Java ArrayOutOfBoundException in ArrayList

Question

The question is Define a cluster in an integer array to be a maximum sequence of elements that are all the same value. For example, in the array {3, 3, 3, 4, 4, 3, 2, 2, 2, 2, 4} there are 5 clusters, {3, 3, 3}, {4, 4}, {3}, {2, 2, 2, 2} and {4}. A cluster-compression of an array replaces each cluster with the number that is repeated in the cluster. So, the cluster compression of the previous array would be {3, 4, 3, 2, 4}. The first cluster {3, 3, 3} is replaced by a single 3, and so on.

public static void main(String[] args) {
 int[] givenArray = {1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1};
 System.out.println("Clustered Array = " + Arrays.toString(isTrivalent(givenArray)));
}
public static int[] isTrivalent (int[] a){
 List<Integer> cluster = new ArrayList<Integer>();
 for (int i=0; i<a.length ; i++ ) {
 if(i == 0){
 cluster.add(a[i]);
 }else{
 if(cluster.get(i-1) != a[i]) cluster.add(a[i]);
 }
 }
 int[] arr = new int[cluster.size()];
 for (int j =0; j<cluster.size() ; j++) {
 arr[j] = cluster.get(j);
 }
 return arr;
}

But I am getting an ArrayOutOfBoundException. What am I doing wrong?

• nomis: On which line are you getting the exception?
• Ruchira Gayan Ranaweera: What do you want to do with this line of code if(cluster.get(i-1) != a[i]) cluster.add(a[i]);

Answer 1

Change

if(cluster.get(i-1) != a[i]) cluster.add(a[i]);

to

if(a[i-1] != a[i]) cluster.add(a[i]);

cluster.get(i-1) may not exist.

Answer 2

This is happening because when you check

if(cluster.get(i-1) != a[i]) 

it is not necessary that the cluster arraylist will actually have size of atleast i-1 since you are skipping a lot of array elements. You need to change your condition to

if(cluster.get(cluster.size()-1) != a[i])

or equivalently (as suggested in previous answer)

if(a[i-1] != a[i])

for this code to work as intended.

public static void main(String[] args) {
 int[] givenArray = {1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1};
 System.out.println("Clustered Array = " + Arrays.toString(isTrivalent(givenArray)));
}
public static int[] isTrivalent(int[] a) {
 List<Integer> cluster = new ArrayList<Integer>();
 for (int i = 0; i < a.length; i++) {
 if (i == 0) {
 cluster.add(a[i]);
 } else {
 if (cluster.get(cluster.size() - 1) != a[i]) {
 cluster.add(a[i]);
 }
 }
 }
 int[] arr = new int[cluster.size()];
 for (int j = 0; j < cluster.size(); j++) {
 arr[j] = cluster.get(j);
 }
 return arr;
}