How to create ArrayList (ArrayList<Integer>) from array (int[]) in Java

Question

I have seen the question: Create ArrayList from array

However when I try that solution with following code, it doesn't quite work in all the cases:

import java.util.ArrayList; import java.util.Arrays; import java.util.Collection; import java.util.List;  public class ToArrayList {      public static void main(String[] args) {         // this works         String[] elements = new String[] { "Ryan", "Julie", "Bob" };         List<String> list = new ArrayList<String>(Arrays.asList(elements));         System.out.println(list);          // this works         List<Integer> intList = null;         intList = Arrays.asList(3, 5);         System.out.println(intList);          int[] intArray = new int[] { 0, 1 };         // this doesn't work!         intList = new ArrayList<Integer>(Arrays.asList(intArray));         System.out.println(intList);     } } 

What am I doing wrong here? Shouldn't the code intList = new ArrayList<Integer>(Arrays.asList(intArray)); compile just fine?

Answer 1

The problem in

intList = new ArrayList<Integer>(Arrays.asList(intArray)); 

is that int[] is considered as a single Object instance since a primitive array extends from Object. This would work if you have Integer[] instead of int[] since now you're sending an array of Object.

Integer[] intArray = new Integer[] { 0, 1 }; //now you're sending a Object array intList = new ArrayList<Integer>(Arrays.asList(intArray)); 

From your comment: if you want to still use an int[] (or another primitive type array) as main data, then you need to create an additional array with the wrapper class. For this example:

int[] intArray = new int[] { 0, 1 }; Integer[] integerArray = new Integer[intArray.length]; int i = 0; for(int intValue : intArray) {     integerArray[i++] = intValue; } intList = new ArrayList<Integer>(Arrays.asList(integerArray)); 

But since you're already using a for loop, I wouldn't mind using a temp wrapper class array, just add your items directly into the list:

int[] intArray = new int[] { 0, 1 }; intList = new ArrayList<Integer>(); for(int intValue : intArray) {     intList.add(intValue); }