class Foo {
static int x = 5;
}
public class Bar {
public static void main(String[] args) {
Bar bar = new Bar();
bar.run();
}
public void run() {
this.what(Foo);
}
public void what(Class<Foo> cls) {
System.out.println(cls.x);
}
}
``` I have this code and (coming from Python) I expected this to print '5', but this does not seem to work in Java and im not sure how I would make this work, I think its pretty clear what im trying to do from the code but if not please ask me its hard to put this in words tbh#Some basic java help
echo token
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echo token
I basically just wanna pass the class as a param and access static variables from that class
I assumed this would be possible since they are static
merry edge
I basically just wanna pass the class as a param and access static variables fro...
classes, types, functions aren't first-class members in java. you can't pass them around.
Class is used for reflection, it doesn't do what you want it to. you could technically access x from it, but again, that's reflection, it's not a mechanic of java.
in a python equivalent, Foo would still not be used as a class, but as an arbitrary object.
try reading about oop
echo token
Yeah I understand OOP
Context for my use case: im not able to create a class instance, I need to access some static variables from said class to create an instance
I understand here you could use Foo.x but in my use case Foo is a superclass that’s a shell for a lot of other classes
merry edge
well.. you can just access those static variables from said class directly
so you want to inherit from Foo?
echo token
Foo inside of Class<Foo> would be a subclass of Foo
merry edge
it wouldn't
echo token
Are there any alternatives for what I’m trying to do
merry edge
/run
class X { public static void main(String[] args) {
Class<Object> cls = String.class;
}}
snow ingotBOT
@merry edge I only received java(15.0.2) error output
file0.code.java:2: error: incompatible types: Class<String> cannot be converted to Class<Object>
Class<Object> cls = String.class;
^
1 error
error: compilation failed
i have no idea what you're trying to do.
echo token
i have no idea what you're trying to do.
I can share a shell of my actual use case in a couple minutes
merry edge
honestly, it sounds like xyproblem
merry edge
I can share a shell of my actual use case in a couple minutes
please do, your description here is pretty vague
echo token
Like I said kind of hard to explain
I can share working python code aswell of the thing I’m trying to do
merry edge
to me, it sounds like you're trying to design something dynamic, which python would do well, but java is a static type system, so it just falls apart in java
so it's an issue of design
echo token
Not at my pc atm tho, give me 15 minutes
merry edge
I can share working python code aswell of the thing I’m trying to do
sure
(please strip it down to just the relevant part though)
echo token
from typing import (
ClassVar,
)
class Super:
x: ClassVar[int]
class Sub1(Super):
x = 1
class Sub2(Super):
x = 2
class Sub3(Super):
x = 3
def foo(sub: type[Super]) -> None:
print(sub.x)
foo(Sub1)
foo(Sub2)
foo(Sub3)
@merry edge I guess that would be it
Id have a subclass, any of Sub1, Sub2, Sub3 and when I would call a function I would be able to get static variables from said class
merry edge
why do Subn have to be classes there to begin with?
echo token
this would print 1 2 3 as expected
echo token
why do `Subn` have to be classes there to begin with?
well they have their use case but thats not really relevant
merry edge
why can't you just
class X:
def __init__(self, x: int):
self.x = x;
sub1 = X(1)
# ...
it very much is relevant
what you use something for kinda heavily influences the design.
echo token
why can't you just
```py
class X:
def __init__(self, x: int):
self.x = x;
...
this is not what I need
its a class var
never would change
it is a class which ALWAYS has some variables, in my example x and an init with other params that can differ
x would never chnage
merry edge
python doesn't have a way to denote things that don't change
echo token
thats why I type hinted it as a class var
well not during runtime but typing.ClassVar is for that
merry edge
if you have multiple classes each with their own class var, what's the difference between that and multiple instances each with their own instance var?
echo token
honestly i cant explain it better than how I have
merry edge
it is a class which ALWAYS has some variables, in my example `x` and an init wit...
you didn't show that in your example, so i don't know what you mean by that. do you mean that you're trying to use Super as an interface?
echo token
you didn't show that in your example, so i don't know what you mean by that. do ...
I mean sort of but as I understood interfaces can only define method shells
merry edge
I mean sort of but as I understood interfaces can only define method shells
don't go there yet; we're still talking about design, not specifically java.
you stripped too much out of your description, you left out the motivation behind your python implementation
again; why can't they be instances?
echo token
because to create said instance I would need to use said x
which differs per sub class
a hashmap would work but I think this would be way cleaner
my acutal use case:
class Animal {
public static int number;
public static HomeType desiredHomeType;
public static Diet diet;
public final String name;
public final Home home;
/**
* .
