Support negative collection index in spec

src/main/java/com/github/leeonky/jfactory/CollectionProducer.java

@@ -38,18 +38,21 @@ protected C produce() {
     @Override
     public Optional<Producer<?>> child(String property) {
         int index = valueOf(property);
-        if (index < 0)
-            index = children.size() + index;
+        index = transformNegativeIndex(index);
         return Optional.ofNullable(index < children.size() ? children.get(index) : null);
     }
 
     @Override
     public void setChild(String property, Producer<?> producer) {
         int index = valueOf(property);
         fillCollectionWithDefaultValue(index);
+        children.set(transformNegativeIndex(index), producer);
+    }
+
+    private int transformNegativeIndex(int index) {
         if (index < 0)
             index = children.size() + index;
-        children.set(index, producer);
+        return index;
     }
 
     public int fillCollectionWithDefaultValue(int index) {
@@ -69,7 +72,7 @@ public int fillCollectionWithDefaultValue(int index) {
     public Producer<?> childOrDefault(String property) {
         int index = valueOf(property);
         fillCollectionWithDefaultValue(index);
-        return children.get(index);
+        return children.get(transformNegativeIndex(index));
     }
 
     @Override

src/test/resources/features/4-spec-property.feature

@@ -899,6 +899,38 @@ Feature: define spec
       ]
       """
 
+    Scenario: support negative index in spec
+      Given the following bean class:
+      """
+      public class Bean {
+        public String value;
+        public Bean setValue(String v) {
+          this.value = v;
+          return this;
+        }
+      }
+      """
+      And the following bean class:
+      """
+      public class Beans {
+        public Bean[] beans;
+      }
+      """
+      When register:
+      """
+      jFactory.factory(Beans.class).spec(instance -> instance.spec()
+        .property("beans[0]").value(new Bean().setValue("world"))
+        .property("beans[-2]").value(new Bean().setValue("hello")));
+      """
+      When build:
+      """
+      jFactory.create(Beans.class);
+      """
+      Then the result should:
+      """
+      beans.value[]= [hello world]
+      """
+
 #    Scenario: collection element property should override collection spec
     Scenario: to be implement
       Given the following bean class:
@@ -935,7 +967,7 @@ Feature: define spec
       """
       class.simpleName: IllegalArgumentException
       """
-#      TODO
+#      TODO ?
 #      Then the result should:
 #      """
 #      beans.value[]: [ hello ]