Java Program to Display Alternate Prime Numbers

public class AlternatePrimeNumberExample1  
{   
//main method  
public static void main(String[] args)    
{   
int num = 20;   
//prints alternate prime numbers  
System.out.print("Alternate prime numbers up to " + num+" are: ");    
//method calling  
printAlternatePrimeNumber(num);   
}  
//method checks that the number is prime or not  
static int checkPrime(int num)   
{   
int i, flag = 0;   
// we have started i=2 because 2 is a known prime number  
for(i = 2; i<= num / 2; i++)   
{   
if(num % i == 0)   
{   
flag = 1;   
break;   
}   
}   
if(flag == 0)   
return 1;   
else  
return 0;   
}   
//method for printing alternate prime numbers  
static void printAlternatePrimeNumber(int n)   
{   
//if the value of temp variable is odd, we do not print the prime number  
//if the value of temp variable is even, we print the prime number  
int temp = 2;   
for(int num = 2; num <= n-1; num++)   
{   
//checking the number is prime or not  
if (checkPrime(num) == 1)   
{    
//prints the prime number if temp is even  
if (temp % 2 == 0)   
System.out.print(num + " ");   
//increments the temp variable by 1  
temp ++;   
}   
}   
}   
}  

Output:

Alternate prime numbers up to 40 are: 2 5 11 17 23 31