Lära Defining Classes and Fields

Definition

Object instantiation is the process of creating a specific instance of a class in memory. When you write Car myCar = Car(); or Book myBook = Book();, you are instantiating an object. This step is crucial in object-oriented programming because it allows you to work with actual data.

When you start learning Dart's object-oriented programming features, classes are your main building blocks. A class defines the structure and behavior of objects, think of it as a blueprint. You use classes to organize code, represent real-world entities, and keep related data and functionality together. Inside a class, you declare fields to represent the state of each object. These fields are like the characteristics or properties that every object of that class will have.

Object state

The object state refers to the specific values stored in an object's fields at a given time. For example, a Car object might have its brand field set to "Toyota" and its year field set to 2022. These values together form the state of that particular car object.

Fields and variables

Fields are variables that belong to a class and describe the properties of its objects. While a variable can exist anywhere in your code, a field is always tied to a class or an object. Fields maintain the state of each object, while regular variables may just hold temporary data or help with calculations.

car.dart


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class Car {
  String brand;
  int year;
}

void main() {
  Car myCar = Car();
  myCar.brand = "Toyota";
  myCar.year = 2022;

  print("Brand: ${myCar.brand}");
  print("Year: ${myCar.year}");
}

In the Car class above, you see two fields: brand and year. These fields are used to store information about a car's brand and the year it was made. When you create a new Car object using Car myCar = Car();, you can assign values to these fields using dot notation, such as myCar.brand = "Toyota";. To access the values, you use the same dot notation: myCar.brand and myCar.year. This approach lets each object maintain its own state, so you can create multiple cars with different brands and years.

book.dart


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class Book {
  String title;
  String author;
}

void main() {
  Book myBook = Book();
  myBook.title = "Dart in Action";
  myBook.author = "Chris Buckett";

  print("Title: ${myBook.title}");
  print("Author: ${myBook.author}");
}

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