为我的Java素数测试处理更大的输入？

Question

因此，对于一个个人项目，我决定进行自己的素性测试。除了AKS测试之外，我对现有的素数测试算法没有做太多研究。我认为这对我来说太高级了，无法用Java实现，所以我自己做了。

问题在于此方法（虽然是确定性的）最多只能使用9位数字。如何更改现有代码以处理较大的输入数字？可以对此代码进行任何更改以加快速度吗？ （请不要推荐全新的算法，我意识到这是一个非常基本的算法，我只想知道我是否可以对其进行改进）

public class PrimeNumberTest {
public static void main(String[] args){
    //Open stream to take user input
    Scanner userinput = new Scanner(System.in);
    //Prompt user for input
    System.out.println("Enter a number you would like to test for primality:");
    //Save user input to a string var.
    String inputNum = userinput.nextLine();
    //Convert inputNum to a primitive int
    int finalInputNum = Integer.parseInt(inputNum);
    //Call method to check primality
    isPrime(finalInputNum);
}
public static boolean isPrime(int finalInputNum){
    //Check if the number is 2. This is an edge case
    if (finalInputNum == 2){
        System.out.println("It's Prime! The number has only 2 factors.");
        return true;
    }
    //Check if number is even and not 2. Not prime
    else if (finalInputNum % 2 == 0){
        System.out.println("Not prime! The number has more than 2 factors.");
        return false;
    }      
    else {
        /* A prime number has no more and no less than 2 factors. If even a single 
           other smaller number evenly divides the user inputed number, then there are
           more than 2 factors, therefore not prime */

        /* We use this to keep track of all number less than the input. 
           It starts at 1 since the input number itself is always a factor */
        int numToSubtract = 1; 
        //Guard stops at 1 since 1 is always a factor
        while ((finalInputNum - numToSubtract) > 1){
            /* If the current num doesn't divide evenly with the number immediately before
               it, then it's not a factor. Increment and check the next one on next loop */
            if (finalInputNum % (finalInputNum - numToSubtract) != 0){
                numToSubtract++;
            }
            else{
                System.out.println("Not prime! The number has more than 2 factors.");
                return false; 
            }
        }
        System.out.println("It's Prime! The number has only 2 factors.");
        return true;
    }
}
}