How does LSP works in OOP

[dusky ginkgo]

I dont understand how it wroks

[clear pasture]

<@&987246399047479336> please have a look, thanks.

[sand mirage]

which part of it is unclear to u

u recently learned about "is-a" relationship. so i guess that's unclear to u as well?

Animal a = new Dog(); is this line unclear to u?

or is it clear but u didnt get the motivation for it yet?

[clear pasture]

@dusky ginkgo

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[dusky ginkgo]

sorry, i didnt saw the nessage, yes, ig LSP says that a class can be replaced by its subclases, but i dont understand why would i do:

"Animal a = new Dog();"

instead of directly saying:

"Dog dog = new Dog();"

like i dont understand for what it is useful for

[timid trout]

Think of it not at this point in the code (that was not the original intent of LSP) but at the edges of the class.

Compare my class offering one of these two methods

public void add(ArrayList<Animal> animals) { ... }

public void add(Collection<? extends Animal> animals) { ... }

One allows anyone to substitute other Collection types, and with generic specialisations of Animal.

Taken to the extreme, anywhere where you have a use-site of a type, if a more general type would be acceptable to the pattern of use, using that type makes it less disruptive and limiting to substitute another type.

Similarly...

   List<Foo> list = new ArrayList<>();

Means that all users of list are assuming nothing about the actual list type, and I could change my mind later and use LinkedList without changing any other code.

Note that the original papers and discussions where about the interfaces between units of code. Often people oversell the LSP intent to mean a strict law that applies to all internal (private) content as well, rather than being more specific about a type-choice - that specificity can express intent (choosing a concrete type for it behavioural, not linguistic properties), and can (somewhat rarely) slightly improve the chances of monomorphic optimisations.

All of these rules are 'good things to consider' not absolute laws - because there's almost always a case in which doing otherwise is the better choice (once all aspects of the project are considered). Software Development does tend to become a little dogmatic though, and people are rewarded for books and videos ranting on the topic which doesn't help.

[sand mirage]

its confusing to u now bc ur not yet at a point where u have to design libraries/apis

especially in bigger code bases these things become relevant

or when u write code for others to use

when ur methods demand a type, it should be the most vague type possible that still fulfills all ur requirements

so for example Iterable instead of ArrayList if all u do with it is iterating it

[dusky ginkgo]

So that means that the LSP is something that helps you make code more readable and useful

[timid trout]

It makes it easier to maintain code.

[dusky ginkgo]

Oh okey

[dusky ginkgo]

ohh

Well i then ig thats something i have to see later right?

[sand mirage]

it makes code more modular and flexible

a method whose parameters use LSP can be used by more classes and make more people happy

a shelter for animals should take any animal

not just dogs

or cats

but both, dogs, cats, also birds, any animal can be taken

addAnimal(Animal a)

and not addAnimal(Dog d)

thats LSP in practice

a method that feeds something should can maybe an interface CanBeFed

instead of Dogs only

bc then it can also feed humans, fish, ...

everything that implements the interface

write the most vague type that still fulfills ur requirements

then it can be used by the most classes

[timid trout]

And in Java, interface-types are less constraining than class-types, so we favour using interface-types when possible.

[dusky ginkgo]

ty

[dusky ginkgo]

ty