How to determine whether a number is prime (prime number) in java

First of all, we must understand what prime numbers (prime numbers) are, and the definition of prime numbers:

For a number greater than 1, if it cannot be any more than 1 and itself, If it is divisible by other positive integers, then we say it is a prime number.

Method to determine whether a number is a prime number (prime number):

If it is an even number, return directly; then start from 3, with a step size of 2, until n If the arithmetic square root cannot be divided, it is a prime number.

The examples are as follows:

public class Main {
    public static void main(String[] args) {

        for (int j = 0; j <= 19; j++) {
            System.out.println(j+":  "+isPrime(j) );
        }

    }

    private static boolean isPrime(int src) {
        double sqrt = Math.sqrt(src);
        if (src < 2) {
            return false;
        }
        if (src == 2 || src == 3) {
            return true;
        }
        if (src % 2 == 0) {// 先判断是否为偶数，若偶数就直接结束程序
            return false;
        }
        for (int i = 3; i <= sqrt; i+=2) {
            if (src % i == 0) {
                return false;
            }
        }
        return true;
    }
}

Recommended tutorial: java introductory tutorial

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