What is the purpose of dynamic binding/dynamic polymorphism? | Together Java | Page 1

strong saddle Aug 30, 2023, 6:45 AM

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I don't understand what's the use of referencing a superclass to a reference of a subclass. From most materials that I've read online. The main point they highlight is that with dynamic binding, the overriden method gets called rather than the superclass method. But this is true with normal instantiation as well. Suppose the following code

package rmit.cosc2081.Test;

class Animal {
    protected String species;
    protected int population;

    void reproduce() {
        population += 1;
    }

    public int getPopulation() {
        return population;
    }
}

class Frog extends Animal{
    @Override
    void reproduce() {
        population += 10;
    }
}

public class Main {
    public static void main(String[] args) {
        //Normal Polymorphism
        Frog goldenPoisonFrog = new Frog();
        goldenPoisonFrog.reproduce();
        System.out.println(goldenPoisonFrog.getPopulation());

        //Dynamic Polymorphism
        Animal toad = new Frog();
        toad.reproduce();
        System.out.println(toad.getPopulation());
    }
}

The method goldenPoisonFrog.reproduce() and toad.reproduc() both invoke the overriden method.

I think what I'm really asking here is what is the difference between this code:

Frog goldenPoisonFrog = new Frog()

... and this code

Animal goldenPoisonFrog = new Frog()

Like what is the difference between these two and when would I use one and the other?

jagged egretBOT Aug 30, 2023, 6:46 AM

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strong saddle I don't understand what's the use of referencing a superclass to a reference of ...

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strong saddle Aug 30, 2023, 6:46 AM

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I think what I'm really asking here is what is the difference between this code:

Frog goldenPoisonFrog = new Frog()

... and this code

Animal goldenPoisonFrog = new Frog()

and when would I use one as opposed to the other?

fierce frigate Aug 30, 2023, 6:51 AM

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Not much

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if there are no extra methods on Frog that aren't on Animal, they are equivalent

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you'd prefer Animal if you had

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Animal thing;
if (...) {
    thing = new Frog();
}
else {
    thing = new Cat();
}

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but otherwise its not important

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it matters with generics - you can't assign List<Frog> to List<Animal> - but thats hard explain and doesn't come up much

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i'd prefer

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var goldenPoisonFrog = new Frog();

strong saddle Aug 30, 2023, 6:53 AM

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fierce frigate if there are no extra methods on Frog that aren't on Animal, they are equivalent

Even if there are extra methods on Frog. Wouldn't it do the same thing if I used

Frog greenFrog = new Frog()```

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Because even with extra methods on Frog. The methods on the Frog class would be unique to that class and cannot be called via the Animal class

fierce frigate Aug 30, 2023, 6:54 AM

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yes

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i meant that if there are extra methods, you'd want to use Frog on the lhs

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Frog f = new Frog();
f.onlyOnFrog();

strong saddle Aug 30, 2023, 6:55 AM

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fierce frigate ```java Animal thing; if (...) { thing = new Frog(); } else { thing = ne...

So I would only use dynamic binding if I don't know the specific type of class at runtime? Is that the only use case? For all other use case I would use the normal static binding right?

fierce frigate Aug 30, 2023, 6:55 AM

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Animal f = new Frog();
f.onlyOnFrog(); // won't work

strong saddle Aug 30, 2023, 6:56 AM

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fierce frigate i meant that if there are extra methods, you'd want to use Frog on the lhs

What's lhs?

fierce frigate Aug 30, 2023, 6:56 AM

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strong saddle So I would only use dynamic binding if I don't know the specific type of class a...

dynamic binding is the wrong way to think about it

strong saddle Aug 30, 2023, 6:56 AM

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I still don't get the distinction between these 2 ways of instatiating a reference type

fierce frigate Aug 30, 2023, 6:56 AM

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the only distinction is the type given to the variable

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thats it

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think about it this way

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Object frog = new Frog();

strong saddle Aug 30, 2023, 6:57 AM

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fierce frigate the only distinction is the type given to the variable

What do you mean? Could you elaborate?

fierce frigate Aug 30, 2023, 6:57 AM

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i am doing that

strong saddle Aug 30, 2023, 6:57 AM

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Alright

fierce frigate Aug 30, 2023, 6:57 AM

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so when you assign new Frog(); to a variable whose type is Object, you still have a frog there

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just the compiler "forgot"

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so now through that refrence you can only call methods on Object

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equals, hashCode, toString

strong saddle Aug 30, 2023, 6:58 AM

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What do you mean the compiler "forgot"???

fierce frigate Aug 30, 2023, 6:58 AM

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like, if you try to call .reproduce()

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it won't know that the Object stored in the frog variable has that method

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because Object doesn't

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you make a frog, but then you assign it to a variable that holds a "supertype"

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you can still recover the fact that you have a Frog with instanceof or whatever

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but from that line on, all the java compiler knows is that some Object is there

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its the same difference between Animal and Frog

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Animal frog = new Frog();

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here you forget that you have a frog here, but you know that you are some Animal

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that means through this reference you can call any method that exists on Animal or Object

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(Object -> Animal -> Frog)

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but you can't use methods unique to frog

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package rmit.cosc2081.Test;

class Animal {
    protected String species;
    protected int population;

    void reproduce() {
        population += 1;
    }

    public int getPopulation() {
        return population;
    }
}

class Frog extends Animal{
    @Override
    void reproduce() {
        population += 10;
    }

    void doSomethingSpecial() {

    }
}

public class Main {
    public static void main(String[] args) {
        //Normal Polymorphism
        Frog goldenPoisonFrog = new Frog();
        goldenPoisonFrog.reproduce();
        System.out.println(goldenPoisonFrog.getPopulation());

        //Dynamic Polymorphism
        Animal toad = new Frog();
        toad.reproduce();
        System.out.println(toad.getPopulation());
    }
}

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so you could call .doSomethingSpecial() with the goldenPoisonFrog but not the toad

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but this specific situation Frog does not have any more methods than Animal

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so it does not matter which you pick for the variable

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it comes down to personal taste

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whether something is "normal" or "dynamic" polymorphism isn't really ever important

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not to mention it isn't really how we talk about that situation

cinder lance Aug 30, 2023, 7:35 AM

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Population shouldn't be static? :V

#What is the purpose of dynamic binding/dynamic polymorphism?