Lists of objects

Creating a list of objects

We can add objects to a list or array just like any other data.

java

import java.util.ArrayList;
import java.util.List;
import models.Book;

public class Main {
  public static void main(String[] args) {
    List<Book> books = new ArrayList<>();

    // Add books to the list
    books.add(new Book("The Handmaid's Tale", 1985, false));
    books.add(new Book("Oryx and Crake", 2003, true));
    books.add(new Book("Brave New World", 1932, false));
    books.add(new Book("The Geometry of Type", 2013, true));
    books.add(new Book("The Design of Everyday Things", 1988, true));

    for (Book book : books) {
      System.out.println(book);
    }
  }
}

output

The Handmaid's Tale
Oryx and Crake
Brave New World
The Geometry of Type
The Design of Everyday Things

Using loops

We can use our knowledge of loops to solve problems on arrays of objects. For example, suppose we need to count how many books were published in the 20th century.

public static int countOldBooks(List<Book> books) {
  int count = 0;

  for (Book book : books) {
    if (book.year < 2000) {
      count++;
    }
  }

  return count; // 3
}

Transforming with .map

It is common to somehow transform the list of objects. For example, we can take the list of books and return a list of just the titles.

java

public static List<String> getBookTitles(List<Book> books) {
  return books.stream().map(book -> book.title).toList();
}

output

[The Handmaid's Tale, Oryx and Crake, Brave New World, The Geometry of Type, The Design of Everyday Things]

Filtering with .filter

We can also filter the list of books based on any criteria we like. For example, we can get a list of only the available books.

public static List<Book> getAvailableBooks(List<Book> books) {
  return books.stream().filter(book -> book.isAvailable).toList();
}