primenumbers.java

输入一个自然数,然后找出所有小于它的素数,并打印出来

package example;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.ArrayList;
import java.util.List;

/**
 * Finds all the prime numbers within a range.
 * 
 */
public class PrimeNumbers {

	public static void main(String[] args) throws IOException {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		System.out.print("Please input the upper bound of the range: ");
		String input = br.readLine();
		if (input != null) {
			try {
				int max = Integer.parseInt(input);
				List<Integer> primeNumbers = findPrimeNumbers(max);
				if (primeNumbers != null && primeNumbers.size() != 0) {
					System.out.println("Found " + primeNumbers.size()
							+ " prime number(s) in this range, which are:");
					for (Integer num : primeNumbers) {
						System.out.print(num + " ");
					}
				} else {
					System.out.println("No prime numbers found.");
				}
				System.out.println();
			} catch (NumberFormatException e) {
				System.err.println("Did you input an integer? Exiting.");
			}
		} else {
			System.out.println("No input is given. Exiting.");
		}
	}

	private static List<Integer> findPrimeNumbers(int range) {
		ArrayList<Integer> primeNumbers =new  ArrayList<Integer>( );
		if(range==1){
		return null;
			}
		if(range==2){
			 primeNumbers.add(range);
		return  primeNumbers ; 
		}
		else{
			primeNumbers.add(2);
		  for (int i = 3; i <=range; i++) {
		    for (int j = 2; j <i; j++) {
		    	if(i%j==0){
		    		break;
		    	}
		    	else{
		    		if(j==i-1){
		            primeNumbers.add(i);
		          }
		        }
			}	
	    } 
		return  primeNumbers ;
		}
	}

}