Java Community | Help. Code. Learn.•11mo ago

why i cant access field in child class when using inheritance?

hey guys. can smb explain this error? i dont understand what im doing wrong:

public class Animal {
    private String name;
    private int age;
    ///getters and setters
}
public class Dog extends Animal {
    String breed;
    //getters and setters
}
public static void main(String[] args) {
        Animal animal = new Animal();
        animal.setAge(123);
        animal.setName("Animal");

        Animal dog1 = new Dog();
        dog1.setAge(111);
        dog1.setName("Max");
        dog1.setBreed("Boxer");//error

        Dog dog2 = new Dog();
        dog2.setAge(222);
        dog2.setName("Rex");
        dog2.setBreed("Pitbull");
    }

public class Animal {
    private String name;
    private int age;
    ///getters and setters
}
public class Dog extends Animal {
    String breed;
    //getters and setters
}
public static void main(String[] args) {
        Animal animal = new Animal();
        animal.setAge(123);
        animal.setName("Animal");

        Animal dog1 = new Dog();
        dog1.setAge(111);
        dog1.setName("Max");
        dog1.setBreed("Boxer");//error

        Dog dog2 = new Dog();
        dog2.setAge(222);
        dog2.setName("Rex");
        dog2.setBreed("Pitbull");
    }

thanks in advance

106 Replies

JavaBot•11mo ago

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dan1st•11mo ago

by creating a variable of type Animal, you say that you know it is an Animal but don't assume further information you could also do

Animal animal = new Dog();
animal = new Animal();

Animal animal = new Dog();
animal = new Animal();

bambyzasOP•11mo ago

ok. but ive seen such cases where you do stuff like Animal dog1=new Dog();. i.e. you create instance of a child class but for some reason assign it to the parent type. whats the purpose of this?

tjoener•11mo ago

breed is only on dog, and you declared it as animal

Unknown User•11mo ago

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dan1st•11mo ago

it means you create a dog and forget that it is a dog you just know about it being an animal after that

Unknown User•11mo ago

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tjoener•11mo ago

It's still a dog, you just reference it as an animal Like, "my friend has a pet, can't remember if it's a dog or a cat, but it's definitely a pet"

Unknown User•11mo ago

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bambyzasOP•11mo ago

ok, but then again. using the more abstract data type wouldnt let you use functinalities from more specific data type

Unknown User•11mo ago

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bambyzasOP•11mo ago

i dont get it

Unknown User•11mo ago

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bambyzasOP•11mo ago

this

dan1st•11mo ago

Ok so there are two things: Variable declaration and variable initialization Declaration means you create a variable, e.g.

List<Integer> list;

List<Integer> list;

This decides what methods are available to you initialization is something else - it sets the variable to another value, e.g.

list = new ArrayList<>();

list = new ArrayList<>();

But this doesn't influence which parts are usable for you new ArrayList() creates an ArrayList object. If you put it somewhetre that expects any List and doesn't care which one it is, that code cannot use ArrayList-specific stuff

bambyzasOP•11mo ago

ok. so it means i can use functionalities, which ArrayList has, right?

Unknown User•11mo ago

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bambyzasOP•11mo ago

i know what polymorphism and inheritance is

Unknown User•11mo ago

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bambyzasOP•11mo ago

wdym?

Unknown User•11mo ago

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bambyzasOP•11mo ago

inheritance - you have a parent class and you can create child class. And child then can receive methods/fields from its parent class. if Animal has eat(), Dog will also have eat(). polymorphism - has two forms. method overloading and overriding. overriding is if in Animal has eat(){yum yum} and Dog overrides this method and it becomes eat(){nom nom}. overloading is having the same name of method but different signature (return type, params type, params numbers). i can have void addition(int a, int b) and void addition (double a, double b) so ur speculation that idk what polymorphism and inheritance is wrong

Unknown User•11mo ago

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bambyzasOP•11mo ago

huh?

