修改 API，使 TreeBuilder 实例可以复用。

src/test/java/xyz/zhouxy/plusone/commons/util/TreeBuilderTests.java

@@ -8,12 +8,17 @@ import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
 import com.google.common.collect.Lists;
+import com.google.gson.Gson;
 
 import lombok.ToString;
 
 class TreeBuilderTests {
 
     private static final Logger log = LoggerFactory.getLogger(TreeBuilderTests.class);
+    private final TreeBuilder<Menu, MenuList, String> treeBuilder = new TreeBuilder<>(
+            Menu::getMenuCode,
+            Menu::getParentMenuCode,
+            MenuList::addChild);
 
     @Test
     void testBuildTree() {
@@ -31,13 +36,10 @@ class TreeBuilderTests {
                 MenuItem.of("C", "C2", "二级菜单C2", "/c/c2"),
                 MenuItem.of("C", "C3", "二级菜单C3", "/c/c3")
         );
-        List<Menu> menuTree = new TreeBuilder<>(
-                menus,
-                Menu::getMenuCode,
-                Menu::getParentMenuCode,
-                MenuList::addChild)
-                .buildTree();
-        log.info("menuTree: {}", menuTree);
+
+        List<Menu> menuTree = treeBuilder.buildTree(menus);
+        log.info("menuTree: {}", new Gson().toJson(menuTree));
+
     }
 }