Questions concerning an interface class

Question

I know this is a question that has been asked a 100 times over but I would like to provide some of my own definitions to see if I understand an interface correctly.

Questions:

What is an interface?

An interface defines the structure for code design. An interface lays the foundation for your design and is made up of a collection of abstract methods and contains behaviors that a class must implement.

When to use an interface?

When similar methods of a design are to be reused across a project. This creates the structure of the behavior within a a project.

Why use an interface?

You use an interface in a project to create the foundation much like the construction of a new home. When a new home is built the frame is built, then the walls and doors and so on are added.

My above answers are what I describe as an interface and would like to know whether I am correct or not? If not, then please explain?

Thanks

Answer 1

I would argue that an interface is more like a an optional contract -- you specify the method names, parameters, and return type, and if a class chooses to implement the interface, it must then conform to the terms of that contract. It's more like an API spec than a design foundation, as interfaces are relatively flexible.

Note that this flexibility gives you considerable leeway in how you choose to use interfaces. As long as the implementing class provides the agreed-upon methods, the implementation is entirely up to you. There are interfaces (e.g. Serializable) that require no methods, and simply act as markers for the programmer's intent regarding a certain class.

Another use of interfaces is to mitigate the disadvantages of the fact that Java doesn't support multiple inheritance. Though each implementing class must contain the actual code, you might use an interface in an 'inheritance-like' way, to indicate that a certain set of classes derives behavior (in name, if not necessarily in implementation) from some common, more abstract pattern.

I would definitely suggest looking through the javadocs (perhaps the Collections framework) for more examples of interfaces. To continue with the contract analogy, the main use of interfaces is as a sort of API that specifies behavior you can count on, without having to know the implementation details.

The problem with the home analogy is that it's too restrictive-- interfaces don't restrict the design of a class as much as a foundation prescribes a certain structure for a building. Additionally, a building can only have one foundation, and in Java, there is no limit to the number of interfaces a class may implement.

Answer 2

You use an interface so that any class can make use of it as long as they implement it. That is why List is so cool and we use it in everyday programming.

Cows and Goats are both animals and they eat, walk, sleep, etc. You do not want to declare the same behaviors separately. If you now have a new animal, you will have to define the similar behaviors again. An interface forces you to implement the required behaviors.

Answer 3

Think of a TV remote analogy.

What

Its a standard way of using a Television(any television...flat screen, CRT, LED, LCD, Plasma etc).

So its basically an interface to a Television. Now, all TV remotes must have some basic common buttons (On, Off, Vol+, Vol-, Ch+, Ch-), these are the methods which should be present in the interface. Different brands implement these using different techs.

When

Now all Television brands want to share this standard way of controlling a Television (which is a big complicated machine).

Why Think about it. It helps the consumer. As far as the consumer is concerned, he/she does not need to know how to manually operate the Television or from inside(you can do that from the circuit board!)