Constructors and named parameters in Flutter - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a constructor with a named parameter called 'name'.

Flutter

class Person {
  String name;
  Person([1]);
}

Drag options to blanks, or click blank then click option'

Arequired this.name

Bthis.name

Cname

Dthis.name = name

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to use curly braces for named parameters.

Not marking the parameter as required when needed.

The constructor uses a named parameter with required this.name to ensure the 'name' is passed when creating an object.

2fill in blank

medium

Complete the constructor to accept an optional named parameter 'age' with a default value of 18.

Flutter

class Person {
  int age;
  Person([1]);
}

Drag options to blanks, or click blank then click option'

Athis.age = 18

Bthis.age = 0

Cthis.age = 18, required

Dthis.age

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to mark default parameters as required.

Not assigning a default value properly.

Using this.age = 18 in the named parameter sets a default value of 18 if no value is provided.

3fill in blank

hard

Fix the error in the constructor to correctly assign the named parameter 'title' to the field.

Flutter

class Book {
  String title;
  Book([1]) {
    this.title = title;
  }
}

Drag options to blanks, or click blank then click option'

AString title

Brequired this.title

Cthis.title

Dtitle

Attempts:

3 left

💡 Hint

Common Mistakes

Declaring parameter without 'this.' and trying to assign manually.

Omitting 'required' for mandatory parameters.

Using required this.title in the constructor parameters automatically assigns the value to the field.

4fill in blank

hard

Fill both blanks to create a constructor with a named parameter 'color' that is optional and has a default value 'blue'.

Flutter

class Car {
  String color;
  Car([1] [2]);
}

Drag options to blanks, or click blank then click option'

Athis.color = 'blue',

Brequired

Cthis.color

Dcolor

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'required' with default values.

Not assigning default value properly.

The constructor uses this.color = 'blue' as an optional named parameter with a default value, and this.color to assign it.

5fill in blank

hard

Fill all three blanks to define a constructor with named parameters 'width' and 'height' both required, and an optional 'color' with default 'red'.

Flutter

class Rectangle {
  int width;
  int height;
  String color;
  Rectangle([1] [2] [3]);
}

Drag options to blanks, or click blank then click option'

Arequired this.width,

Brequired this.height,

Cthis.color = 'red',

Dthis.color

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting 'required' for mandatory parameters.

Not setting default value for optional parameters.

The constructor requires width and height using required this.field, and sets an optional color with a default value.

Practice

1. What is the main purpose of named parameters in Flutter constructors?

easy

A. To prevent object creation

B. To force all arguments to be positional

C. To make constructors private

D. To make arguments optional and improve code readability

Solution

1. Step 1: Understand named parameters

   Named parameters are enclosed in curly braces and can be optional or required, making code easier to read.

2. Step 2: Identify their purpose in constructors

   They allow passing arguments by name, improving clarity and safety by avoiding positional mistakes.

3. Final Answer:

   To make arguments optional and improve code readability -> Option D

4. Quick Check:

   Named parameters = optional and readable [OK]

Hint: Named parameters use braces and improve clarity [OK]

Common Mistakes:

• Thinking named parameters force positional arguments
• Believing named parameters prevent object creation
• Confusing named parameters with private constructors

2. Which of the following is the correct syntax for a Flutter constructor with named parameters?

easy

A. MyClass({this.name, this.age});

B. MyClass(this.name, this.age);

C. MyClass[name, age];

D. MyClass(name: this.name, age: this.age);

Solution

1. Step 1: Identify named parameter syntax

   Named parameters are declared inside curly braces in the constructor parameter list.

2. Step 2: Match syntax to options

   MyClass({this.name, this.age}); uses curly braces with this.name and this.age, which is correct syntax.

3. Final Answer:

   MyClass({this.name, this.age}); -> Option A

4. Quick Check:

   Named parameters use curly braces { } [OK]

Hint: Look for curly braces around parameters [OK]

Common Mistakes:

• Using square brackets instead of curly braces
• Forgetting curly braces for named parameters
• Using colon syntax incorrectly in constructor declaration

3. What will be the output of this Flutter code?

class Person {
  String name;
  int age;
  Person({required this.name, this.age = 30});
}

void main() {
  var p = Person(name: 'Anna');
  print('${p.name} is ${p.age} years old');
}

medium

A. Anna is 30 years old

B. Anna is null years old

C. Error: Missing required parameter age

D. Anna is 0 years old

Solution

1. Step 1: Analyze constructor parameters

   The constructor requires name and has an optional age with default 30.

2. Step 2: Check object creation and print output

   Person is created with name 'Anna' only, so age defaults to 30. The print shows 'Anna is 30 years old'.

3. Final Answer:

   Anna is 30 years old -> Option A

4. Quick Check:

   Default age = 30 used [OK]

Hint: Default values fill missing optional named parameters [OK]

Common Mistakes:

• Assuming missing optional parameters cause errors
• Thinking default values are ignored
• Confusing required and optional parameters

4. Identify the error in this Flutter constructor code:

class Car {
  String model;
  int? year;
  Car({this.model, this.year});
}

void main() {
  var c = Car(model: 'Tesla');
  print(c.year.toString());
}

medium

A. model is missing and causes compile error

B. year is null and calling toString() causes runtime error

C. No error, code runs fine

D. Constructor syntax is invalid

Solution

1. Step 1: Check constructor parameters and defaults

   Both model and year are optional named parameters without defaults, so year can be null.

2. Step 2: Analyze print statement

   Calling toString() on null year does NOT cause runtime error because toString() is defined on nullable int? and returns 'null' string.

3. Final Answer:

   No error, code runs fine -> Option C

4. Quick Check:

   toString() on null returns 'null' string [OK]

Hint: Optional parameters without defaults can be null but toString() handles null safely [OK]

Common Mistakes:

• Assuming missing optional parameters have default values
• Ignoring null safety and runtime errors
• Thinking constructor syntax is wrong here

5. You want to create a Flutter class Book with a required named parameter title and an optional named parameter author with default 'Unknown'. Which constructor is correct?

hard

A. Book(this.title, {this.author = 'Unknown'});

B. Book({required this.title, this.author = 'Unknown'});

C. Book({this.title, required this.author = 'Unknown'});

D. Book({this.title = 'Unknown', required this.author});

Solution

1. Step 1: Identify required and optional named parameters

   Required named parameters use 'required' keyword inside curly braces; optional can have default values.

2. Step 2: Match correct syntax

   Book({required this.title, this.author = 'Unknown'}); correctly marks title as required and author as optional with default 'Unknown'.

3. Final Answer:

   Book({required this.title, this.author = 'Unknown'}); -> Option B

4. Quick Check:

   Required + optional with default = correct syntax [OK]

Hint: Use 'required' inside braces for required named params [OK]

Common Mistakes:

• Placing required outside braces
• Assigning default to required parameter
• Mixing positional and named parameters incorrectly