Question

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Interfaces: Why cant I seem to grasp them?

What is the purpose of interfaces in C#?
How would they enable extendable,modular design in c#,Java?

As far as my experience with the interfaces is concerned ,we used in a gridview scenario where columns values are brought from disparate objects.

(e.g:

List<IPub> list = new List<IPub>();
gridview.DataSource = list;
gridview.DataBind();

IPub has 4 methods which is implemented by 4 or 5 disparate classes.
)
What are the cases they come in handy compared to their class counterparts,apart from above?

I heard Java creator despised of interfaces or saying like “If i were a given a chance to design java again;I would never make interfaces into the language”.
Does this applies to C# as well?
What implications made him to say that ?
I am feeling like i never understood interfaces completely.
Please somebody take pains to explain.
EDIT: Here is the Goslings quote, see the Java section

Answer 1

you should see the answers here or here or here or here

An interface provides a way to use something without having to worry about how that thing is implemented.

If you have an interface ITextGetter with a method GetText(string textKey) which you are using, you don’t know if where the text comes from, all you know is that when you ask for the text with a particular key, you get the text. If you write you app using this interface then you can use different classes which all implement the interface to get the text without having to change your app. This makes it easy to switch from file based text getting to database based or webservice based text getting without having to change any of the code that uses the interface