fix: simplify children() with safe casting and null handling

Author: ADITYA-KUMAR-2358

abstract-document/src/main/java/com/iluwatar/abstractdocument/AbstractDocument.java

@@ -24,7 +24,6 @@
  */
 package com.iluwatar.abstractdocument;
 
-import java.util.Collection;
 import java.util.List;
 import java.util.Map;
 import java.util.Objects;
@@ -52,16 +51,19 @@ public Object get(String key) {
     return documentProperties.get(key);
   }
 
-  @Override
-  public <T> Stream<T> children(String key, Function<Map<String, Object>, T> childConstructor) {
-    return Stream.ofNullable(get(key))
-        .filter(Objects::nonNull)
-        .map(el -> (List<Map<String, Object>>) el)
-        .findAny()
-        .stream()
-        .flatMap(Collection::stream)
+@SuppressWarnings("unchecked")
+@Override
+public <T> Stream<T> children(String key, Function<Map<String, Object>, T> childConstructor) {
+  Object value = get(key);
+  if (value instanceof List<?>) {
+    return ((List<?>) value).stream()
+        .filter(e -> e instanceof Map) // Only keeping items that are actually maps before casting
+        .map(e -> (Map<String, Object>) e)
         .map(childConstructor);
   }
+  return Stream.empty();
+}
+
 
   @Override
   public String toString() {