Can you help me understand how to use this class.

[AcousticBruce]

I wrote a small program that found the nth prime. Then I decided to make separate classes and see if I can understand methods and objects.

So I made a class called IsThisAPrime(). I got it working and it would display the information in system out. But then I wanted to make it return a boolean value so another class can use it.

The problem is probably with the IsThisAPrime() constructor.. but I havnt figured it out yet.

1) How can I make IsThisAPrime() return a boolean to FindSpecificPrime() ?

2) Is it ok to create code like this in a loop? Or will this create a new Object each time and use lots of memory? if (new IsThisAPrime())

Main

public class PrimeTest {
    public static void main(String[] args) {
        new FindSpecificPrime(10001);
    }
}

IsThisAPrime()

public class IsThisAPrime {

    private boolean isPrime;
    private int testNum;

    [COLOR="Red"]public boolean IsThisAPrime(int isItPrime) {[/COLOR]

        testNum = isItPrime;
        if (searchIfPrime()) {
        return true;
        } else {
            return false;
        }
    }

    private boolean searchIfPrime(){

        for (int i = 2; i < testNum; i++) {
            if (testNum % i == 0) {
                return false;
            }
        }
        return true;
    }
}

FindSpecificPrime()

public class FindSpecificPrime {

    int testNum, primeAt, n;
    int[] primes;


    public FindSpecificPrime(int primeNum) {
        primes = new int[primeNum];
        primeAt = 0;
        n = 0;
        while (true) {
            n += 1;
            if ([COLOR="Red"]new IsThisAPrime(n)[/COLOR]) {
                primes[primeAt] = n;
                primeAt +=1;
            }
            if (primeAt == primeNum){
                displayPrimes();
                break;
            }
        }
    }

    private void displayPrimes() {
        for (int i = 0; i < primes.length; i++) {
            System.out.print(primes[i] + " ");
        }
        System.out.println("\nPrime number " + primes.length + " is " + primes[primes.length - 1]);
    }
}

[Zack]

You can't do it like that. You've basically overriden the constructor with a method, and that's not allowed. What you have to do instead is something like this:

public class IsThisAPrime {

    private boolean isPrime;
    private int testNum;

    [color=red]public IsThisAPrime(int isItPrime) {
        testNum = isItPrime; // This line comes up here into the constructor, as it makes more sense to initialize the variable here.
    }

    // I've axed the entire second function as it's not necessary. We need a separate method ANYway, so we can just use searchIfPrime.[/color]

    private boolean searchIfPrime(){
        for (int i = 2; i < testNum; i++) {
            if (testNum % i == 0) {
                return false;
            }
        }
        return true;
    }
}

...then, in your other class, something like this:

if (new IsThisAPrime(n).searchIfPrime()) {

[AcousticBruce]

I guess I do not understand. That search was a private method.

If I take out the boolean and rewrite it to manualy display the boolean value in system.out.print method and in the main method "new IsThisPrime(45)" It works. I would like to have FindSpecificPrime() call the IsThisPrime class send it a number and IsThisPrime returns a boolean value.

The point of this was to have code that I can reuse (I was exercising that idea at least)

So basically FindSpecificPrime will use the IsThisPrime class to test each number.

public class IsThisAPrime {

    private boolean isPrime;
    private int testNum, pot;

    public IsThisAPrime(int isItPrime) {

        testNum = isItPrime;
        searchIfPrime();
        System.out.println(isPrime);

    }

    private void searchIfPrime(){

        pot = 0;
        for (int i = 2; i < testNum; i++) {
            if (testNum % i == 0) {
                pot = 1;
                break;

            }
        }
        if (pot > 0) {
        isPrime = false;
        } else {
            isPrime = true;
        }
    }
    
}

[Zack]

You can't have a constructor return a value other than object itself. I apologize for not realizing the method was private; however this is honestly the best way to achieve what you want to do.

If you use the code you have now, you'll have a call like: if (new IsThisAPrime(n).isPrime) {, except that isPrime is still private.

So either way, you need a get method, or a method has to be made public to achieve what you want to do.

If you use the non-private 2nd method idea, you can still reuse the code later.

[Gordy.Liang]

For this kind of thing, you do not need to construct any object instances to invoke the method of IsThisAPrime. So, this method should be defined as static method, so that it can be invoke without any instance of this class.

public class IsThisAPrimeTool {

...

public static IsThisAPrime(int isItPrime) {

...
}

When you want to invoke it, call IsThisAPrimeTool.IsThisAPrime(xxx)

Many standard java APIs use this kind of definition mechanism.