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}));
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
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.
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.
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.
new Integer[] {1,1}creates a new integer object array, that is NOT saved at the same place where you first assigned it. It is not exactly the same and thus not in your list. (It's like asking where your neighbour John is working but the list only contains Johns from the neighbouring city, he is not in that list).