SOLID principles by examples: Liskov Substitution Principle

In this post we’re going to explore the third of the SOLID principles: the Liskov Substitution Principle (LSP).

The most practical definition of this principle was written by Robert C. Martin in his book Agile Software Development, Principles, Patterns, and Practices.

Subtypes must be substitutable for their base types.

The concept was introduced by Barbara Liskov in 1987. The formal definition is:

Let q(x) be a property provable about objects x of type T. Then q(y) should be provable for objects y of type S where S is a subtype of T.

For our daily activities we must remember that a subclass should override the parent class’ methods in a way that doesn’t break functionality from a consumers’s point of view.

Example


abstract class MusicalInstrument
    {
        public abstract void PlayANote();
    }

class Piano : MusicalInstrument
    {
        public override void PlayANote()
        {
            PressKey();
        }

        private void PressKey()
        {
            //Press a piano key.
        }
    }

class Saxophone : MusicalInstrument
    {
        public override void PlayANote()
        {
            Blow();
        }

        private void Blow()
        {
            //Blow air into the instrument.
        }
    }

The evergreen example

To better underestand LSP let’s examine this classic example. It’s a classic because it’s easy to understand and very meaningful. We start with this question:

Is a square a special rectangle in OOP?

We try to answer this question with this simple class hierarchy: a Rectangle as base class and a Square class that inherits from it. In the Square class we override the behavior of the setters to enforce that the the Heigth and Width properties have the same value.

class Rectangle
    {
        public virtual float Heigth { get; set; }
        public virtual float Width { get; set; }
        public virtual float Area
        {
            get { return Heigth * Width; }
        }
    }

class Square : Rectangle
    {
        private float _heigth;

        private float _width;

        public override float Heigth
        {
            get
            {
                return _heigth;
            }
            set
            {
                _heigth = value;
                _width = value;
            }
        }

        public override float Width
        {
            get
            {
                return _width;
            }
            set
            {
                _width = value;
                _heigth = value;
            }
        }
    }

Now we have to remember that a subclass must override the base class in a way that it doesn’t break functionality from a client’s POV. We write these two tests to check.


[TestClass]
    public class UnitTest1
    {
        [TestMethod]
        public void TestWithRectangle()
        {
            Rectangle sut = new Rectangle();
            sut.Heigth = 3;
            sut.Width = 7;

            Assert.AreEqual(21, sut.Area);
        }

        [TestMethod]
        public void TestWithSquare()
        {
            Rectangle sut = new Square();
            sut.Heigth = 3;
            sut.Width = 7;

            Assert.AreEqual(21, sut.Area); //This test will fail. Area equals 49.
        }

    }

So it’s clear that in OOP a square isn’t a particulare case of a rectangle. How can we do to better organize these classes? The typical approach consists of creating an abstract class (or an interface). For example

abstract class Shape
    {
        public abstract float Area { get; }
    }

 class Rectangle : Shape
    {
        public float Heigth { get; set; }
        public float Width { get; set; }
        public override float Area
        {
            get { return Heigth * Width; }
        }
    }

    class Square : Shape
    {
        public float Edge { get; set; }

        public override float Area
        {
            get { return Edge * Edge; }
        }
    }

Now our code states that both the Rectangle class and the Sqare class are Shapes which is true. Our code is also safer and we don’t have any situation where a client will receive unexpected values.

TL;DR

In this post we explored the Liskov Substituion Principle (LSP) and we learned that it’s not true that real-life objects always maps to the same OOP structure / class ecosystem. When also tried to improve the wrong example with the simple technique of adding an abstraction layer (the Shape class).

1 anno di blogging

Oggi questo blog compie 1 anno.

Nonostante tante cose è sempre rimasto attivo e ne sono orgoglioso. Ok, non è così attivo come vorrei (ha vissuto un paio di mesi di pause) ma ha attraversato eventi decisamente importanti come:

  1. La nascita del mio secondo figlio;
  2. Picchi lavorativi dovuti a nuovi clienti;
  3. Un trasloco verso una casa nuova;
  4. Notti insonni dovute al punto 1.