*/
public Animal(
String name,
Home home
) {
this.name = name;
this.home = home;
}
public static final Class<Animal> classFromNumber(int number) {
return Animal.class;
}
}
final class Lion extends Animal {
public final int number = 1;
public final HomeType desiredHomeType = HomeType.BOTH;
public final Diet diet = new Diet(
DietType.CARNIVORE
);
public Lion(
String name,
Home home
) {
super(name, home);
}
}
final class Tiger extends Animal {
public final int number = 2;
public final HomeType desiredHomeType = HomeType.BOTH;
public final Diet diet = new Diet(
DietType.CARNIVORE
);
public Tiger(
String name,
Home home
) {
super(name, home);
}
}
final class Leopard extends Animal {
public final int number = 3;
public final HomeType desiredHomeType = HomeType.BOTH;
public final Diet diet = new Diet(
DietType.CARNIVORE
);
public Leopard(
String name,
Home home
) {
super(name, home);
}
}```there are a lot more animals, its for homework
merry edge
because to create said instance I would need to use said `x`
that's... not what im talking about though
no, the instance itself defines x.
merry edge
instead of having Subn extend Super, they can be instances of Super
echo token
I need to use desiredHomeType to check if there is a valid Home
merry edge
why can't you just
```py
class X:
def __init__(self, x: int):
self.x = x;
...
like i showed here
echo token
in that example they can but thats not my point
echo token
```java
class Animal {
public static int number;
public static HomeType ...
in here they cannot be instances
merry edge
in that example they can but thats not my point
then you made a bad example that doesn't show your case well
echo token
because a Lion would never have a differnent Diet
and I dont wanna have to pass that in as a param when I create a Lion
merry edge
in here they cannot be instances
i mean, they could. seriously.
echo token
I dont really see how
echo token
then you made a bad example that doesn't show your case well
I made a minimal example like you asked lol
merry edge
because a Lion would never have a differnent Diet
yeah, but you wouldn't have a definition of lion there. the definition of lion comes with the definition of the diet
merry edge
I made a minimal example like you asked lol
no, again, you stripped out too much
"minimal" is not least code, minimal is least code to explain your issue
echo token
honestly if you have a better way to do it wiht my actual use case (the java code I just sent) im all ears
merry edge
if you strip out a part that causes problems, how are we supposed to identify the problems we can't see?
echo token
the python code worked fine
perfectly as expected
I wanna (sort of) replicate that in java
merry edge
honestly if you have a better way to do it wiht my actual use case (the java cod...
i'll get to that in a sec; but i think you are misunderstanding how instances work
merry edge
Animal lion = new Animal(DietType.Carnivore); // lion will always have the same diet
python OOP is pretty far from any other language's OOP.
echo token
I dont want the user to have to specify that a lion will have a carnivore diet
merry edge
and the user doesn't have to do that.
echo token
beacuse frankly a Lion in my code will always have that
merry edge
just like the user doesn't have to define the class Lion. you can do that for them.
merry edge
beacuse frankly a Lion in my code will always have that
then you can define it to be such.
echo token
then you can define it to be such.
well thats what im trying to do
merry edge
and i just told you you can do it with no issue
just that it wouldn't be constructible as its own class, since it wouldn't be a class there.
echo token
youve told me but not shown me
merry edge
i literally did show you
echo token
I have this function
public boolean addAnimal(
Class<Animal> animalClass,
String name,
HomeType homeType
) {
if (this.nameExists(name)) {
return false;
}
Home home = this.findHome(animalClass, homeType);
if (home == null) {
return false;
}
Animal animal = new Animal(name, home);
home.animals.add(animal);
return true;
}```which will TRY to add an animal
but will return false early if its not allowed
to hceck if there is a home for the animal, I need to access Animal.desiredHomeType
but not for the actual Animal
but for the subclass class passed into AddAnimal
merry edge
and that's why i asked if it was for defining an interface.
dude you aren't listening to me
echo token
I am
is this defining an interface? I thought interface you meant an actual interface
interface Foo {}```again; talking about design. the class Super in your python code defines something which subclasses have to implement. that's an interface.
echo token
again; talking about design. the `class Super` in your python code defines somet...
yeah well sort of then I guess
but I thought in Java interfaces were solely for methods
which mine is not
mine would be solely for variables
merry edge
but I thought in Java interfaces were solely for methods
they are
echo token
and it would need a constructor aswell
merry edge
...no, you're construing ideas
echo token
they are hard coded variables which will never change
merry edge
methods... can do that.
they don't need to be variables. they just need to be accessible
although, i think you could make them variables with an abstract class.
let me reiterate what i said before (which isn't the fix to your problem, but just an explanation) {
under the premise that the separate Sub1, etc don't actually have to be classes (since that was omitted at first)
from some superclass, you can either:
make extending classes, where each has a class variable. 1 new thing - 1 new value. (which you did.)
make instances, where each has an instance variable. 1 new thing - 1 new value. (which i was trying to explain was possible.)
}
so about your actual problem;
echo token
make instances, where each has an instance variable. 1 new thing - 1 new value. (which i was trying to explain was possible.)
I know its possible but I would like to refrain from using this
use case as per the homework assignment:
I get input as a number
1-7
merry edge
> make instances, where each has an instance variable. 1 new thing - 1 new value...
....you were completely denying it before.
echo token
based on that, I create an Animal class
merry edge
uhhhhh no?
echo token
....you were completely denying it before.
I meant in my use case its not possible
possible as in not allowed by the homework assignment
merry edge
you did not describe your use case accurately; i was trying to explain that without the part you left out, it was possible.
but we're just going in circles there so whatever
echo token
what im trying to do with this number 1-7:
some method that returns the actual class of the animal corresponding to the number:
that is: Lion, Tiger, Leopard etc. etc.
merry edge
ok, that part is not possible. at least not cleanly.
echo token
then I call addAnimal with this class and a home
home I also get with a number from input
merry edge
ok that's far besides the point now
so the core of your problem is;
you need to define a class that represents a type of animal, and specifically a class because it needs to be instantiated with a home/name.
each type of animal should have its own unchanging fields, namely diet and desiredHomeType.
am i understanding that correctly?
echo token
yes
what I was trying to do, in python terms because am way more familiar with said syntax:
merry edge
i have an implementation idea in mind, but give me a sec to see what other stuff is possible.
echo token
def get_animal_class_based_on_number(number: int) -> type[Animal]: # Never actually type[Animal], always type of a subclass of Animal
...
class HomeType(Enum):
doesnt matter
def get_home_type_based_on_number(number: int) -> HomeType:
...
def add_animal(
animal_class: type[Animal],
home_type: HomeType,
name_of_animal: str,
):
actual logic to add the animal
n1 = ...
n2 = ...
animal_cls = get_animal_class_based_on_number(n1)
home_type = get_home_type_based_on_number(n2)
add_animal(animal_cls, home_type, 'some name')
alright thanks
noble perchBOT
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If you are finished with your post, please close it.
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echo token
lol does it have some filter that checks for the word thanks
thanks
also I timer I suppose
merry edge
on cooldown i guess
ok so your python design doesn't work in normal OOP at all it seems, or at least it isn't exactly logically sound within typical OOP
but it's working in python since python is insanely dynamic
think of this; for any arbitrary instance of Animal, should diet be constant?
echo token
yes
every species of animal eats the same diet
all these variables will never cahnge
merry edge
ok i phrased that horribly sec
echo token
should I make those final aswell or is that bad?
merry edge
Animal a; Diet d;
a = getRandomAnimal();
d = a.diet;
a = getRandomAnimal();
d == a.diet ?
should any 2 arbitrary animals always have the same diet
merry edge
so should Animal.diet be static?
merry edge
but it does change, across species. you just said so.
echo token
no I mean like
every species has a differnet diet
which never changes
after the classes are defined, they never change
not sure if java caches those variables but in my understanding they get cached and should remain the same forever
merry edge
ok but say i have an animal a of species A
and an animal b of species B
and they have different diets
they're both animals. do arbitrary animals have different diets?
echo token
like I said animals from different species can have different diets
animals from the same species have the same diet
it does not change
merry edge
a static Animal.diet means that every single animal shares the same diet
so that's a contradiction
echo token
youre looking at my code
merry edge
a static `Animal.diet` means that *every single animal* shares the same diet
that's what it means to be static.
echo token
look what im saying
merry edge
im looking at both your code and your words
echo token
im not very familiar with java, tahts why im asking for help
echo token
for the third time:
every animal has the SAME DIET as animals that have the SAME SPECIES
merry edge
you're getting mixed up about every animal of a given species having the same diet, and that diet being static on the animal class
echo token
if an animal is not of the same species it does not have to have the same diet
sometimes they do, but doenst have to
merry edge
for the third time:
every animal has the SAME DIET as animals that have the S...
dude. i understand. im trying to poke you in the direction of understanding
of the same species.
