The Liskov Substitution Principle is theL inSOLID object oriented design, and despite having one of the more intimidating sounding names of the 5 principles, it's actually pretty easy to wrap your head around.

It's so simple, in fact, that you're going to understand it in about 3 minutes.

What Is It?

The LSP, in simple terms, states that objects of the same superclass should be able to be swapped with each other without breaking anything.

If we have aCat and aDog class derived from anAnimal class, any functions using theAnimal class should be able to useCat orDog and behave normally.

Let's do an example!

Even that definition doesn't quite explain it very well, so let's put it into practice. All will become clear with the code.

In true OOP fashion, let's make anAnimal superclass, and aDog andCat subclass and capture their favorite kinds of food.

publicstaticclassAnimal{publicStringfavoriteFood;publicAnimal(StringfavoriteFood){this.favoriteFood=favoriteFood;}}publicstaticclassDogextendsAnimal{publicDog(StringfavoriteFood){super(favoriteFood);}}publicstaticclassCatextendsAnimal{publicCat(StringfavoriteFood){super(favoriteFood);}}

Now we want to make a method that will let us give these cuties some treats. I don't want to put this method in theAnimal classes though, because it'sme feeding the animals, not a behavior intrinsic to the animals. Something likeDog.feed might get the point across, but I thinkfeed(Dog) is better, don't you?

Let's make this method and call itGiveTreatTo:

publicstaticvoidGiveTreatTo(Animalanimal){Stringmsg="You fed the "+animal.getClass().getSimpleName()+" some "+animal.favoriteFood;System.out.println(msg);}

See here thatGiveTreatTo takes anyAnimal as a parameter. Since ourAnimal constructors assign the animal's favorite food, we can pretty much count on that data always being there.

This means we don't have to make a method for each animal, i.e.,GiveTreatToDog andGiveTreatToCat. Because we implemented LSP, we have one method. Let's see it in action:

publicstaticvoidmain(String[]args){Dogrover=newDog("bacon");Catbingo=newCat("fish");GiveTreatTo(rover);GiveTreatTo(bingo);}

Now, if we properly implemented the LSP, this program should run just fine. Let's check the output:

You gave the Dog some baconYou gave the Cat some fish

Amazing. Another benefit of this principal is we can add more and more subclasses ofAnimal and theGiveTreatTo method won't need any tinkering.

Wrap Up

Pretty non-intimidating right? As with many things in programming and computer science, the name belies its simplicity. Now, go out there and make your subclasses swappable, and thank Dr. Barbara Liskov for such a useful principle.