program to print 1-100 prime number and throw exception for composite number in given range

问题

i have made a program to print 1-100 prime numbers. please help me to throw exception for composite number in range of 1-100 numbers. i am a beginner so any help will be appreciated.

  public static void main(String[] args) {

    System.out.println("Prime numbers from 1 - 100 are :");
    int i = 0;
    int x = 0;

    for (i = 1; i <= 100; i++) {

        int ctr = 0;
        for (x = i; x >= 1; x--) {
            if (i % x == 0) {
                ctr = ctr + 1;
            }
        }
        if (ctr == 2) {
            System.out.println(i);
        }

    }

}

回答1:

I'd rather implement isPrime method and call it

public static boolean isPrime(int value) {
  if (value <= 1)
    return false;

  // There's only one even prime: that is two 
  if ((value % 2) == 0) 
    return (value == 2);

  int from = (int) (Math.sqrt(value) + 1);

  // You have to check possible divisors from 3 to sqrt(value)
  for (int i = 3; i <= from; i += 2)
    if ((value % i) == 0)
      return false;

  return true;
}

public static void main(String[] args) {
  ...

  for (int i = 1; i <= 100; ++i) {
    if (isPrime(i)) 
      System.out.println(i);
    else {
      // i is not prime. You can do nothing, throw an exception etc
      // throw new MyException("Not a prime"); 
    }
  }
}

回答2:

You should add an else clause to if (ctr == 2) { and throw an exception in it. Have a look at the documentation on how to throw an exception.

回答3:

put one more condition like. else if(ctr!=2){ throw new CompositeException("composite exception occurs");}

回答4:

public void primeNumber(int n1, int n2){

    String primeNo= "";
    System.out.print("Prime number between " + n1 +" and "+ n2 + "is/are - ");
    for(int i = n1;i <=n2; i++)
    {
        int count = 0;
        for(int j=1; j<=i; j++)
        {

           if(i%j==0)
           {
               count = count + 1;
           }

        }
        if(count == 2) {
            primeNo = primeNo + i + ", ";
        }

    }

System.out.print(primeNo);

}

来源：https://stackoverflow.com/questions/20840396/program-to-print-1-100-prime-number-and-throw-exception-for-composite-number-in