EAN13.java

import java.math.BigInteger;
import java.security.InvalidAlgorithmParameterException;
import java.util.Scanner;
/**
 * @version 1
 * @author ChloeWake
 *
 */
public class EAN13 {
	public static void main (String[] args){
		
		String input = GetInput(); //get input from the user
		
		if (input.length() < 12) { //check to see if the input is too short
			try {
				throw new InvalidAlgorithmParameterException("That's not a long enough barcode! Give me the rest of the numbers!");
			} catch (InvalidAlgorithmParameterException e) {
				e.printStackTrace(); 
			    return;
			}
		}
		if (input.length() > 12 ) { //check to see if the input is too long
			try{
				throw new InvalidAlgorithmParameterException("That's too long! You only need 12 numbers!");
			} catch (InvalidAlgorithmParameterException e){
				e.printStackTrace(); 
			    return;
			}

		}
		int ans = checkSum(input); //pass that input to the checkSum function
		System.out.println(ans); //print out the checksum digit
		
		/**
		 * 
		 */
	}
	
	public static String GetInput(){
		Scanner console = new Scanner(System.in);
		System.out.println("This program will take the first 12 numbers of a EAN13 barcode and compute the check number at the end of the code.");
		System.out.println("Please enter a EAN13 code: ");
		String EanCode = console.next();
		return EanCode;
	}
	
	public static int checkSum (String Input) {
		int evens = 0; //initialize evens variable
		int odds = 0; //initialize odds variable
		int checkSum = 0; //initialize the checkSum
		for (int i = 0; i < Input.length(); i++) {
			//check if number is odd or even
			if ((int)Input.charAt(i) % 2 == 0) { // check that the character at position "i" is divisible by 2 which means it's even
				evens += (int)Input.charAt(i);// then add it to the evens
			} else {
				odds += (int)Input.charAt(i); // else add it to the odds
			}
		}
		odds = odds * 3; //multiply odds by three
		int total = odds + evens; //sum odds and evens
		if (total % 10 == 0){ //if total is divisible by ten, special case
			checkSum = 0;//checksum is zero
		} else { //total is not divisible by ten
			checkSum = 10 - (total % 10); //subtract the ones digit from 10 to find the checksum
		}
		return checkSum;
	}
}
		
	
	

I think this is one of method
private static int checkSum(String input) {
int evens = 0; //initialize evens variable
int odds = 0; //initialize odds variable
int checkSum = 0; //initialize the checkSum
for (int i=0;i<12; i++) {// fixed because it is fixed in pratices but you can use length() insted
int digit = Integer.parseInt(input.substring(i,i+1));
if (i%2==0) {
evens += digit;// then add it to the evens
}else {
odds += digit; // else add it to the odds
}
}
odds = odds * 3; //multiply odds by three
int total = odds + evens; //sum odds and evens
if (total % 10 == 0){ //if total is divisible by ten, special case
checkSum = 0;//checksum is zero
} else { //total is not divisible by ten
checkSum = 10 - (total % 10); //subtract the ones digit from 10 to find the checksum
}
return checkSum;
}

Here is a workable code from Apache Commons Validator:
https://gist.github.com/Rody66/3ae9898fbac690407c5e6b975fb9f15c
Use it like this: digit = EAN13CheckDigit.EAN13_CHECK_DIGIT.calculate(ean)