Chapter 12 — Null Safety

Level: beginner / intermediate Estimated duration: 5 hours Prerequisites: Chapter 11 — Advanced OOP (abstract, mixins, enums) What you'll be able to do by the end: write Dart code in which a missing value can no longer crash the program, by distinguishing nullable types from non-nullable types and using ?., ??, ??=, !, and late appropriately.

12.0 — Chapter objectives

By the end of this chapter, you will be able to:

• explain what null is and why it has cost the industry so much;
• distinguish a non-nullable type (String) from a nullable type (String?);
• read and understand compiler error messages related to null safety;
• use the safe access operator ?.;
• use the fallback value operator ??;
• use the conditional assignment operator ??=;
• explain why the ! operator is a last resort;
• use late for deferred initialization;
• use late final for a value written only once;
• recognize and fix a LateInitializationError;
• understand type promotion (flow analysis);
• explain why promotion fails on a class field;
• fix this problem with an intermediate local variable;
• distinguish List<int>? from List<int?>;
• read a Map without crashing the program;
• write nullable parameters, default values, and required parameters;
• apply a best-practices checklist to a real project.