È nato per sfida personale sull’argomento costanza: spesso mi dico che non sono bravo a portare avanti le cose per lunghi periodi di tempo e volevo migliorare. È nato anche perché sono stato influenzato da Jeff Atwood, Scott Hanselman, Frank Bettger e da tantissimi blog-post che indicano come scrivere articoli, anche semplici, ci rende più ferrati sull’argomento.

Ho iniziato così a sperimentare per trovare la mia voce, il mio modo di raccontare le cose e di trasmetterle.

Un grosso cambiamento è avvenuto a maggio quando sono stato alla Microsoft House per un evento sul Dekstop Bridge. Parlando con Matteo Pagani e altri ragazzi ho scoperto quanto un blog sia un esercizio svolto da molti. È stata un’iniezione di motivazione e allo stesso una scoperta perché ha cambiato il contenuto dei miei articoli. Ho scoperto quanto mi piaccia scrivere dei miei esperimenti che svolgo per curiosità su tecnologie informatiche che non utilizzo nella mia quotidianità al lavoro: mi aiuta a fissare i concetti e ciò che scopro può essere utile a qualcun altro.
Alcuni esperimenti con UWP si sono trasformati in un possibile business per un conoscente che mi ha chiesto informazioni. Le prove con Prism sono state impiegate per un prodotto nuovo in azienda portato avanti da dei colleghi. Gli articoli su DZone condivisi da questo blog raggiungono migliaia di visite e questo blog ha raggiunto 200 visitatori unici al mese in luglio. Certo altri blogger fanno queste cifre al giorno ma per me è già tanto.

Il mio caloroso consiglio è quello di cominciare un blog anche voi se siete indecisi: condividete il vostro interesse per qualcosa per il puro piacere di insegnare, per divertirvi e per imparare. A me ha portato solo cose buone. Trovate un ritmo che siete in grado di mantenere e mantenetelo a tutti i costi. Non traete conclusioni prima di un anno di blogging più o meno continuo.

Queste sono le mie conclusioni: ne vale la pena e continuerò più che posso.

SOLID principles by examples: open/closed

This post continues the analisys of the SOLID principles started some blog posts ago. This is the turn for the Open Closed Priciple (OCP).

The definition

An object/entity should be open for extension but closed for modification.

What we are basically talking about is to design our modules, classes and functions in a way that when a new functionality is needed, we should not modify our existing code but rather write new code that will be used by existing code.

Read More »

SOLID principles by examples: single responsability

This blog post will explain with simple examples the Singe Responsabily Principle of SOLID agiles principles to better understand how we can improve our daily coding activites. In future post I’ll cover the other four priciples.

The Definition

A class should have only one reason to change.

Defined by Robert C. Martin in his book Agile Software Development, Principles, Patterns, and Practices it is the first of the five SOLID agile principles. What it states is very simple, however achieving that simplicity is not so obvious.

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SOLID principles by examples: introduction

SOLID is a common acronym in the software development world. It’s useful to remeber five best practices to design classes in a object oriented language. It means:



Initial

Stands for

Concept

SRP Single Responsability Principle A class should have only a single responsability.
OCP Open Closed Principle Software entities should be open for extension but closed for modification.
LSP Liskov Substitution Principle Objects in a program should be replaceable with instances of theri subtypes without altering the correctness of that program.
ISP Interface Segregation Principle Many client-specific interfaces are better than one general purpose-interface.
DIP Dependency Injection Principle

One should depend upon abstractions, not concretions.

These concepts are easy to understand when we read them, but what does it mean to apply them in our daily coding activities? What do we have to do?

In the next posts we’ll go through each concept with coding examples because as someone much smarter than me said:

Tell me and I’ll foget, show me and I might remember. Involve me and I will remember. – Confucius

UWP Prism Unit Test

I’m sure we all hear about unit testing, test driven development and so on. These practices are useful and provide a long list of benefit like writing low-coupled and mantainable code to name a few.

We want to write unit test for our UWP Prism application and we’d like, for example, to put our ViewModels under tests to make sure the logic they implement is correct. Unit testing is possible only if we can get rid of all the dependencies in our class under test because we want to run our test every time we want (for exemple when the service library our teammate is writing is not yet finished) and as fast as we can.

Read More »