CODEWITHSUNDEEP
javaarrayslistfindindexof
user3803086
user3803086

Reputation: 35

Find index of array in ArrayList

Is it possible to find the index of an array within an ArrayList?

The method indexOf doesn´t seem to work. For instance, for a given list of arrays with size two, this code prints "-1"

List<Integer[]> nodes = new ArrayList<Integer[]>();
nodes.add(new Integer[] {1,1});
System.out.println(nodes.indexOf(new Integer[] {1, 1}));

Upvotes: 3

Views: 1175

Answers (4)

Aditya Singh
Aditya Singh

Reputation: 2443

No you cannot do that. new Integer[]{1, 1} is an integer object and your code creates to separate objects of the same.

To do what you want to do, 

 // You need to save a reference of your array in a variable
 Integer[] arr = new Integer[]{1, 1};

 // Add it to your list
 nodes.add(arr);  

 // Finally retrieve the index 
 System.out.println(nodes.indexOf(arr));

When you print out the index of the variable arr, it will refer to the same integer array object created before.

Upvotes: 0

user2413972
user2413972

Reputation: 1355

You need to understand that Java compare by reference rather than by value in this case. The following code is displays the correct solution:

    List<Integer[]> nodes = new ArrayList<>();
    Integer[] a = new Integer[] {1,1};
    nodes.add(a);
    System.out.println(nodes.indexOf(a)); //0
    System.out.println(a == a); //true
    System.out.println(a == new Integer[] {1,1}); //false
    System.out.println(Arrays.equals(a, new Integer[]{1, 1})); //true

Ordinary objects for comparison is to use the method equals(). (etc. a.equals(b)) For arrays and some class need static method.

Upvotes: 2

uoyilmaz
uoyilmaz

Reputation: 3105

indexOf method "returns the lowest index i such that (o==null ? get(i)==null : o.equals( get(i))), or -1 if there is no such index", as stated in the Java API. And for arrays, array1.equals( array2) is the same as array1 == array2, therefore you get -1 as those two arrays have different references.

Upvotes: 0

Craig Otis
Craig Otis

Reputation: 32054

The equals() implementation of the primitive Java array performs a reference equality check. Meaning, they're only considered equal if the references point to the same instance of the array.

You could write your own index-seeking method that uses the Arrays.equals() static method like so:

List<Integer[]> nodes = new ArrayList<Integer[]>();
nodes.add(new Integer[] {1,1});
Integer[] lookingFor = new Integer[] {1,1};
int index = -1;
for (int i = 0; i < nodes.size(); i++) {
    Integer[] array = nodes.get(i);
    if (Arrays.equals(lookingFor, array)) {
        index = i;
        break;
    }
}
System.out.println(index); // 0

Upvotes: 6

Related Questions