isbn-verifier: implement exercise in Java track (#1004)

* isbn: implement exercise in Java track

* isbn-verifier: Update reference solution.

* isbn-verifier: Add starter implementation.

* Add @FridaTveit 's improvements

* Add some more improvements

config.json

     {
       "core": false,
       "difficulty": 4,
+      "slug": "isbn-verifier",
+      "topics": [
+        "integers",
+        "loops",
+        "strings"
+      ],
+      "unlocked_by": "hamming",
+      "uuid": "838bc1d7-b2de-482a-9bfc-c881b4ccb04c"
+    },
+    {
+      "core": false,
+      "difficulty": 4,
       "slug": "sum-of-multiples",
       "topics": [
         "arrays",

exercises/isbn-verifier/.meta/src/reference/java/IsbnVerifier.java

+class IsbnVerifier {
+
+    boolean isValid(String stringToVerify) {
+
+        String isbn = stringToVerify.replace("-", "");
+        int total = 0;
+
+        if (isbn.length() != 10) {
+            return false;
+        }
+
+        for (int i = 0; i < isbn.length() - 1; i++) {
+            char currentChar = isbn.charAt(i);
+            if(Character.isDigit(currentChar)) {
+                int currentCharVal = Character.getNumericValue(currentChar);
+                total += currentCharVal * (10 - i);
+            } else {
+                return false;
+            }
+        }
+
+        char finalChar = isbn.charAt(isbn.length() - 1);
+        if (Character.isDigit(finalChar)) {
+            total += Character.getNumericValue(finalChar);
+        } else if (finalChar == 'X') {
+            total += 10;
+        } else {
+            return false;
+        }
+
+        return total % 11 == 0;
+
+    }
+
+}

exercises/isbn-verifier/.meta/version

+2.0.0

exercises/isbn-verifier/README.md

+Check if a given ISBN-10 is valid.
+
+## Functionality
+
+Given an unknown 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 allow for ISBN-10 without the separating dashes to be verified as well.
+
+## ISBN
+
+Let's take a random ISBN-10 number, say `3-598-21508-8` for this.
+The first digit block indicates the group where the ISBN belongs. Groups can consist of shared languages, geographic regions or countries. The leading '3' signals this ISBN is from a german speaking country.
+The following number block is to identify the publisher. Since this is a three digit publisher number there is a 5 digit title number for this book.
+The last digit in the ISBN is the check digit which is used to detect read errors.
+
+The first 9 digits in the ISBN have to be between 0 and 9.
+The check digit can additionally be an 'X' to allow 10 to be a valid check digit as well.
+
+A valid ISBN-10 is calculated with this formula `(x1 * 10 + x2 * 9 + x3 * 8 + x4 * 7 + x5 * 6 + x6 * 5 + x7 * 4 + x8 * 3 + x9 * 2 + x10 * 1) mod 11 == 0`
+So for our example ISBN this means:
+(3 * 10 + 5 * 9 + 9 * 8 + 8 * 7 + 2 * 6 + 1 * 5 + 5 * 4 + 0 * 3 + 8 * 2 + 8 * 1) mod 11 = 0
+
+Which proves that the ISBN is valid.
+
+## Caveats
+
+Converting from string to number can be tricky in certain languages.
+It's getting even trickier since the check-digit of an ISBN-10 can be 'X'.
+
+## 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
+
+# Running the tests
+
+You can run all the tests for an exercise by entering
+
+```sh
+$ gradle test
+```
+
+in your terminal.
+
+## 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](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.

exercises/isbn-verifier/build.gradle

+apply plugin: "java"
+apply plugin: "eclipse"
+apply plugin: "idea"
+
+repositories {
+    mavenCentral()
+}
+
+dependencies {
+    testCompile "junit:junit:4.12"
+}
+
+test {
+    testLogging {
+        exceptionFormat = 'full'
+        events = ["passed", "failed", "skipped"]
+    }
+}

exercises/isbn-verifier/src/main/java/IsbnVerifier.java

+class IsbnVerifier {
+
+    boolean isValid(String stringToVerify) {
+        throw new UnsupportedOperationException("Delete this statement and write your own implementation.");
+    }
+
+}

exercises/isbn-verifier/src/test/java/IsbnVerifierTest.java

+import org.junit.Assert;
+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("3-598-2K507-0"));
+    }
+
+    @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 isbnWithoutCheckDigit() {
+        assertFalse(isbnVerifier.isValid("3-598-21507"));
+    }
+
+    @Ignore("Remove to run test")
+    @Test
+    public void tooLongIsbn() {
+        assertFalse(isbnVerifier.isValid("3-598-21507-XX"));
+    }
+
+    @Ignore("Remove to run test")
+    @Test
+    public void checkDigitOfXShouldNotBeUsedForZero() {
+        assertFalse(isbnVerifier.isValid("3-598-21515-X"));
+    }
+
+}

exercises/settings.gradle

 include 'hexadecimal'
 include 'hello-world'
 include 'house'
+include 'isbn-verifier'
 include 'isogram'
 include 'kindergarten-garden'
 include 'largest-series-product'