implementazione di linked tree. manca il tostring

exceptions/NonEmptyTreeException.java

@@ -0,0 +1,18 @@
+package exceptions;
+
+/**
+ * Signals that the boundaries of a data structure have been illegally
+ * traversed (e.g. past the end of a list).
+ * @author Roberto Tamassia
+ */
+//Copyright (c) 2003 Brown University, Providence, RI
+//Additional modifications and methods by xgiovio
+
+public class NonEmptyTreeException extends RuntimeException {
+  public NonEmptyTreeException(String message) {
+    super (message);
+  }
+    public NonEmptyTreeException() {
+        super ("Tree already has a root");
+    }
+}