Java Design Patterns

This repository contains examples of Design Patterns implemented in Java. The problems were given as assignments in a course at Università di Pisa.

Project files can be opened with the BlueJ development environment.

Assignment descriptions

Game of Life

The application of various design patterns (Observer, State, Singleton, Command, and Visitor) to the classic Game of Life program. Based on the paper by Michael R. Wick "Teaching Design Patterns in CS1: a Closed Laboratory Sequence based on the Game of Life" (2005).

Strategy

PizzaCap is a company that sells pizzas and gives discounts to clients. But there are many kinds of discount calculation methods such as: 10% off, reduce a fixed amount, happy hour, no discount at all, etc.

Now Pizza Cap asks you to develop a sales management system, they want you to design a schema to calculate the discount when selling pizzas.Your design should be capable of selecting the discount calculation methods. Furthermore, when they need new discount calculation methods or want to modify old methods, it should be very easy and will not affect the existing system.

State 1 & 2

Consider a class with two methods, push() and pull(), whose behaviour changes depending on the object state.

To send the push and pull requests to the object, we'll use a GUI with "Push" and "Pull" buttons. The state of the object will be indicated by the colour of the canvas in the top part of the GUI. The states are: black, red, blue and green

Create State objects ahead of time and never destroy them. Implement different variants, accordingly to the following implementation strategies:

Letting Context to decide the flow of state transitions.
Using a table to define state transitions.

Observer

You will build a point of sale GUI for a restaurant staff.

Consider a menu: Hamburger 3.99, Cheeseburger 4.99, Milk Shake 5.00, Soda 2.00, Fries 2.50. The program should load the menu and create a panel full of large buttons (ideal for a touchscreen) for each menu item.

A waiter should be able to click on each menu item to add it to the current order (the subject). This should add the item to a receipt panel which displays the full order and the total cost. The total cost should be accurate at all times, updated as each item is added (not only when the order is finalised).

The waiter should be able to click a Place Order (notification needed now) button that simulates transmitting the order to the kitchen and cashier. Simply, a screen in the kitchen shows the list of the order and the number of the order. The screen of the cashier shows the order number, names and costs of the ordered items, and the total cost (two different observers).

Decorator

The winter holidays will be here (again) before you know it! Being the organised individual you are, you have a plan for next year's holiday tree. Implement a software system that allows you to calculate the price of any tree plus any combination of decorations. The system must be easily extendable in the sense that whenever new decorations are added in the store you will have to at most add one class.

A very important requirement is that a tree can only have one star (not trivial to implement this requirement). When a user wants to decorate a tree with a star with a new star you must print a warning that the tree already has a star and not add the price of a star to tree. Users must be able to continue decorating their tree if they add another star to it.

Factory Patterns 1 & 2

Apply the factory patterns to produce TVs and remote controls of two types: Samsung and Philips.

With factory method the creator builds a TV and its RC, then packs it.
With abstract factory a client chooses the factory and asks for the product(s) he needs.

Command

Using command, decorate a Christmas tree (you might have problems in composing with decorator, figure out a solution, the requirement being: use command). Define two standard decorations that can be invoked by lazy or non-creative people. Undecorate the tree after Epiphany (and put everything in a box).

Adapter 1 & 2

Extend the ugly duckling example to adapt turkeys too:

Using strategy.
Using façade.

Façade 1 & 2

Modify the home theatre example along the following dimensions:

CD is substituted with a french one.
The system prevents you from watching a movie you saw.

Create and example where Façade is an abstract class with different concrete subclasses.

Template Method 1 & 2

Use the Template Method pattern to:

Refine a sorting algorithm for sorting in a different (ascending vs descending) order.
Write your favorite recipe with measures in the metric system and in the anglosaxon one.

Bridge 1 & 2

Using the Bridge pattern:

Design a soda machine with two abstractions (regular and supersize soda) and three implementors (cola, ginger ale, beer).
Model the software for switched devices in a home with a UML class diagram. Switches come in several varieties, such as
- a ceiling light two-position switch,
- a ceiling fan pull chain (i.e., each pull increases the fan’s speed until the maximum speed is reached, where upon the next pull turns the fan off), and
- a light dimmer switch (to permit continuous adjustment to the light’s brightness).

Visitor & Composite

Given three colour classes (Red, Blue, Green), assume an array colours contains mixed objects from those three classes. We want to the following:

To find number of Red, Blue, and Green objects, respectively, in the array.
To create another array sortedColours which contains the same objects as in colours but sorted according to their colours in red-blue-green order.

Using Visitor pattern to implement the above two operations. You need to write abstract and concrete Visitor classes, modify Red, Blue, Green classes, and write a main program to use a Visitor object to perform each of the operations.

Builder

In most available (on the web) examples of use of Builder there is only one Director, and the Director only has a construction method. This way the full power of the pattern is not exploited. Invent an example (or extend an existing one) of use of builder with this observation in mind.

Chain of Responsibility

When the client of a bank asks for a loan, (s)he asks to:

the desk employee, who is entitled of approving loans up to $10.000, otherwise (s)he asks to...
the deputy director of the bank branch, who is entitled of approving loans up to $50.000, otherwise (s)he asks to...
The director of the bank branch, who is entitled of approving loans up to $200.000, otherwise (s)he asks to...
...

Dynamically change the chain (no more deputy director of the bank branch).

Chain of Responsibility & Builder

Use CoR to write a program that, given a number n < 50, is able to return:

the number of primes smaller than n, and
the decomposition in prime factors of n.

Use Builder to build the chain, in a situation where there are two kinds of handlers, and hence two chains (the client decides which chain to build).

Flyweight

A Flyweight object has intrinsic state that cannot be changed. This also means that a Flyweight object cannot have any public-accessible set() method to set a new value for some instance variable of the object. Consider using the Flyweight pattern to allow a Flyweight object to have set() methods. When a set() method of a Flyweight object is called, the object becomes a non-Flyweight, non-shared object. This idea is similar to copy-on-write.

Build an example of copy-on-write. For instance: decorate the Christmas tree and then change colour to some decorations.

Implement the Chatroom, having the following interface:

public interface AbstractChatroom {
  public abstract void register(Participant participant);
  public abstract void send(String from, String to, String msg);
}

At this point only one-to-one communication is implemented in the Chatroom. Optional: experiment with one-to-many (communication to a group).

gvinciguerra / TDP-2017