First Recurring Number

import java.util.HashSet;

class Main {
   
    public static void main(String[] args) {
        printFirstRecurringNumber(new int[] {1,2,3,4,5,2,6,7,6});
    }
    
    public static void printFirstRecurringNumber(int[] arr){
        
        if(arr != null && arr.length > 1){
            HashSet<Integer> set = new HashSet<>();
            for(int i = 0; i < arr.length; i++){
                if(set.contains(arr[i])){
                    System.out.println(arr[i]);
                    return;
                } else {
                    set.add(arr[i]);
                }
            }
        }
        
        throw new IllegalArgumentException("Supplied input is not valid or has no recurrences.");
        
    }
}