Question

Define a public method that is called isPrime() that returns a boolean and implements the Sieve of Eratosthenes method.

Define a public method that is called numberOfPrimes() that returns the number of prime numbers between 2 and the private attribute value.

Show this object in a main method that allows the user to interact with all the public methods of your class.

Make a class called MyNumber with an integer private attribute.

Make a constructor that defines an integer parameter to set the private integer attribute.

Make a setter that validates the attribute does not accept a value lower than 2 or the method will throw a IllegalArgumentException.

Make a getter to return the private integer attribute value.

An example done in Java would help in comparing it to mine.

Answer 1

// MyNumber.java
public class MyNumber {
  
   // private field to store the value
   private int value;
  
   // constructor
   public MyNumber(int value)
   {
       // if argument value < 2 throw illegal argument exception
       if(value < 2)
           throw new IllegalArgumentException();
       this.value = value; // else set value to input value
   }
  
   // method to set the value
   public void setValue(int value)
   {
       // if argument value < 2 throw illegal argument exception
       if(value < 2)
           throw new IllegalArgumentException();
       this.value = value;
   }
  
   // method to return the value
   public int getValue()
   {
       return value;
   }
  
   // method to return if n is prime or not using Sieve of Eratosthenes method
   public boolean isPrime(int n)
   {
       // Create a list of consecutive integers from 2 through n: (2, 3, 4, ..., n).
       boolean mark[] = new boolean[n+1];
      
       // set them initially to true
       for(int i=0;i<=n;i++)
           mark[i] = true;
      
       // loop from p=2 till square root of n
       for(int p=2;p*p<=n;p++)
       {
           if(mark[p]) // if mark[p] is true(p is a prime number)
           {
               for(int j=p*2;j<=n;j=j+p) // update the multiples of p as false
                   mark[j]= false;
          
               // if n is not prime return false
               if(!mark[n])
                   return false;
           }
       }
      
       return mark[n]; // return the value stored in mark[n] for the input n
   }
  
   // method to count and return the number of prime numbers between 2 and value using isPrime method
   public int numberOfPrimes()
   {
       int count = 0;
      
       for(int i=2;i<=value;i++)
       {
           if(isPrime(i))
               count++;
       }
      
       return count;
   }

   public static void main(String[] args) {

       // test the class
       MyNumber num = new MyNumber(25);
       System.out.println("Number of prime numbers between 2 and "+num.getValue()+" : "+num.numberOfPrimes());
   }

}

//end of MyNumber.java

Output: