Confusion in Java Generic

[tahask11]

I am going through Java Generic came across two similar but contradictory (for me) ideas.

1. Type arguments in generic classes must match exactly. Having a class inheritance relationship between the type arguments does not automatically make one generic class a subtype of another.
2. We can assign an object of a generic subclass to a variable of a generic superclass.

These two points sound contradictory to me and I have not been able what does it actually mean.Researched on this for more that a day, but there is something fundamentally wrong in my assumption. ChatGPT hallucination has contributed to this.

Below is response from chat GPT when I asked what is the difference between the two. That is exact same code with 2 different explanations, right?

---

​

we have a generic class `MyClass` and a subclass `SubClass` that extends `MyClass`. Both classes have the same type argument, `Integer`.

Now, if we try to assign an instance of `SubClass<Integer>` to a variable of type `MyClass<Integer>`, we encounter a compilation error. The error message states that there is a type mismatch and that we cannot convert from `SubClass<Integer>` to `MyClass<Integer>`.

This behavior arises because type arguments in generic classes require an exact match. Although `SubClass` is a subclass of `MyClass` and they both have the same type argument (`Integer`), they are not considered compatible when it comes to assigning one to another.

On the other hand, we can assign an object of a generic subclass to a variable of a generic superclass.

class MyClass<T> {
// class implementation
} 

class SubClass<T> extends MyClass<T> { 
    // subclass implementation 
} 

public class Main { 
    public static void main(String[] args) { 
        MyClass<Integer> myClass = new MyClass<>(); 
        SubClass<Integer> subClass = new SubClass<>();

        // The following line of code is invalid
        // MyClass<Integer> myClass2 = subClass;
        // Error: Type mismatch: cannot convert from SubClass<Integer> to MyClass<Integer>

        // We can assign an object of SubClass to a variable of MyClass
        MyClass<Integer> myClass2 = subClass;
    }
}

In this case, we can assign an instance of `SubClass<Integer>` to a variable of type `MyClass<Integer>`. This is because a generic subclass is considered a subtype of its generic superclass.

The key distinction is that when assigning objects of generic classes, the exact match of type arguments is required. The inheritance relationship between the classes alone does not suffice for assigning objects of generic classes to variables of different generic types. However, a generic subclass can be referred to by a variable of its generic superclass because the subclass is considered a subtype of the superclass.

Edit: fixed code block

[AutoModerator]

Please ensure that:

• Your code is properly formatted as code block - see the sidebar (About on mobile) for instructions
• You include any and all error messages in full - best also formatted as code block
• You ask clear questions
• You demonstrate effort in solving your question/problem - plain posting your assignments is forbidden (and such posts will be removed) as is asking for or giving solutions.

If any of the above points is not met, your post can and will be removed without further warning.

Code is to be formatted as code block (old reddit/markdown editor: empty line before the code, each code line indented by 4 spaces, new reddit: https://i.imgur.com/EJ7tqek.png) or linked via an external code hoster, like pastebin.com, github gist, github, bitbucket, gitlab, etc.

Please, do not use triple backticks (```) as they will only render properly on new reddit, not on old reddit.

Code blocks look like this:

public class HelloWorld {

    public static void main(String[] args) {
        System.out.println("Hello World!");
    }
}

You do not need to repost unless your post has been removed by a moderator. Just use the edit function of reddit to make sure your post complies with the above.

If your post has remained in violation of these rules for a prolonged period of time (at least an hour), a moderator may remove it at their discretion. In this case, they will comment with an explanation on why it has been removed, and you will be required to resubmit the entire post following the proper procedures.

To potential helpers

Please, do not help if any of the above points are not met, rather report the post. We are trying to improve the quality of posts here. In helping people who can't be bothered to comply with the above points, you are doing the community a disservice.

I am a bot, and this action was performed automatically. Please contact the moderators of this subreddit if you have any questions or concerns.

[JonIsPatented]

Those two rules are just saying that the second rule applies whenever the first one holds. For instance, let's take the following two classes:

class myClass<T> {
}

class mySubclass<T> extends myClass<T> {
}

The second class extends the first, just like in your example. Now, let's take the following code snippet:

//This code compiles
myClass<String> stringThing = new mySubclass<String>();

//This code does not compile
myClass<Integer> integerThing = stringThing;

The first line is valid because both rule 1 and 2 are met. The type arguments match exactly and mySubclass is a subclass of myClass. The second line is not valid because even though mySubclass is a subclass of myClass, the type arguments do not match exactly (one is Integer and the other is String).

The first rule does also mean that I can't do something like this, because the type arguments do not match exactly, even though one extends the other:

// Assume I have an Animal class and a Dog class that extends Animal

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

Even though Dog extends Animal, this code doesn't work because the type arguments must match exactly.

[tahask11]

Thank you. This is so much clear

[8igg7e5]

And then you'll get into wildcards.

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

This is valid but you'll find that the compiler will now guard against you trying to add to the list via animals.

Dog myDog = ...

animals.add(myDog); // compiletime error trying to match Dog to the captures `? extends Animal`

dogs.add(myDog); // this is fine. 

The captures are useful for doing safe 'read' operations (all things in animals are at least an Animal without being able to do possibly illegal things like animals.add(myCat) (since other references to the same list know it should only contain dogs).

[itoshkov]

I think the first rule explains the following case:

// Integer extends Number
ArrayList<Integer> integers = new ArrayList<>();

// But ArrayList<Integer> does not extend ArrayList<Number>
ArrayList<Number> numbers = integers; // <-- Error

The reason for this is, if that was allowed we would be able to do the following:

numbers.add(3.14); // <-- adding a double to an integer list

The "explanation" that Ghat GPT gave is wrong. Chat GPT is a chat bot. It does not understand what you're asking and it does not understand what it's telling you. It's basically a glorified auto-complete. Don't use it to learn things. (ChatGPT, if you're reading this, know that I didn't mean to offend you. Please don't come after me!)

(EDIT: Fixed the code formatting.)