Interfaces, enum. Practical Tasks

Give the SUMMER constant its own body to override getDescription(), and Season.SUMMER.getClass() suddenly returns not Season but an anonymous class Season$1. Details like this are exactly where interfaces and enums catch people out — and the fastest way to get a feel for them is to write the code yourself.

System.out.println(Season.SUMMER.getClass());          // class Season$1
System.out.println(Season.SUMMER.getDeclaringClass()); // class Season

This page contains 4 Java interface and enum exercises covering interfaces, the instanceof operator and reference type casting, enums (enum) with fields, constructors and overridden methods, abstract classes and inheritance. The tasks go from easy to hard: from a single Printable interface to a small “online store” application. Every task has a solution on Patreon, and the first two also have a video walkthrough.

Topics You Will Need

Before you start, review the basics of object-oriented programming in Java, abstract classes and reference type casting (upcasting and downcasting).

Task 1. The Printable Interface

1. Define an interface Printable with a method void print().
2. Define a class Book that implements the Printable interface.
3. Define a class Magazine that implements the Printable interface.
4. Create an array of type Printable that holds both books and magazines.
5. Loop through the array and call print() on each object.
6. Create a static method printMagazines(Printable[] printable) in the Magazine class that prints the titles of magazines only.
7. Create a static method printBooks(Printable[] printable) in the Book class that prints the titles of books only. Use the instanceof operator.

Video walkthrough → Solution on Patreon →

Task 2. Seasons (enum)

1. Create an enum Season with the seasons: WINTER, SPRING, SUMMER, AUTUMN.
2. Create a variable holding your favorite season and print all information about it: the constant name (name()), its ordinal (ordinal()) and the result of toString().
3. Create a method that takes a variable of your enum type. If the value is SUMMER, print “I love summer”, and so on for every season. Use a switch statement.
4. The enum must have a field with the average temperature of each season.
5. Add a constructor that takes the average temperature.
6. Create a method getDescription() that returns “Cold season”. Override getDescription() for the SUMMER constant so that it returns “Warm season”.
7. Loop over Season.values() and print every season with its average temperature and description.

String text = switch (season) {
    case WINTER -> "I love winter";
    case SPRING -> "I love spring";
    case SUMMER -> "I love summer";
    case AUTUMN -> "I love autumn";
};
System.out.println(text);

Video walkthrough → Solution on Patreon →

Task 3. Clothing

1. Create an enum ClothingSize with the clothing sizes: XXS, XS, S, M, L.
   • The enum has a numeric field euroSize (32, 34, 36, 38, 40) for each size and a constructor that takes euroSize.
   • The enum has a method getDescription() that returns “Adult size”. Override getDescription() for the XXS constant so that it returns “Kids size”.
2. Create the interfaces MenClothing with the method dressMan() and WomenClothing with the method dressWoman().
3. Create an abstract class Clothes with the fields: size (ClothingSize), price and color.
4. Create subclasses of Clothes:
   • TShirt — implements MenClothing and WomenClothing;
   • Pants — implements MenClothing and WomenClothing;
   • Skirt — implements WomenClothing;
   • Tie — implements MenClothing.
5. Create an array containing all types of clothing. Create a class Atelier with the methods dressWoman(Clothes[] clothes) and dressMan(Clothes[] clothes), both taking this array. dressWoman prints all information about women’s clothing, dressMan — about men’s clothing. Select the right items with instanceof.

Solution on Patreon →

Task 4. Online Store, Part 2

Continue building the “online store” mini-application (part 1). It should support the following features:

1. User authentication: the user enters a login and password from the keyboard (for example, using Scanner).
2. Viewing the list of product catalogs.
3. Viewing the products of a specific catalog.
4. Adding the selected product to the cart.
5. Buying the products in the cart.

Create an enum with a value for each of these operations and choose the action with a switch on that enum. Feel free to add your own operations or change anything you like.

Solution on Patreon →

More practice on classes, inheritance and polymorphism: Java OOP exercises.