added TreeNode

bean/src/main/java/io/github/mmm/entity/bean/TreeNode.java

@@ -0,0 +1,23 @@
+/* Copyright (c) The m-m-m Team, Licensed under the Apache License, Version 2.0
+ * http://www.apache.org/licenses/LICENSE-2.0 */
+package io.github.mmm.entity.bean;
+
+import io.github.mmm.bean.AbstractInterface;
+import io.github.mmm.entity.property.link.LinkProperty;
+
+/**
+ * {@link EntityBean} that represents a node in a tree. The tree hierarchy is defined by the {@link #Parent() parent}
+ * relation.
+ *
+ * @param <T> type of the tree-nodes.
+ * @since 1.0.0
+ */
+@AbstractInterface
+public abstract interface TreeNode<T extends TreeNode<T>> extends EntityBean {
+
+  /**
+   * @return the parent node or {@code null} if this is the root node.
+   */
+  LinkProperty<T> Parent();
+
+}