Why is indexOf failing to find the object?

java indexof
rishimaharaj picture rishimaharaj · Apr 2, 2012 · Viewed 15.9k times · Source

I created an integer list and am trying to return the index of a specific value. The array is 3,8,2,5,1,4,7,6 and I want to return the indexOf(3), which should be 0.

I've tried the following in the Eclipse Java Scrapbook after importing java.util.*:

int[] A = {3,8,2,5,1,4,7,9};
Arrays.asList(A).indexOf(3)

I have also tried:

int[] A = {3,8,2,5,1,4,7,6};
ArrayList<Integer> l = new ArrayList(Arrays.asList(A));
l.indexOf(3)

Both are returning -1. Why? How to get this to work as expected?

Answer

Louis Wasserman picture Louis Wasserman · Apr 2, 2012

Arrays.asList(A) returns a List<int[]>. This is because it expects an array of objects, not primitive types. Your options include:

  • use Integer[] instead of int[]
  • inline the array, and let autoboxing take care of it; Arrays.asList(3,8,2,5,1,4,7,9) will work fine
  • use Guava's Ints.asList(int...) method to view the primitive array as a List<Integer>. (Disclosure: I contribute to Guava.)
  • use Guava's Ints.indexOf(int[], int), which works directly on primitive arrays.

Related questions

Where is Java's Array indexOf?

I must be missing something very obvious, but I've searched all over and can't find this method.

Read More
Getting Index of an item in an arraylist;

I have a Class called AuctionItem. The AuctionItem Class has a method called getName() that returns a String. If I have an ArrayList of type AuctionItem, what is the best way to return the index of an item in the …

Read More
Finding second occurrence of a substring in a string in Java

We are given a string, say, "itiswhatitis" and a substring, say, "is". I need to find the index of 'i' when the string "is" occurs a second time in the original string. String.indexOf("is") will return 2 in this case. …

Read More