primes_within_range.java

import java.util.ArrayList;
import java.util.Scanner;
	public class primes_within_range {public static void main(String[] args) {
		primes();
	}
	public static void primes() {
		//open input scanner
		Scanner in = new Scanner(System.in);
		//Defensive program for selection between range and amount
		System.out.println("Do you need the primes within a range, or an amount from a starting position ( '1' for range or '2' for amount?");
		boolean isGoodInput = false;
		int choice = 0;
		while(!isGoodInput){
			try {
				choice = in.nextInt();
				if (choice == 1 || choice == 2){
					isGoodInput=true;
				} else {
					System.out.println("Invalid input for choice. Please try again:");
					continue;	
				}
			} catch (java.util.InputMismatchException e) {
				System.out.println("Invalid input for choice. Please try again:");
				in.next();
				continue;
			}
		}
		if (choice == 1){
			//Defensive program for selection of range
			System.out.println("Where should the primes start from (pick a natural number larger than 1)?");
			long min = 2;
			long max = 2;
			try {
				min = in.nextLong();
				if (min<2) {
					System.out.println("Invalid input for value from minimum prime.");
					System.out.println("Minimum prime set to 2.");
					min = 2;
				}
			} catch (java.util.InputMismatchException e) {
				System.out.println("Invalid input for value from minimum prime.");
				System.out.println("Minimum prime set to 2.");
				min = 2;
			}
			System.out.println("Where should the primes go up to (pick a natural number larger than your minimum)?");
			//ensure max>min
			try {
				max = in.nextLong();
				if (max<min) {
					System.out.println("Invalid input for value from maximum prime.");
					System.out.println("Minimum prime set to minimum.");
					max = min;
				}
			} catch (java.util.InputMismatchException e) {
				System.out.println("Invalid input for value from maximum prime.");
				System.out.println("Minimum prime set to minimum.");
				max = min;
			}
			//Close scanner
			in.close();
			//Creation of prime list
			long numPrimesFound = 0;
			ArrayList<Long> primes = new ArrayList<Long>();
			boolean isPrime;
			//check factors up to square root of checked number
			for (long index = min; index<=max; index++) {
				isPrime = true;
				
				long maxFactorOfIndex = (long)Math.sqrt(index);
				long factorToCheck = maxFactorOfIndex;
				while (factorToCheck>1) {
					if (index%factorToCheck == 0) {
						isPrime = false;
						break;
					} else {
						factorToCheck--;
					}	
				}
				//add to list of primes if while loop exited naturally
				if (isPrime) {
					numPrimesFound++;
					primes.add(index);
				}	
			}
			//print results
			if (numPrimesFound==0) {
				System.out.printf("No primes found in the range from %d to %d.", min, max);
			} else {
				System.out.printf("The following primes in the range from %d to %d are:\n", min, max);
				for (int idx = 0; idx < primes.size(); idx++) {
					System.out.println(primes.get(idx) + ", ");
				}
			}
		} else if (choice == 2) {
			//Defensive program for selection of range
			System.out.println("Where should the primes start from (pick a natural number larger than 1)?");
			long start = 2;
			long amount = 1;
			//need a valid start point
			try {
				start = in.nextLong();
				if (start<2) {
					System.out.println("Invalid input for starting value.");
					System.out.println("Start point set to 2.");
					start = 2;
				}
				//catching input exceptions
			} catch (java.util.InputMismatchException e) {
				System.out.println("Invalid input for starting value.");
				System.out.println("Start point set to 2.");
				start = 2;
			}
			System.out.println("How many primes do you want (pick a natural number)?");
			//ensure valid amount
			try {
				amount = in.nextLong();
				if (amount<1) {
					System.out.println("Invalid input for value of amount.");
					System.out.println("Amount set to 1.");
					amount=1;
				}
			} catch (java.util.InputMismatchException e) {
				System.out.println("Invalid input for value of amount.");
				System.out.println("Amount set to 1.");
				amount=1;
			
			}
			//Close scanner
			in.close();
			long numPrimesFound = 0;
			ArrayList<Long> primes = new ArrayList<Long>();
			boolean isPrime;
			//check factors up to square root of checked number
			long index = start;
			while (numPrimesFound < amount){
				isPrime = true;
				long maxFactorOfIndex = (long)Math.sqrt(index);
				long factorToCheck = maxFactorOfIndex;
				while (factorToCheck>1) {
					if (index%factorToCheck == 0) {
						isPrime = false;
						break;
					} else {
						factorToCheck--;
					}	
				}
				//add to list of primes if while loop exited naturally
				if (isPrime) {
					numPrimesFound++;
					primes.add(index);
				}
				index++;	
			}
			//print results
			System.out.printf("The first %d primes starting from %d are:\n", amount, start);
			for (int idx = 0; idx < primes.size(); idx++) {
				System.out.println(primes.get(idx) + ", ");
			}
			
		}

	}
}