Break code into multiple files.

lib/models/expense.dart

@@ -0,0 +1,18 @@
+import '/models/frequency.dart';
+
+class Expense {
+  final String name;
+  final double cost;
+  final Frequency frequency;
+  final String description;
+
+  const Expense(
+      {required this.name,
+      required this.cost,
+      required this.frequency,
+      required this.description});
+
+  String toString() {
+    return "$name, $cost, ${frequency.title}, $description";
+  }
+}

lib/models/frequency.dart

@@ -0,0 +1,12 @@
+// https://www.tutorialspoint.com/dart_programming/dart_programming_enumeration.htm
+enum Frequency {
+  daily(title: "Daily"),
+  weekly(title: "Weekly"),
+  biweekly(title: "Biweekly"),
+  montly(title: "Monthly"),
+  yearly(title: "Yearly");
+
+  const Frequency({required this.title});
+
+  final String title;
+}