Handle null arguments when throwing DeserializationException

Fixes #164

CHANGELOG.md

@@ -1,6 +1,13 @@
 CHANGELOG
 =========
 
+3.1.1
+------------------
+
+* When handling a deserialization exception, the decoder now avoids
+  throwing a `NullPointerException` when one of the constructor arguments
+  is `null`. Reported by Keith Massey. GitHub #164.
+
 3.1.0 (2023-12-05)
 ------------------
 

src/main/java/com/maxmind/db/Decoder.java

@@ -447,7 +447,8 @@ private <T> Object decodeMapIntoObject(int size, Class<T> cls)
             StringBuilder sbErrors = new StringBuilder();
             for (String key : parameterIndexes.keySet()) {
                 int index = parameterIndexes.get(key);
-                if (!parameters[index].getClass().isAssignableFrom(parameterTypes[index])) {
+                if (parameters[index] != null &&
+                    !parameters[index].getClass().isAssignableFrom(parameterTypes[index])) {
                     sbErrors.append(" argument type mismatch in " + key + " MMDB Type: "
                         + parameters[index].getClass().getCanonicalName()
                         + " Java Type: " + parameterTypes[index].getCanonicalName());

src/test/java/com/maxmind/db/ReaderTest.java

@@ -944,6 +944,31 @@ public void testDecodeWithDataTypeMismatchInModel() throws IOException {
         assertThat(ex.getCause().getCause().getClass(), equalTo(ClassCastException.class));
     }
 
+
+    static class TestConstructorMismatchModel {
+        @MaxMindDbConstructor
+        public TestConstructorMismatchModel(
+            @MaxMindDbParameter(name = "other")
+            String other,
+            @MaxMindDbParameter(name = "utf8_string")
+            double utf8StringField
+        ) {
+        }
+    }
+
+    @Test
+    public void testDecodeWithDataTypeMismatchInModelAndNullValue() throws IOException {
+        this.testReader = new Reader(getFile("MaxMind-DB-test-decoder.mmdb"));
+
+        DeserializationException ex = assertThrows(DeserializationException.class,
+            () -> this.testReader.get(
+                InetAddress.getByName("::1.1.1.0"),
+                TestConstructorMismatchModel.class));
+
+        assertThat(ex.getMessage(), containsString("Error creating object of type"));
+        assertThat(ex.getCause().getCause().getClass(), equalTo(IllegalArgumentException.class));
+    }
+
     static class TestWrongModelSubdivisions {
         List<TestWrongModelSubdivision> subdivisions;