Update Luhn tests and sample implementation

Fixes #245.

Tracking issue: exercism/problem-specifications#491

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

@@ -0,0 +1,40 @@
+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.getNumericValue(sanitizedCandidate.charAt(charIndex));
+
+            // Character.getNumericValue returns a negative int if the supplied character does not represent a digit.
+            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 +1,39 @@
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.Parameterized;
+
+import java.util.Arrays;
+import java.util.Collection;
+
+import static org.junit.Assert.assertEquals;
+
+@RunWith(Parameterized.class)
+public class LuhnValidatorTest {
+
+    @Parameterized.Parameters(name="When checking if {0} is valid, the answer should be {1}")
+    public static Collection<Object[]> data() {
+        return Arrays.asList(new Object[][]{
+                {"1", false},
+                {"0", false},
+                {"046 454 286", true},
+                {"046 454 287", false},
+                {"8273 1232 7352 0569", false},
+                {"046a 454 286", false},
+        });
+    }
+
+    private final String candidate;
+
+    private final boolean expectedToBeValid;
+
+    public LuhnValidatorTest(final String candidate, final boolean expectedToBeValid) {
+        this.candidate = candidate;
+        this.expectedToBeValid = expectedToBeValid;
+    }
+
+    @Test
+    public void test() {
+        assertEquals(expectedToBeValid, new LuhnValidator().isValid(candidate));
+    }
+
+}