fix:[uno-core] data-structure tree sentinel element

Author: ClearXs

uno-core/src/main/java/cc/allio/uno/core/datastructure/tree/ComparableElement.java

@@ -3,6 +3,13 @@
 import java.io.Serializable;
 import java.util.Comparator;
 
+/**
+ * comparable tree element base on {@link Comparator}
+ *
+ * @author j.x
+ * @date 2023/4/11 22:21
+ * @since 1.1.4
+ */
 public class ComparableElement<T extends ComparableElement<T>> extends DefaultElement<T> {
 
     private final Comparator<T> comparator;
@@ -16,7 +23,12 @@ public ComparableElement(Serializable id, Comparator<T> comparator) {
     public void addChildren(T element) {
         super.addChildren(element);
         if (comparator != null) {
-            getChildren().sort((o1, o2) -> comparator.compare((T) o1, (T) o2));
+            getChildren().sort(comparator);
         }
     }
+
+    @Override
+    public Element obtainSentinel() {
+        return new ComparableElement(-1, comparator);
+    }
 }

uno-core/src/main/java/cc/allio/uno/core/datastructure/tree/Element.java

@@ -157,7 +157,7 @@ default void accept(Visitor<T> visitor) {
     /**
      * 获取Sentinel结点
      */
-    static Element getRootSentinel() {
+    default Element obtainSentinel() {
         return new DefaultElement<>(-1, -1);
     }
 }

uno-core/src/main/java/cc/allio/uno/core/datastructure/tree/TreeSupport.java

@@ -88,15 +88,22 @@ public static <R extends Element<R>> List<R> adjust(List<R> elements) {
             }
         }
 
-        Element sentinel = Element.getRootSentinel();
-        for (R virtual : idElement.values()) {
-            if (virtual.getDepth() == Element.ROOT_NODE) {
-                // 触发Element添加结点的特性，如排序
-                sentinel.addChildren(virtual);
-            }
-        }
+        // base on first element (if empty then return empty list) get sentinel element then traversal each element and add children (maybe trigger element feature, like as sort)
+        return idElement.values()
+                .stream()
+                .findFirst()
+                .map(Element::obtainSentinel)
+                .map(sentinel -> {
+                    for (R virtual : idElement.values()) {
+                        if (virtual.getDepth() == Element.ROOT_NODE) {
+                            // 触发Element添加结点的特性，如排序
+                            sentinel.addChildren(virtual);
+                        }
+                    }
+                    return sentinel.getChildren();
+                })
+                .orElse(Collections.emptyList());
 
-        return sentinel.getChildren();
     }
 
     /**