java program To find the coprimes of a number

// coprimes of any number n are numbers bw 1 and n, which are not divisible by any factors of n
public class Coprimes
{

public static void main(String args[])
{
int sum=0;
int p=0;
int y;
for(int i=18; i<=23; i++)
{
sum=0;
p=0;
int[]a=new int[i/2];
for(int j=2; j<=i/2; j++)
{
if(i%j==0 && IsPrime(j)==true)
a[p++]=j;
}

System.out.println("For "+i+", the co-primes are:");
for(int x=1; x{
for(y=0; y{
if(x%a[y]==0)
break;
}
if(y==p)
{
sum+=x;
System.out.println(x);
}
}
System.out.println("Their sum is "+sum);
System.out.println("");
System.out.println("");
}
}

public static boolean IsPrime(int a)
{
a=Math.abs(a);
for(int i=2; i<=a/2; i++)
if(a%i==0)
return false;
return true;
}
}

java program: Recursive function to multiply two numbers

// to multiply 2 nos. using recursive functions
import java.io.*;
class recursiveMultiplication
{
public static void main(String args[])throws IOException
{
BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
System.out.print("Enter a number: ");
String s=br.readLine();
System.out.print("Enter another number: ");
String S=br.readLine();
int a=Integer.parseInt(s);
int b=Integer.parseInt(S);
if((a<0&&b>0)||(a>0&&b<0))
System.out.print(-Mult(Math.abs(a),Math.abs(b)));
else System.out.print(Mult(Math.abs(a),Math.abs(b)));
}
public static int Mult(int a, int b)
{
if(a==0||b==0)
return 0;
else if(b==1)
return a;

return(a+Mult(a,(b-1)));
}

}

Shortest Function to find if a number is prime

class prime
{
public static boolean PrimeCheck(int a)//a should be greater than 0
{
for(int i=2; i<=a/2; i++)
if(a%i==0)
return false;
return true;
}
}