late

Learn deferred initialization with late.

late

Sometimes you want to give a field its initial value later, not in the constructor. The late keyword declares that a field will be initialized later; by the first read, its value must be set.

Example: class Config { late String apiKey; void load(String key) { apiKey = key; } }. Here apiKey is declared late; the load method assigns its value when called. If apiKey is read before load runs, a LateInitializationError is thrown. This shape fits heavy resources that should be prepared only when needed and helps resolve circular dependencies.

late and lazy initialization

The late keyword can also take a body; then the field is initialized lazily: on first read the body runs and the result is cached.

Example: late final heavy = _compute();. Here heavy runs _compute() on first read and caches the result; later reads return the same value. If heavy is never read, _compute never runs. This shape is a clean way to do expensive calculations only when needed. The late final combo guarantees that the value is computed once and cached.

Exercise Add a late field to Config

Define a class named Config. Add a late String apiKey field. Write a void load(String key) { apiKey = key; } method. In main, create an instance with var c = Config();, call c.load('abc123') and print with print(c.apiKey).

Show example solution

// Define the Config class
class Config {
  late String apiKey;
  void load(String key) {
    apiKey = key;
  }
}

void main() {
  var c = Config();
  c.load('abc123');
  print(c.apiKey);
}

Test yourself

When does a late field throw an error?
- When the constructor is called
- When declared as static
- When used with final
- When read before being set
Show answer

When read before being set — A late field throws LateInitializationError if read before being set.
Match the late parts

late

Keyword
apiKey

Field assigned later
load

Method that assigns

Add the late keyword

class Config {
  ____ String apiKey;
  void load(String key) {
    apiKey = key;
  }
}

Show answer

class Config {
  late String apiKey;
  void load(String key) {
    apiKey = key;
  }
}

Next lesson: static

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