Added better exception for null collection elements; fixes gh-703

spring-cloud-contract-verifier/src/main/java/org/springframework/cloud/contract/verifier/assertion/CollectionAssert.java

@@ -30,14 +30,21 @@ public class CollectionAssert<ELEMENT> extends IterableAssert<ELEMENT> {
 		isNotNull();
 		isNotEmpty();
 		for (Object anActual : this.actual) {
+			if (anActual == null) {
+				failWithMessageRelatedToRegex(regex, anActual);
+			}
 			String value = anActual.toString();
 			if (!value.matches(regex)) {
-				failWithMessage("The value <%s> doesn't match the regex <%s>", value, regex);
+				failWithMessageRelatedToRegex(regex, value);
 			}
 		}
 		return this;
 	}
 
+	private void failWithMessageRelatedToRegex(String regex, Object value) {
+		failWithMessage("The value <%s> doesn't match the regex <%s>", value, regex);
+	}
+
 	/**
 	 * Flattens the collection and checks whether size is greater than or equal to the provided value
 	 * @param size - the flattened collection should have size greater than or equal to this value

spring-cloud-contract-verifier/src/test/groovy/org/springframework/cloud/contract/verifier/assertion/CollectionAssertTests.java

@@ -33,6 +33,18 @@ public class CollectionAssertTests {
 		}
 	}
 
+	@Test
+	public void should_throw_an_exception_when_element_is_null() {
+		Collection collection = collectionWithNulls();
+
+		try {
+			SpringCloudContractAssertions.assertThat(collection).allElementsMatch("[0-9]");
+			Assertions.fail("should throw exception");
+		} catch (AssertionError e) {
+			Assertions.assertThat(e).hasMessageContaining("The value <null> doesn't match the regex <[0-9]>");
+		}
+	}
+
 	@Test
 	public void should_throw_an_exception_when_collection_is_null() {
 		Collection collection = null;
@@ -265,6 +277,12 @@ public class CollectionAssertTests {
 		return list;
 	}
 
+	private Collection<String> collectionWithNulls() {
+		List<String> list = new ArrayList<>();
+		list.add(null);
+		return list;
+	}
+
 	private Collection nestedCollection() {
 		List list = new ArrayList<>();
 		List list1 = new ArrayList<>();