Interface question in java

Hi
i have a question regarding interface

interface Igroup {

Igroup getAllGroups();


}

if i want to implement this interface i need to create new class and add
an implemenation to the getAllGroups()
i take this method without changing it;s signiture.

Class GroupImpl implements Igroup {

String name;

//define link list of the type Igroup.
LinkList<Igroup> group;


GroupImpl (String name) {

this.name=name;
this.group= new LinkList<Igroup>();

}


Igroup getAllGroups(){

return this.group;

}


My question is what is the idea behind defining the member of GroupImpl
as Igroup type.
why i can;t define it like this


Class GroupImpl implements Igroup {
String name;
LinkList<GroupImpl> group;
GroupImpl (String name) {
this.name=name;
this.group= new LinkList<GroupImpl>();
}

Igroup getAllGroups(){
return this.group;

}
this is not working because getAllGroups() expected Igroup type not GroupImpl
thougth GroupImpl implements Igroup.

what is the idea behind the concept of OOD to use the interface itself and not using the
son (that implements this interface)

Best Regards

Hi, I am still learning myself, but I will try and explain it to you.

Coding to an interface you can know in the future you will be able to deal with objects that implement that interface, as they will have at least implemented the method declared. I will try to give an example to explain:

Code:

---

//Here declare a List reference (interface) to an ArrayList
List arrayList = new ArrayList();

---

In the above code you can see arrayList is a list object, but at compile time it is a List reference so the only methods you can call on arrayList are the List methods that it has overridden as at compile time the compiler doesnt know that this might be an ArrayList object.

If you declare a method that takes as an input parameter a List object then you know that your method will always be able to take a future object that implements List (one that doesnt even exist right now) and your method will always be able to call those List methods.

I guess you need to make a decision about whether doing this is the right thing in your current situation. Sorry I cant explain it better but I expect someone else will answer.

Thought I would just add something about your example above, basically the top example declares the list objects type as the interface, this means ANY object that implements Igroup can be put in this list. In the second example it is not as flexible, basically you are saying ONLY GroupImpl objects can be placed into the list. This maybe what you want.

If you had a setter method for instance, your version would only ever allow people to pass in GroupImpl to set to the list because thats the only object the list can hold, but if you make the method take a type Igroup, and make the list take objects of type Igroup then anyone in future can use your method, all they have to do is implement Igroup. Your code always knows it can call the methods defined in the interface because anything coming in is an IGroup and must have implemented at least those I group methods.

The reason your method cannot work is that you say it returns a type Igroup, so anyone calling it expects to get an Igroup, right? So it wouldnt make sense to give them GroupImpl because they might not even know about it, THEIR reference that they try to assign the object returned from your method might have implemented Igroup but never extended GroupImpl.

If I remember rightly methods that are available to an object are always based on the reference type. Sorry its a bit rambling.

Unfortunately neither code example will work because the getAllGroups method is declared to return an IGroup but you're trying to return a LinkedList of IGroups from it.

The concept of OOD where you use the interface instead of the implementation of it is called 'Polymorphism'. The interface specifies a contract that the object must satisfy and the user can rely on, but different objects can satisfy it in their own way without the user having to know the details. For example, user code that needs some data might ask a DataManager class for a DataSupplier. The DataManager returns an object that implements the DataSupplier interface.The user code can call the DataSupplier methods to get the data without knowing or caring whether the object implementing the DataSupplier interface is getting the data from a database or a file or an internet connection. The DataManager decides which type of DataSupplier object to return to the user code, and the user code doesn't need to know. There are plenty of better explanations if you search online ;)

A good programming language is a conceptual universe for thinking about programming...
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