Looping Clarification

Question

Hi, I'm new to the whole Java game and need some clarification and help understanding some of this code I've adapted.
Firstly I think I understand how the 'for' loops and their parameters work, however I'm unsure how "i" is being used, it doesn't seem to be declared anywhere?
Secondly, the "return prime" function, what exactly does it do? I thought prime was a boolean? What is it returning? Is it simply resetting the above boolean status to the current one?
Thirdly 'int num' is an object in the 'isPrime' class? What is it being used for? Where does it get it's value from?
Finally and probably most importantly, by calling the the class "IsPrime(i)" in the main, what does the "i" parameter depict? I was thinking "i" is simply a counter used for the 'for' loop?
import java.util.Scanner;
public class PrimeTest{
  // This method tests whether a given number is prime or not.
     public static boolean isPrime ( int num ){
     boolean prime = true;
   int limit = (int) Math.sqrt ( num ); 
   for ( int i = 2; i <= limit; i++ ){
          if ( num % i == 0 ){
               prime = false;
               break;
     return prime;
public static void main(String[] args){
     Scanner input = new Scanner(System.in);
     System.out.print("Enter the number: ");
     int number = input.nextInt();
     // This loop writes out all the prime numbers less than given number.
   for ( int i = 2; i < number; i++ ){
          if ( isPrime ( i ) )
               System.out.print ( i+"," );
}Any help would be greatly apreciated!
Thanks, Dan.

Answer 1

Dan.Cody wrote:
Firstly I think I understand how the 'for' loops and their parameters work, however I'm unsure how "i" is being used, it doesn't seem to be declared anywhere?It is declared inside the for loop. The following two pieces of code are (mostly) identical:
int i;
for (i = 0; i < SOME_MAXIMUM; i++) {and
for (int i = 0; i < SOME_MAXIMUM; i++) {Not that the second piece of code has the "int i" embedded inside the for-statement.
Secondly, the "return prime" function, what exactly does it do? I thought prime was a boolean? What is it returning? Is it simply resetting the above boolean status to the current one?prime is a boolean. isPrime(int) is a method that should return a boolean. "return prime" leaves the method and sets the return value to that of the variable prime.
Thirdly 'int num' is an object in the 'isPrime' class? What is it being used for? Where does it get it's value from?No, "int num" is a parameter in the isPrime method. by calling isPrime(3) you are basically asking the method "is the number 3 a prime number?" and the method "answers" (returns) true (if it's implemented correctly).
You only define a single class in your code and that's PrimeTest.
Finally and probably most importantly, by calling the the class "IsPrime(i)" in the main, what does the "i" parameter depict? I was thinking "i" is simply a counter used for the 'for' loop?i is the counter of the loop, that's correct. Calling the method isPrime with the parameter i is basically asking "is the number stored in the variable i a prime number?". If that method returns true (i.e. the value in i is a prime number), then the next line will print it.
Again: isPrime (note the lower-case i, Java is case sensitive and so should you be, when writing about Java) is a method and not a class.
You can call methods.
You can instantiate classes to get objects.
You can't call a class, but you can call a method on an object.