Project Euler #7 in Java

Project Euler #7:

By listing the first six prime numbers: 2, 3, 5, 7, 11, and 13, we can see that the 6th prime is 13.

What is the 10 001st prime number?

Here is my solution:

public class PrimeFinder {

    public static void main(String[] args) {
        long time = System.nanoTime();
        int result = getNthPrime(10001);
        time = System.nanoTime() - time;
        System.out.println("Result: " + result
                + "\nTime used to calculate in nanoseconds: " + time);
    }
    
    private static int getNthPrime(int n) {
        int max = (int) (1.4 * n * Math.log(n));
        boolean[] isPrimeArray = new boolean[max + 1];
        for (int i = 2; i <= max; i++) {
            isPrimeArray[i] = true;
        }
        for (int i = 2; i * i <= max; i++) {
            if (isPrimeArray[i]) {
                for (int j = i; i * j <= max; j++) {
                    isPrimeArray[i * j] = false;
                }
            }
        }
        // Find the nth prime
        int nthPrime = 0;
        int index = 0;
        for(boolean isPrime : isPrimeArray) {
            if(isPrime) {
                nthPrime++;
            }
            if(nthPrime == n) {
                return index;
            }
            index++;
        }
        throw new IllegalArgumentException("n is not valid");
    }
}

It simply performs a sieve, and then find the 10001st true.

Output:

Result: 104743

Time used to calculate in nanoseconds: 13812289

Questions:

• This obviously is not efficient. How can I improve it?

• Are there any bad practices?

Instead of looping one more time through your isPrime array, consider expanding the iterations on your first for loop and then exiting from there once you reach the desired result:

...
    int counter = 0;
    for (int i = 2; i <= max; i++) {
        if (isPrimeArray[i]) {
            if (++counter == n) {
                return i;
            }
            for (int j = i; i * j <= max; j++) {
                isPrimeArray[i * j] = false;
            }
        }
    }
    throw new IllegalArgumentException("n is not valid");

StackExchange Code Review Q#78367, answer score: 2