Classes

Set up the data model with class and constructor.

Classes

Now we will add the Book class. A book has a title; so we will write a String title field and a Book(this.title) constructor. This way we can represent books in our library.

Classes model real-world objects in code. Book represents a book, Library a library. Each class carries its own fields and behavior. In an application many classes work side by side; one uses another. In this lesson we will use both Library and Book together.

Using two classes together

Many classes can be defined side by side in a single file. The Library and Book classes can work together. In main you create objects from both and use them together. For example you can create a book and print its title below the library info.

This approach is the foundation of large applications. Each class does one thing well; classes talk to each other. In this lesson Library represents the library, Book a book; main brings them together. In the next lesson we will gather books in a list.

Exercise Add the Book class

The Library class is already defined in the code below. Now define the Book class: a String title field and a Book(this.title) constructor. In main create a book with final b = Book("Crime and Punishment"); and print "New book: ${b.title}".

Show example solution

class Library {
  String name;
  Library(this.name);
  void info() {
    print("Library: $name");
  }
}

class Book {
  String title;
  Book(this.title);
}

void main() {
  final lib = Library("City");
  lib.info();
  final b = Book("Crime and Punishment");
  print("New book: ${b.title}");
}

Test yourself

The Book class

____ Book {
  String title;
  Book(this.title);
}

Show answer

class Book {
  String title;
  Book(this.title);
}

How is the Book constructor written?
```
class Book {
  String title;
  ____
}
```
- Book(this.title)
- Book()
- new Book()
- title()
Show answer

Book() — the Book(this.title) constructor assigns the title field.
Match class usage

Library

carries the library
Book

carries a book
main

brings together
lib.info()

prints library info

Next lesson: Collections

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