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# coma123's solution

## to ISBN Verifier in the Java Track

Published at Feb 12 2021 · 0 comments
Instructions
Test suite
Solution

The ISBN-10 verification process is used to validate book identification numbers. These normally contain dashes and look like: 3-598-21508-8

## ISBN

The ISBN-10 format is 9 digits (0 to 9) plus one check character (either a digit or an X only). In the case the check character is an X, this represents the value '10'. These may be communicated with or without hyphens, and can be checked for their validity by the following formula:

(x1 * 10 + x2 * 9 + x3 * 8 + x4 * 7 + x5 * 6 + x6 * 5 + x7 * 4 + x8 * 3 + x9 * 2 + x10 * 1) mod 11 == 0

If the result is 0, then it is a valid ISBN-10, otherwise it is invalid.

## Example

Let's take the ISBN-10 3-598-21508-8. We plug it in to the formula, and get:

(3 * 10 + 5 * 9 + 9 * 8 + 8 * 7 + 2 * 6 + 1 * 5 + 5 * 4 + 0 * 3 + 8 * 2 + 8 * 1) mod 11 == 0

Since the result is 0, this proves that our ISBN is valid.

## Task

Given a string the program should check if the provided string is a valid ISBN-10. Putting this into place requires some thinking about preprocessing/parsing of the string prior to calculating the check digit for the ISBN.

The program should be able to verify ISBN-10 both with and without separating dashes.

## Caveats

Converting from strings to numbers can be tricky in certain languages. Now, it's even trickier since the check digit of an ISBN-10 may be 'X' (representing '10'). For instance 3-598-21507-X is a valid ISBN-10.

## Bonus tasks

• Generate a valid ISBN-13 from the input ISBN-10 (and maybe verify it again with a derived verifier).

• Generate valid ISBN, maybe even from a given starting ISBN.

## Setup

Go through the setup instructions for Java to install the necessary dependencies:

https://exercism.io/tracks/java/installation

# Running the tests

You can run all the tests for an exercise by entering the following in your terminal:

\$ gradle test

In the test suites all tests but the first have been skipped.

Once you get a test passing, you can enable the next one by removing the @Ignore("Remove to run test") annotation.

## Source

Converting a string into a number and some basic processing utilizing a relatable real world example. https://en.wikipedia.org/wiki/International_Standard_Book_Number#ISBN-10_check_digit_calculation

## Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

### IsbnVerifierTest.java

import org.junit.Before;
import org.junit.Ignore;
import org.junit.Test;

import static org.junit.Assert.assertTrue;
import static org.junit.Assert.assertFalse;

public class IsbnVerifierTest {
private IsbnVerifier isbnVerifier;

@Before
public void setUp() {
isbnVerifier = new IsbnVerifier();
}

@Test
public void validIsbnNumber() {
assertTrue(isbnVerifier.isValid("3-598-21508-8"));
}

@Ignore("Remove to run test")
@Test
public void invalidIsbnCheckDigit() {
assertFalse(isbnVerifier.isValid("3-598-21508-9"));
}

@Ignore("Remove to run test")
@Test
public void validIsbnNumberWithCheckDigitOfTen() {
assertTrue(isbnVerifier.isValid("3-598-21507-X"));
}

@Ignore("Remove to run test")
@Test
public void checkDigitIsACharacterOtherThanX() {
assertFalse(isbnVerifier.isValid("3-598-21507-A"));
}

@Ignore("Remove to run test")
@Test
public void invalidCharacterInIsbn() {
assertFalse(isbnVerifier.isValid("3-598-P1581-X"));
}

@Ignore("Remove to run test")
@Test
public void xIsOnlyValidAsACheckDigit() {
assertFalse(isbnVerifier.isValid("3-598-2X507-9"));
}

@Ignore("Remove to run test")
@Test
public void validIsbnWithoutSeparatingDashes() {
assertTrue(isbnVerifier.isValid("3598215088"));
}

@Ignore("Remove to run test")
@Test
public void isbnWithoutSeparatingDashesAndXAsCheckDigit() {
assertTrue(isbnVerifier.isValid("359821507X"));
}

@Ignore("Remove to run test")
@Test
public void isbnWithoutCheckDigitAndDashes() {
assertFalse(isbnVerifier.isValid("359821507"));
}

@Ignore("Remove to run test")
@Test
public void tooLongIsbnAndNoDashes() {
assertFalse(isbnVerifier.isValid("3598215078X"));
}

@Ignore("Remove to run test")
@Test
public void tooShortIsbn() {
assertFalse(isbnVerifier.isValid("00"));
}

@Ignore("Remove to run test")
@Test
public void isbnWithoutCheckDigit() {
assertFalse(isbnVerifier.isValid("3-598-21507"));
}

@Ignore("Remove to run test")
@Test
public void checkDigitOfXShouldNotBeUsedForZero() {
assertFalse(isbnVerifier.isValid("3-598-21515-X"));
}

@Ignore("Remove to run test")
@Test
public void emptyIsbn() {
assertFalse(isbnVerifier.isValid(""));
}

@Ignore("Remove to run test")
@Test
public void inputIsNineCharacters() {
assertFalse(isbnVerifier.isValid("134456729"));
}

@Ignore("Remove to run test")
@Test
public void invalidCharactersAreNotIgnored() {
assertFalse(isbnVerifier.isValid("3132P34035"));
}

@Ignore("Remove to run test")
@Test
public void inputIsTooLongButContainsAValidIsbn() {
assertFalse(isbnVerifier.isValid("98245726788"));
}
}
class IsbnVerifier {

boolean isValid(String stringToVerify) {
stringToVerify = stringToVerify.replaceAll("-", "");

if(stringToVerify.length() != 10) {
return false;
}

String[] arrStringToVerify = stringToVerify.split("");

int result = 0;
int numToMultiply = 1;

for(int i = 9; i >= 0; i--) {
Integer n = getAsInteger(arrStringToVerify[i]);

if(i == 9 && arrStringToVerify[i].equals("X")) {
n = 10;
}

if(n == null) {
return false;
}
result += n * numToMultiply;
numToMultiply++;
}

return result % 11 == 0;
}

private Integer getAsInteger(String s) {
try {
return Integer.parseInt(s);
} catch (NumberFormatException e) {
return null;
}
}

}

## Community comments

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### What can you learn from this solution?

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