import java.util.ArrayList;

public class Application {

public static void main(String[] args)
{
ArrayList<Integer> someInts = new ArrayList<>();
int[] nums = {5, 7, 2, 15};
for (int i = 0; i < nums.length; i++) {
  someInts.add(nums[i]);
}
someInts.set(3, 100);

// Display the sum
int sum = 0;
for (int i = 0; i < someInts.size(); i++) {
  sum += someInts.get(i);
}

System.out.println("sum is " + sum);
System.out.println("Index of 100: " + someInts.indexOf(100));
System.out.println("Index of 4: " + someInts.indexOf(4));
}
}

Output:

sum is 114
Index of 100: 3
Index of 4: -1