Added isNumber Method to Administration class

src/main/java/org/example/Administration.java

@@ -208,6 +208,22 @@ public class Administration {
         }
     }
 
+    public boolean isNumber(String substring)
+    {
+        if(substring == null || substring.isEmpty())
+        {
+            return false;
+        }
+        for(char c : substring.toCharArray())
+        {
+            if(!Character.isDigit(c))
+            {
+                return false;
+            }
+        }
+        return true;
+    }
+
 }