return integers.length;
        }
    };
}

---

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;
}

---

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.

---

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!