Q:

Calling method of another class JAVA

I'm using an external class that contains the method: public static boolean isPrime(int n).
I'm new in java and I don't know how to call a method from another class, I've been reading in other posts and they suggest to create an object, so that's what I did creating TestPrime objeto=new TestPrime(); but the if(objeto.isPrime(arrayInt[i])==true) isn't working.   
import com.utad.idcd.redes.PrimeNumber;
public class TestPrime {
    public static void main(String[] args) {
        TestPrime objeto = new TestPrime();
        int arrayInt[] = { 1, 2, 3, 4, 5, 6, 7, 8, 9 };
        for (int i = 0; i < 9; i++) {

            System.out.print("El" + arrayInt[i] + "es: ");

            if (objeto.isPrime(arrayInt[i]) == true) {
                System.out.print("primo\n");
            } else {
                System.out.print("no primo\n");
            }
        }    
    } 
}

A:

You have to create object of the PrimeNumber Class
import com.utad.idcd.redes.PrimeNumber;
public class TestPrime {
    public static void main(String[] args) {
        int arrayInt[] = { 1, 2, 3, 4, 5, 6, 7, 8, 9 };
        for (int i = 0; i < 9; i++) {

            System.out.print("El" + arrayInt[i] + "es: ");

            if (PrimeNumber.isPrime(arrayInt[i]) == true) {
                System.out.print("primo\n");
            } else {
                System.out.print("no primo\n");
            }
        }    
    } 
}

Now it should run fine. 
Edit:
sorry, I did not see the method was static;
since your method is static and public you can also use static import like:
import static com.utad.idcd.redes.PrimeNumber.isPrime;

and then directly use the function as
if (isPrime(arrayInt[i]) == true) { 

