Classes and instances

Creating a class

We create a class by using the class keyword, defining the properties, and providing a constructor method.

public class Author {
  String name;
  int birthYear;

  public Author(String name, int birthYear) {
    this.name = name;
    this.birthYear = birthYear;
  }
}

INFO

You can think of this as representing the object currently under construction. You don't need to return this in the constructor as that will happen by default.

Using a class

We make new instances of a class by passing in values. These will be used by the constructor.

Author maggie = new Author("Margaret Atwood", 1936);

Composing classes

Objects can be nested inside objects. For example, Book objects can contain an Author object as a property.

We update the Book class.

public class Book {
  public String title;
  public int year;
  public boolean isAvailable;
  public Author author;

  public Book(String title, int year, boolean isAvailable, Author author) {
    this.title = title;
    this.year = year;
    this.isAvailable = isAvailable;
    this.author = author;
  }
}

Now, we can pass an instance of Author to its constructor.

Author maggie = new Author("Margaret Atwood", 1936);
Book book = new Book("The Handmaid's Tale", 1985, true, maggie);

System.out.printl(book.author.name) // Margaret Atwood