Unknown User•11mo ago

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bambyzasOP•11mo ago

doesnt matter. Animal a=new Dog(); means that var a can have access to Dog class fields and methods. bc a var point to frickin Dog object

Unknown User•11mo ago

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bambyzasOP•11mo ago

bambyzasOP•11mo ago

yes

Unknown User•11mo ago

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bambyzasOP•11mo ago

ok. then ill ask it from another angle. these two are valid: Animal animal=new Dog(); Dog dog=new Dog(); so are there any cases where id need to use animal var, that has less methods/fields?

Unknown User•11mo ago

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bambyzasOP•11mo ago

for example?

Unknown User•11mo ago

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bambyzasOP•11mo ago

but vice versta its compatible, right?

Unknown User•11mo ago

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bambyzasOP•11mo ago

All dogs are animals, but not all animals are dogs crap

Unknown User•11mo ago

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bambyzasOP•11mo ago

but i wasnt talking about generics yet

Unknown User•11mo ago

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bambyzasOP•11mo ago

nope

tjoener•11mo ago

Here we go with the full blown statement: Generics in Java are invariant (a pet peeve of mine). So what that means is that Java doesn't know if you're putting something in, or taking something out of a generic. If I had a List<Dog> I can add only dogs to it, but only get dogs out. If I have a list<animal> I can add dogs and cats, but only get animals out So a List<Dog> is not a List<Animal> So if I were to assign a List<Dog> to a List<Animal> all of a sudden I can put cats in there, which I shouldn't be able to do

dan1st•11mo ago

In case you want to read more about that: https://stackoverflow.com/q/2745265/10871900

Stack Overflow

Is List a subclass of List? Why are Java generics not implicitly po...

I'm a bit confused about how Java generics handle inheritance / polymorphism. Assume the following hierarchy - Animal (Parent) Dog - Cat (Children) So suppose I have a method doSomething(List<

tjoener•11mo ago

I miss in/out generics 😐

dan1st•11mo ago

just do extends/super when necessary

tjoener•11mo ago

It works (mostly) but it's a crutch

bambyzasOP•11mo ago

holy cow. thanks

JavaBot•11mo ago

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bambyzasOP•11mo ago

@dan1st | Daniel but wait. i dont understand the issue here.

List<Dog> dogs = new ArrayList<Dog>(); 
List<Animal> animals = dogs; 
animals.add(new Cat());
Dog dog = dogs.get(0);

List<Dog> dogs = new ArrayList<Dog>(); 
List<Animal> animals = dogs; 
animals.add(new Cat());
Dog dog = dogs.get(0);

dan1st•11mo ago

you put a cat in the list and then you do Dog dog = dogs.get(0); so you take out the cat and assume it's a dog

bambyzasOP•11mo ago

the first two lines mean this?

dan1st•11mo ago

the first line means: create a list of dogs The second one means: assume it's a list of animals (which is the issue)

bambyzasOP•11mo ago

but they both point to arraylist of dogs

dan1st•11mo ago

it's the same list but in one variable, you use it as a list of dogs which is fine but in the other one, you use it as a list of animals which isn't ok because you can use it to insert a cat into the list of dogs and if you have a list of dogs, you really don't want a cat in there

bambyzasOP•11mo ago

List<Dog> dogs = new ArrayList<Dog>(); 
List<Animal> animals = dogs;

List<Dog> dogs = new ArrayList<Dog>(); 
List<Animal> animals = dogs;

is the same as :

List<Animal> animals =new ArrayList<Dog>();

List<Animal> animals =new ArrayList<Dog>();

right?

dan1st•11mo ago

Neitheŕ of them is allowed

bambyzasOP•11mo ago

wouldnt jvm do casting or smth like that under the hood? bc in my notes i have this fragment from the book:

dan1st•11mo ago

int/double is one thing but it doesn't work with generics

bambyzasOP•11mo ago

but i dont use generics. theres no <T> stuff

dan1st•11mo ago

the List<Cat>/List<Dog> is about generics

tjoener•11mo ago

Yes, people have explained this in your last 4 questions

bambyzasOP•11mo ago

again. i didnt use <T> syntax anywhere

Unknown User•11mo ago

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bambyzasOP•11mo ago

But if i do List<String> thats also generics?

