Fix NPE in ReflectionUtils when scaning interfaces

Author: vladimirfx

modules/swagger-core/src/main/java/io/swagger/util/ReflectionUtils.java

@@ -145,7 +145,7 @@ public static boolean isConstructorCompatible(Constructor<?> constructor) {
      * @return list of Fields
      */
     public static List<Field> getDeclaredFields(Class<?> cls) {
-        if (cls.equals(Object.class)) {
+        if (cls == null || Object.class.equals(cls)) {
             return Collections.emptyList();
         }
         final List<Field> fields = new ArrayList<Field>();

modules/swagger-core/src/test/java/io/swagger/ReflectionUtilsTest.java

@@ -3,6 +3,7 @@
 import io.swagger.annotations.ApiOperation;
 import io.swagger.annotations.ApiResponses;
 import io.swagger.reflection.Child;
+import io.swagger.reflection.IParent;
 import io.swagger.reflection.Parent;
 import io.swagger.util.ReflectionUtils;
 
@@ -12,6 +13,7 @@
 import java.lang.reflect.Method;
 import java.lang.reflect.Type;
 import java.util.Arrays;
+import java.util.Collections;
 
 import javax.ws.rs.Path;
 
@@ -114,4 +116,10 @@ public void testDerivedAnnotation() {
         Assert.assertNotNull(annotation);
         Assert.assertEquals(annotation.value(), "parentInterfacePath");
     }
+
+    @Test
+    public void getDeclaredFieldsFromInterfaceTest() throws NoSuchMethodException {
+        final Class cls = IParent.class;
+        Assert.assertEquals(Collections.emptyList(), ReflectionUtils.getDeclaredFields(cls));
+    }
 }