Record element type of first element in LazyDynaList.transform

src/main/java/org/apache/commons/beanutils2/LazyDynaList.java

@@ -659,7 +659,18 @@ private DynaBean transform(final Object element) {
 
         // Check the new element type, matches all the
         // other elements in the List
-        if (elementType != null && !newElementType.equals(elementType)) {
+        if (elementType == null) {
+            // The first element populated defines the element type (see class
+            // Javadoc). Record it so later elements are type-checked and
+            // toArray()/toDynaBeanArray() know the element type.
+            this.elementType = newElementType;
+            this.elementDynaBeanType = newDynaBeanType;
+            if (WrapDynaBean.class.isAssignableFrom(newDynaBeanType)) {
+                this.wrapDynaClass = (WrapDynaClass) dynaBean.getDynaClass();
+            } else {
+                this.elementDynaClass = dynaBean.getDynaClass();
+            }
+        } else if (!newElementType.equals(elementType)) {
             throw new IllegalArgumentException("Element Type " + newElementType + " doesn't match other elements " + elementType);
         }
 

src/test/java/org/apache/commons/beanutils2/LazyDynaListTest.java

@@ -374,6 +374,29 @@ void testNullType() {
         lazyList.add(new HashMap<>());
     }
 
+    /**
+     * Test that the element type is set from the first element populated on an untyped List: a later
+     * element of a different type is rejected, and toArray() returns an array of the element type.
+     */
+    @Test
+    void testFirstElementSetsElementType() {
+        final LazyDynaList lazyList = new LazyDynaList();
+        lazyList.add(new HashMap<>());
+        assertThrows(IllegalArgumentException.class, () -> lazyList.add("a POJO String"),
+                "Element of a different type must be rejected once the type is set");
+
+        final TreeMap<String, Object>[] source = new TreeMap[2];
+        source[0] = new TreeMap<>();
+        source[0].put("key0", "val0");
+        source[1] = new TreeMap<>();
+        source[1].put("key1", "val1");
+        final LazyDynaList mapList = new LazyDynaList(source);
+        final TreeMap<?, ?>[] array = (TreeMap[]) mapList.toArray();
+        assertEquals(2, array.length);
+        assertEquals("val0", array[0].get("key0"));
+        assertEquals("val1", array[1].get("key1"));
+    }
+
     /**
      * Test Pojo Create
      */