Unknown User•11mo ago

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bambyzasOP•11mo ago

but if i dont use objects. then its not generics, like List<int>?

Unknown User•11mo ago

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dan1st•11mo ago

(which is using generics)

bambyzasOP•11mo ago

one last question fellas. in java theres List interface and ArrayList class. as we know, if u go up the inheritance hierarchy, u lose functionality/methods/fields. so List<String>stringList=new ArrayList<>(): seems like a less capable solution. but for some reason its used more often than ArrayList<String>stringList=new ArrayList<>();. why is that? why use less capable/powerful solution?

Unknown User•11mo ago

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bambyzasOP•11mo ago

you misunderstand my question on purpose

tjoener•11mo ago

He might but that stack overflow answer is exactly why

dan1st•11mo ago

that explains exactly what you just asked

bambyzasOP•11mo ago

i know what does programming to an interface means but it doenst answer my question and you keep telling me that it does

dan1st•11mo ago

it's the reason you use List for the declaration instead of ArrayList

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bambyzasOP•11mo ago

again, i said that you lose some functinality

dan1st•11mo ago

yes, that's the point of programming against the interface or a part of it you only use the functionality specified in the interface

bambyzasOP•11mo ago

this is the same logic

bambyzasOP•11mo ago

why use String, when you can use less powerful interface?

dan1st•11mo ago

depends on which functionality you need

bambyzasOP•11mo ago

doesnt matter. yet still everyone uses String class all the time

dan1st•11mo ago

if you just need a CharSequence, use that

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dan1st•11mo ago

because in many cases, you actually need a String and different CharSequences do vastly different things

bambyzasOP•11mo ago

i dont get it when using ArrayList, u need for some reason to use less capable solution? doesnt make sense oh wow. my code will be more flexible, but ill lose functionality.

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dan1st•11mo ago

Do you need functionality of ArrayList that isn't in List? if you used CharSequence instead of String, you wouldn't be able to use substring any more

bambyzasOP•11mo ago

its the same like asking do u need functionality of String that isnt in CharSequence. ofc. u always use String, instead of less capable CharSequence. yet u r telling me that I should use less capable List instead of ArrayList

dan1st•11mo ago

I need String specific functionality in most cases ArrayList doesn't provide more methods you'd need often and then there's the thing that if you have a String, you can be sure it's immutable etc there isn't a good interface for String and there's also no need for it since there are not alternative implementation of the same functionality (StringBuilder has different functionality but if you want to write something working for both, use CharSequence if possible)

bambyzasOP•11mo ago

ArrayList has 31 method. List has 28 methods . u dont know ur stuff lol

Unknown User•11mo ago

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dan1st•11mo ago

I know

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bambyzasOP•11mo ago

mhm. u cant even count lol

dan1st•11mo ago

I haven't used any of the extra ArrayList methods for quite some time

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dan1st•11mo ago

You need to use methods like ensureCapacity very rarely and most uses of it are useless/make something worse

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dan1st•11mo ago

No, that isn't necessary here

bambyzasOP•11mo ago

so if u use List, u get flexible code + basic/mostly needed functionalit. but if u use ArrayList u just get a couple of more extra methods. so that is the reasoning one would choose List over ArrayList?

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dan1st•11mo ago

When was the last time you actually needed ArrayList functionality that isn't in List?

bambyzasOP•11mo ago

huih?

dan1st•11mo ago

I think it's never because if you needed it at some point, you wouldn't say that

JavaBot•11mo ago

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why i cant access field in child class when using inheritance?