adds null safety codelab (#2983)

Co-authored-by: Parker Lougheed <[email protected]>
Co-authored-by: Filip Hracek <[email protected]>
Co-authored-by: Kathy Walrath <[email protected]>

null_safety_examples/null_safety_codelab/analysis_options.yaml

@@ -0,0 +1 @@
+include: ../analysis_options.yaml

null_safety_examples/null_safety_codelab/bin/assertion_operator.dart

@@ -0,0 +1,14 @@
+int? couldReturnNullButDoesnt() => -3;
+
+void main() {
+  int? couldBeNullButIsnt = 1;
+  List<int?> listThatCouldHoldNulls = [2, null, 4];
+
+  int a = couldBeNullButIsnt;
+  int b = listThatCouldHoldNulls.first!; // first item in the list
+  int c = couldReturnNullButDoesnt()!.abs(); // absolute value
+
+  print('a is $a.');
+  print('b is $b.');
+  print('c is $c.');
+}

null_safety_examples/null_safety_codelab/bin/conditional_access.dart

@@ -0,0 +1,14 @@
+class BigThing {
+  LittleThing little = LittleThing();
+}
+
+class LittleThing {
+  int fetchInt() => 12;
+}
+
+void main() {
+  BigThing? big = BigThing();
+
+  print('The value is:');
+  print(big?.little.fetchInt());
+}

null_safety_examples/null_safety_codelab/bin/definite_assignment.dart

@@ -0,0 +1,12 @@
+void main() {
+  String text;
+
+  if (DateTime.now().hour < 12) {
+    text = "It's morning! Let's make aloo paratha!";
+  } else {
+    text = "It's afternoon! Let's make biryani!";
+  }
+
+  print(text);
+  print(text.length);
+}

null_safety_examples/null_safety_codelab/bin/late_circular_references.dart

@@ -0,0 +1,16 @@
+class Team {
+  late final Coach coach;
+}
+
+class Coach {
+  late final Team team;
+}
+
+void main() {
+  final myTeam = Team();
+  final myCoach = Coach();
+  myTeam.coach = myCoach;
+  myCoach.team = myTeam;
+
+  print('All done!');
+}

null_safety_examples/null_safety_codelab/bin/late_keyword.dart

@@ -0,0 +1,13 @@
+class Meal {
+  late String description;
+
+  void setDescription(String str) {
+    description = str;
+  }
+}
+
+void main() {
+  final myMeal = Meal();
+  myMeal.setDescription('Feijoada!');
+  print(myMeal.description);
+}

null_safety_examples/null_safety_codelab/bin/late_lazy.dart

@@ -0,0 +1,16 @@
+int _computeValue() {
+  print('In _computeValue...');
+  return 3;
+}
+
+class CachedValueProvider {
+  final _cache = _computeValue();
+  int get value => _cache;
+}
+
+void main() {
+  print('Calling constructor...');
+  var provider = CachedValueProvider();
+  print('Getting value...');
+  print('The value is ${provider.value}!');
+}

null_safety_examples/null_safety_codelab/bin/more_nullable_types.dart

@@ -0,0 +1,9 @@
+void main() {
+  List<String> aListofStrings = ['one', 'two', 'three'];
+  List<String?> aNullableListOfStrings = [];
+  List<String?> aListofNullableStrings = ['one', null, 'three'];
+
+  print('aListofStrings is $aListofStrings.');
+  print('aNullableListOfStrings is $aNullableListOfStrings.');
+  print('aListofNullableStrings is $aListofNullableStrings.');
+}

null_safety_examples/null_safety_codelab/bin/non_nullable_types.dart

@@ -0,0 +1,5 @@
+void main() {
+  int a;
+  a = 145;
+  print('a is $a.');
+}

null_safety_examples/null_safety_codelab/bin/nullable_types.dart

@@ -0,0 +1,5 @@
+void main() {
+  int? a;
+  a = null;
+  print('a is $a.');
+}

null_safety_examples/null_safety_codelab/bin/promotion_exceptions.dart

@@ -0,0 +1,11 @@
+int getLength(String? str) {
+  // Try throwing an exception here if `str` is null.
+  if (str == null) {
+    throw Exception("String is null");
+  }
+  return str.length;
+}
+
+void main() {
+  print(getLength(null));
+}

null_safety_examples/null_safety_codelab/bin/type_promotion.dart

@@ -0,0 +1,11 @@
+int getLength(String? str) {
+  // Add null check here
+  if (str == null) {
+    return 0;
+  }
+  return str.length;
+}
+
+void main() {
+  print(getLength('This is a string!'));
+}

null_safety_examples/null_safety_codelab/pubspec.yaml

@@ -0,0 +1,10 @@
+name: null_safety_codelab
+description: dart.dev example code for null safety codelab
+
+environment:
+  sdk: ">=2.12.0-0 <3.0.0"
+
+dev_dependencies:
+  examples_util: {path: ../util}
+  pedantic: ^1.10.0-nullsafety.3
+  test: ^1.16.0-nullsafety.13

src/_data/side-nav.yml

@@ -16,6 +16,8 @@
           permalink: /codelabs/iterables
         - title: Asynchronous programming
           permalink: /codelabs/async-await
+        - title: Null safety
+          permalink: /codelabs/null-safety
     - title: Tutorials
       urlExactMatch: true
       match-page-url-exactly: true