Update Luhn tests and sample implementation

Fixes #245.

Tracking issue: exercism/problem-specifications#491

exercises/luhn/src/example/java/LuhnValidator.java

@@ -0,0 +1,43 @@
+import java.util.ArrayList;
+import java.util.List;
+import java.util.regex.Pattern;
+
+final class LuhnValidator {
+
+    private static final Pattern SPACE_PATTERN = Pattern.compile("\\s+");
+
+    boolean isValid(final String candidate) {
+        final String sanitizedCandidate = SPACE_PATTERN.matcher(candidate).replaceAll("");
+
+        final List<Integer> computedDigits = new ArrayList<>();
+
+        for (int charIndex = 0; charIndex < sanitizedCandidate.length(); charIndex++) {
+            int inputDigit = Character.digit(sanitizedCandidate.charAt(charIndex), 10);
+
+            /*
+             * Character.digit returns a negative int if the supplied character does not represent a digit with respect
+             * to the given radix.
+             */
+            if (inputDigit < 0) {
+                return false;
+            }
+
+            if (charIndex % 2 == 1) {
+                /*
+                 * Since our doubled input digit must lie in [2, 18], the operation
+                 *
+                 *   "subtract 9 from the doubled input digit if it exceeds 9 in value"
+                 *
+                 * is equivalent to applying the modulo operation below universally.
+                 */
+                inputDigit = (2 * inputDigit) % 9;
+            }
+
+            computedDigits.add(inputDigit);
+        }
+
+        final int digitSum = computedDigits.stream().mapToInt(Integer::intValue).sum();
+        return digitSum > 0 && digitSum % 10 == 0;
+    }
+
+}

exercises/luhn/src/test/java/LuhnValidatorTest.java

@@ -0,0 +1,44 @@
+import org.junit.Ignore;
+import org.junit.Test;
+
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+
+public class LuhnValidatorTest {
+
+    @Test
+    public void testThatSingleDigitStringIsNotValid() {
+        assertFalse(new LuhnValidator().isValid("1"));
+    }
+
+    @Ignore
+    @Test
+    public void testThatTheStringConsistingOfASingleZeroIsInvalid() {
+        assertFalse(new LuhnValidator().isValid("0"));
+    }
+
+    @Ignore
+    @Test
+    public void testThatAValidCanadianSocialInsuranceNumberIsIdentifiedAsValid() {
+        assertTrue(new LuhnValidator().isValid("046 454 286"));
+    }
+
+    @Ignore
+    @Test
+    public void testThatAnInvalidCanadianSocialInsuranceNumberIsIdentifiedAsInvalid() {
+        assertFalse(new LuhnValidator().isValid("046 454 287"));
+    }
+
+    @Ignore
+    @Test
+    public void testThatAnInvalidCreditCardIsIdentifiedAsInvalid() {
+        assertFalse(new LuhnValidator().isValid("8273 1232 7352 0569"));
+    }
+
+    @Ignore
+    @Test
+    public void testThatAddingANonDigitCharacterToAValidStringInvalidatesTheString() {
+        assertFalse(new LuhnValidator().isValid("046a 454 286"));
+    }
+
+}