Animal, is not for the same species
diet and desiredHomeType should not be static.
there, i said it.
merry edge
which doesn't work.
echo token
am too unfamiliar with static to fully understand why not tbh
I thought static meant
Class.var
instead of Class().var
so it would be a ClassVar instead of a variables of an instance
merry edge
that's the syntactic (how you use it) meaning, not the semantic (what it means) meaning
echo token
oh well I thought you meant how you use it
merry edge
semantically, a static variable is a property that is common to all instances of the class, or a property of the class itself
echo token
well in that case no
merry edge
yeah
echo token
but removing the static keyword does not make it work obviously
merry edge
now apply that logic down a level. imagine that you had finer categories.
maybe, say, class Feline extends Animal, class Cat/Lion/Tiger/Bobcat extends Feline
does every Feline necessarily share the same diet?
merry edge
that's why i said imagine
echo token
I just have plain classes that are subclasses of 1 class
merry edge
take this logically, not within the bounds of your problem
objects in OOP are meant to represent real things, thinking of the real things can help you with design
what about a broader category? do all fish necessarily have the same diet?
no, right? so down a level, the extensions of Animal, also shouldn't have static diets.
echo token
sure
merry edge
echo token
but how is this relevant though
merry edge
again, OOP models real things, so the real relationships will be represented as well
it'll click once we go along, i hope.
let's go back to Animal.
echo token
I understand OOP trust me
merry edge
each instance of Animal, which would represent a single animal, has a specific diet, right?
echo token
this is not an issue of me not understanding concepts of programming as a whole
more me not understanding syntax and behaviour of java itself
merry edge
I understand OOP trust me
...imma be honest, understanding python OOP is kinda not the same as understanding OOP everywhere else
echo token
and like I said trust me I know OOP
merry edge
but well, if you say so
merry edge
imma just skip ahead, check if there's anything unexpected?
dietis an instance field ofAnimaldesiredHomeTypeis an instance field ofAnimal- both of those are inherited to subclasses
Animalneeds some way to initializediet/desiredHomeType, via its constructor- subclasses of
Animalcan pass the specificdiet/desiredHomeTypeto the superconstructor, while propagating thehomeandnamearguments
echo token
Animal needs some way to initialize diet/desiredHomeType, via its constructor
no the opposite it needs to be hard coded in like in my example
merry edge
> Animal needs some way to initialize diet/desiredHomeType, via its constructor
...
no, the class itself.
echo token
im just trying to mimic the behaviour of Animal.diet
but then I dont have Animal
but any of my subcalsses of Animal
merry edge
it is hardcoded. it's hardcoded in the subclasses, not in Animal.
echo token
and in the code it doenst need to know which one
echo token
it is hardcoded. it's hardcoded in the *subclasses*, not in `Animal`.
yes
the constructor is the public Animal() part right?
merry edge
and in the code it doenst need to know which one
which... is the point of doing all this?
yeah
echo token
that doenst have anything to do with those variables
echo token
im talkinb about Animal not animal
the class itself
becasue I dont have the instance
merry edge
Animal.diet is a logical error.
echo token
I need the class variables to create the instance
merry edge
you do not.
echo token
public boolean addAnimal(
Class<Animal> animalClass,
String name,
HomeType homeType
) {
if (this.nameExists(name)) {
return false;
}
Home home = this.findHome(animalClass, homeType);
if (home == null) {
return false;
}
Animal animal = new Animal(name, home);
home.animals.add(animal);
return true;
}``` how would I do this without having an instanceim not sure what exactly you're trying to do there.
merry edge
is it trying to instantiate some arbitrary Animal?
merry edge
then you don't need it.
merry edge
```java
public boolean addAnimal(
Class<Animal> animalClass,
Str...
you would make one. without using Animal nor Class
classes in pythons are just functions that return nice objects
echo token
how would I do that
merry edge
you can do that in java too
echo token
I think I just dont really know how to anotate Class<Animal>
bc that isnt correct
but I dont know what is
I basically need this I guess
merry edge
a BiFunction<String, Home, ? extends Animal> could be passed.
the BiFunction is how you say in java, some function that takes 2 arguments and returns a new value
you could pass (name, home) -> new Lion(name, home) as that, which is a lambda function
echo token
??? animalClass
if (number == 1) {
animalClass = Lion
} else if (number == 2) {
animalClass = Giraffe
} etc etc```what would be the ??? in my example
merry edge
you could pass `(name, home) -> new Lion(name, home)` as that, which is a lambda...
more succinctly, you could do Lion::new, which references the constructor
merry edge
what would be the ??? in my example
nothing. classes are not first-class members.
that design does not work in java.
you can use the BiFunction to represent the constructors instead, like i showed above
echo token
would that be like a lambda
merry edge
that's what i just said
echo token
idk the exact syntax of lambdas in python mate
merry edge
....come again?
echo token
java*
merry edge
lamdba x: v?
merry edge
you could pass `(name, home) -> new Lion(name, home)` as that, which is a lambda...
ok. well i just showed you that.
merry edge
more succinctly, you could do `Lion::new`, which references the constructor
and a version that acts kinda like the class is a first-class member.
this partly depends on knowledge of java-specific stuff, namely how lambdas work in java.
echo token
a `BiFunction<String, Home, ? extends Animal>` could be passed.
the `BiFunction`...
waht would the ? be here
would a BiFunction be equivallent to Callable in python?
BiFunction<arg1, ..., argn, return>
merry edge
waht would the ? be here
the ? there is used literally, it's a wildcard.
java's generic system defines that even if T extends U, X<T> does not extend X<U>
wildcards are used to resolve this. ? extends Animal means any type can be used here, as long as it extends Animal.
further reading
merry edge
would a BiFunction be equivallent to Callable in python?
no, java's type system is static
there's a different interface for each signature; or if there isn't, make one yourself
echo token
the `?` there is used literally, it's a wildcard.
java's generic system defines ...
why would you need that instaed of just Animal
does that not allow Animal subclasses
merry edge
that's how java's type system works
it doesn't matter here, but it absolutely does elsewhere
echo token
what would be the difference?
merry edge
List<Object> allows any object to be added
if you construct a List<String>, that should only have strings, but if you then assign that to a variable of type List<Object>, suddenly you can add any object
so java does not define that List<String> extends List<Object>
echo token
Are you talking about ArrayList or is that something else
merry edge
java arrays get this wrong, actually. String[] is a subtype of Object[], and methods/indexing are bivariant, so you could do the issue performed above with no compiler error, however you'd get a runtime exception, ArrayStoreException.
merry edge
Are you talking about ArrayList or is that something else
List is an interface that ArrayList implements using an array
LinkedList also implementst that interface, using linked nodes
echo token
`List<Object>` allows any object to be added
if you construct a `List<String>`, ...
btw would that just be for method annotations
if I were to have
Animal animal = ...;```
could that also be ```java
? extends Animal animal = ...;```annotations are a specific thing in java, and this certainly isn't it. im not sure what you mean by "method annotations" here.
merry edge
```java
Animal animal = ...;```
could that also be ```java
? extends Animal anim...
no, since ? is a syntax exclusive to generics
merry edge
I mean like the type part
since java's type system is static, type hints aren't optional, so they aren't called hints/annotations, fyi
they'd be "declared types" here
merry edge
no, since `?` is a syntax exclusive to generics
and it's exclusive to generics to solve a problem exclusive to generics (not counting arrays)
echo token
no, since `?` is a syntax exclusive to generics
this doenst seem to be working in interfaces
merry edge
using types directly doesn't have this issue, since methods get overloaded and not overridden
merry edge
this doenst seem to be working in `interface`s
? is only valid when passed, not when defining generics. there's no point to defining a wildcard generic, since it's just unused
echo token
oh right
public ArrayList<? extends Animal> animals = new ArrayList<>();
public ArrayList<Animal> animals = new ArrayList<>();```
the first would be better than the latter correct?not better, just different
with the latter, any animal will be allowed to be added. you can't guarantee that with the former
but if you need to accept some unknown list, that might all be of the same subclass, and have the list declared as such, then it'd only be assignable to the former
echo token
im not sure I understand this
do you have a quick example of what would be allowed in the former and not in the latter
or the other way around
merry edge
ArrayList<? extends Animal> wildcard = new ArrayList<Lion>();
ArrayList<Animal> exact = new ArrayList<>();
wildcard.add(new Tiger()); // error
exact.add(new Tiger()); // fine
wildcard = new ArrayList<Tiger>(); // fine
exact = new ArrayList<Tiger>(); // error
what would be the difference between if you were to replace ? extends Animal with Lion in the first line
oh wait
merry edge
other subtypes of animal wouldn't be usable
red shell
```java
ArrayList<? extends Animal> wildcard = new ArrayList<Lion>();
ArrayList<...
Is there any difference between
ArrayList<? extends Animal> wildcard = new ArrayList<Lion>();```
and
```java
ArrayList<Animal> wildcard = new ArrayList<Lion>();```
Isnt saying "? extends Animal" equivelant to just "Animal"?no, and the message you're replying to demonstrates the difference. the latter is invalid.
red shell
gotcha