Design a class that tells whether a number is prime or not

Question

My homework is to Design a class named MyInteger with the following conditions:

An int data field named value that stores the int value of an integer.
A constructor that creates a MyInteger object for the specified int value.
A get method that returns the int value.
A method, isPrime() that returns true if the value is a prime number. See -section 4.10 of the text for Java code that detects prime numbers (this may vary depending on the edition you have).\
A static, isPrime(MyInteger) that returns true if the value is a prime number. Note that this method takes an object reference variable (rather than the value) as a parameter.

My problems come up in the static boolean isPrime method, stating that "/" and "%" are undefined for arguement type and in the main method in my if statement: isPrime() == true. It says to change it to static, but i already have a static boolean isPrime method and I'm supposed to have two isPrime methods according to my conditions. Thank you if you are able to help.

public class MyInteger {


    public MyInteger(int value){

    }
    public static int getValue(){
        int value = 997;
        return value;
    }
    public boolean isPrime(){
        int value = 997;
        for (int i=2; i<=value/2; i++){
            if(value % i == 0) {
                return false;
                }
        }
        return true;
    }
    public static boolean isPrime(MyInteger value){
        for(int i=2; i<=value/2; i++){
            if(value%i == 0){
                return false;
            }
        }
        return true;
    }

    public static void main(String[] args) {
            MyInteger value = new MyInteger(MyInteger.getValue());
            if (isPrime()==true && isPrime(value)==true){
                System.out.println("Testiwng Instance method, is Prime");
                System.out.println("isPrime: " + value + " is prime");
                System.out.println("--------------------------------");
                System.out.println("Testing Class method (That takes a reference variable) is Prime");
                System.out.println("isPrime: " + value + " is prime");
            }
            else{
                System.out.println("Testiwng Instance method, is Prime");
                System.out.println("isPrime: " + value + " is not prime");
                System.out.println("--------------------------------");
                System.out.println("Testing Class method (That takes a reference variable) is Prime");
                System.out.println("isPrime: " + value + " is not prime");
            }

      } 
}

What instance `isPrime()` in `main()` is called for? `static` methods have no instance... — , Apr 08 '15 at 17:07
If your number is not divisible by 2, do you need to check if it is divisible by 4? A potential optimization is to use Eratosthenes' Sieve to calculate the primes less than SquareRoot(value), then preform remainder checks against that list. — Andreas, Apr 08 '15 at 17:13

score 3 · Accepted Answer · edited May 23 '17 at 11:58

You don't have to go till the half of the number to check whether it is prime. You can have a loop that checks only the numbers from 2 to the square root of your number. See this - StackOverflow question about checking prime numbers

I believe you need something like this:

public class Main {
    public static void main(String[] args) throws IOException {
        Scanner inp = new Scanner(System.in);
        int someValue = inp.nextInt();
        MyInteger myInt = new MyInteger(someValue);
        System.out.println("Testing instance method:");
        System.out.println(myInt.isPrime());
        System.out.println("Testing static method:");
        System.out.println(MyInteger.isPrime(myInt));
    }
}

class MyInteger {
    private int value;

    public MyInteger(int value) {
        this.value = value;
    }
    public int getValue() {
        return value;
    }
    public boolean isPrime() {
        int sqrt = (int) Math.sqrt((double)value);
        for(int i = 2; i <= sqrt; i++) {
            if (value % i == 0) return false;
        }
        return true;
    }

    public static boolean isPrime(MyInteger myInt) {
        return myInt.isPrime();
    }
}

score 2 · Answer 2 · edited May 23 '17 at 12:14

Here's a good reference for testing prime numbers What would be the fastest method to test for primality in Java?

Primarily, change your isPrime() to

boolean isPrime(long n) {
    if(n < 2) return false;
    if(n == 2 || n == 3) return true;
    if(n%2 == 0 || n%3 == 0) return false;
    long sqrtN = (long)Math.sqrt(n)+1;
    for(long i = 6L; i <= sqrtN; i += 6) {
        if(n%(i-1) == 0 || n%(i+1) == 0) return false;
    }
    return true;
}

score 1 · Answer 3 · answered Apr 08 '15 at 16:56

1

Your value variable in the method you mentioned is of the type MyInteger, but you're trying to use it as an int. you probably want to use value.getValue() instead.

answered Apr 08 '15 at 16:56

Johannes Larsson

11
2

score 1 · Answer 4 · answered Apr 08 '15 at 17:20

You should consider using class level variable to make use of constructor to initialize it. Also in main() method you are trying to access non-static method (isPrime()), use it as value.isPrime().

In case you don't want to use class variable, make use of static getValue() method inside your static boolean isPrime(MyInteger value) method which solves your problem

score 0 · Answer 5 · answered Apr 09 '15 at 08:22

Another way to check the prime number

 public String isPrime(int number) {
    if (number < 0) {
        return "not a valid number";
    }
    if (number == 0 || number == 1) {
        return "not a prime number";
    }
    if (number == 2 || number == 3) {
        return "prime number";
    }
    if ((number * number - 1) % 24 == 0) {
        return "prime number";
    } else {
        return "not a prime number";
    }
}

Design a class that tells whether a number is prime or not

5 Answers5