Classes and objects in Flutter - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

What is the output of this Flutter widget?

Consider this Flutter code defining a class and using it in a widget. What text will appear on the screen?

Flutter

class Person {
  final String name;
  Person(this.name);
}

class MyWidget extends StatelessWidget {
  final Person person = Person('Alice');

  @override
  Widget build(BuildContext context) {
    return Text(person.name);
  }
}

A Alice

B person.name

C null

D Error: name not found

💡 Hint

Look at how the Person class stores the name and how the Text widget uses it.

The Person class stores the name 'Alice'. The Text widget displays person.name, so it shows 'Alice'.

🧠 Conceptual

intermediate

What does this Dart class constructor do?

Look at this Dart class constructor. What is its purpose?

Flutter

class Car {
  String model;
  int year;

  Car(this.model, this.year);
}

A It creates a Car object but leaves model and year null.

B It creates a Car object and sets model and year from parameters.

C It creates a Car object with default model and year values.

D It creates a Car object but requires setters to assign model and year.

💡 Hint

The constructor uses 'this' to assign parameters to fields.

The constructor Car(this.model, this.year) assigns the passed arguments to the object's fields.

❓ lifecycle

advanced

What happens when this Flutter StatefulWidget is created?

Examine this Flutter StatefulWidget code. What is printed when the widget is first inserted into the widget tree?

Flutter

class CounterWidget extends StatefulWidget {
  @override
  State<CounterWidget> createState() => _CounterWidgetState();
}

class _CounterWidgetState extends State<CounterWidget> {
  int count = 0;

  @override
  void initState() {
    super.initState();
    print('Init count: $count');
  }

  @override
  Widget build(BuildContext context) {
    return Text('Count: $count');
  }
}

A No output printed

B Count: 0

C Init count: null

D Init count: 0

💡 Hint

initState runs once when the widget is created before build.

initState prints 'Init count: 0' because count is initialized to 0 before build runs.

📝 Syntax

advanced

Which option correctly defines a Dart class with a named constructor?

Select the correct Dart code that defines a class with a named constructor called 'fromJson'.

A

class User {
  String name;
  User.fromJson(Map<String, dynamic> json) {
    name = json['name'];
  }
}

B

class User {
  String name;
  User.fromJson(Map<String, dynamic> json) => name = json['name'];
}

C

class User {
  String name;
  User.fromJson(Map<String, dynamic> json) {
    return name = json['name'];
  }
}

D

class User {
  String name;
  User.fromJson(Map<String, dynamic> json) => {
    name = json['name'];
  };
}

💡 Hint

Named constructors use the class name dot constructor name syntax with a body.

Option A correctly defines a named constructor with a body that assigns the name field.

🔧 Debug

expert

Why does this Flutter code cause a runtime error?

This Flutter code tries to display a person's age but crashes at runtime. Why?

Flutter

class Person {
  int? age;
  Person(this.age);
}

class AgeWidget extends StatelessWidget {
  final Person person = Person(null);

  @override
  Widget build(BuildContext context) {
    return Text('Age: ' + person.age!.toString());
  }
}

A The '+' operator cannot concatenate String and int.

B The Person class must initialize age with a non-null value.

C person.age is null, so person.age!.toString() causes a runtime error.

D StatelessWidget cannot have final fields.

💡 Hint

Consider null safety and the non-null assertion operator when accessing nullable fields.

person.age is null. The expression person.age!.toString() uses the non-null assertion operator '!' on a null value, causing a runtime error: "Null check operator used on a null value".