Instance variables
- Dart course
- Beginner
- Lesson 79/116
- 5 min
Learn to declare instance variables in a class.
Instance variables
Instance variables are pieces of data held by each instance of a class. They are declared inside the class body. Example: class User { String name; int age; }.
You reach them through the instance with a dot: u.name, u.age. Each instance has its own values; two User objects may have different name fields.
An instance variable can have an initial value: String name = 'Guest';. Or it is set in the constructor. If the type is non-nullable, either an initial value or a constructor assignment is required.
Non-nullable instance variable
If an instance variable is non-nullable, you must give it an initial value or assign it in the constructor. Otherwise it is a compile error. Example: class User { String name = 'Guest'; } — name always has a value.
A constructor assignment is enough too: class User { String name; User(this.name); }. Here the constructor forces name, so it can stay non-nullable.
Making it nullable lifts the requirement: String? name; starts as null. Pick based on your data; null safety was the topic of the previous group.
Practice User with two fields
Define a User class with String name and int age fields. Write the constructor User(this.name, this.age). Inside main create a User('Ayşe', 28) instance and print name and age on separate lines.
Starter code
void main() {
// define the User class
// create an instance and print both fields
}Sample solution
class User {
String name;
int age;
User(this.name, this.age);
}
void main() {
var u = User('Ayşe', 28);
print(u.name);
print(u.age);
}Expected output
Ayşe 28
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Two User instances' name?
- Each has its own value Correct answer
- Throws
- Share the same value
- Becomes null
Explanation: each instance has its own values.
2 Two fields
class User {
String name;
____ age;
}class User {
String name;
int age;
}Explanation: Use the integer type for age.
3 Match the field types
| Term | Meaning |
|---|---|
String name |
Text |
int age |
Integer |
double score |
Decimal |
bool active |
Boolean |