Interfaces and Abstract Classes

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Interfaces and Abstract Classes

Interfaces and Abstract Classes
Understanding contracts
Interface, Inheritance, and Composition
What is an interface?
Why discuss inheritance, interfaces, abstract classes, and composition in one module?
Interface, Inheritance, and Composition Example
Contract Enforcement using Interfaces
Java and Multiple Inheritance
Java and Multiple Inheritance, Cont.

1. Interfaces and Abstract Classes

More about
- Interface
- Inheritance
- Composition

2. Understanding contracts

Contract is an interface: what is to be done, not how.
Interface is like a class with a set of public operations but no attributes and no implementation.
A UML diagram of a Java interface:


public interface Nameable {
    String getName();
    void setName (String aName);
}

Compare to:


public abstract class Nameable
{
    public abstract  String getName();
    public abstract void setName (String aName);

}

3. Interface, Inheritance, and Composition

A UML example of the interface, inheritance, and composition:

4. What is an interface?

An interface is similar to a class, except...
1. interface has no attributes
2. interface has only method declarations (headers) without implementation.
We already know that...
- ...a class in Java extends from abstract super class.
Similarly,
- a class in Java implements an interface.

5. Why discuss inheritance, interfaces, abstract classes, and composition in one module?

OO programming languages provide good tools to develop reusable code.
Interfaces can only be used with inheritance relationship.
Interfaces cannot be used with a composition relationship.
Java does not support multiple inheritance of classes...
- ...Java supports multiple inheritance of interfaces.

6. Interface, Inheritance, and Composition Example

Inheritance: Dog is-a Mammal.
Interface: Dog implements Nameable.
Composition: Dog has-a Head.
Note that Nameable interface could also be implemented by a Planet class, a Car class, etc.

UML example of the interface, inheritance, and composition relationships, nameable dog:

7. Contract Enforcement using Interfaces

Any class that implements an interface must also implement concrete code for each method listed in the interface.
If class fails to implement a method declared in the interface, the class will not compile...
- ...because in Java the interface contract must be enforced.

8. Java and Multiple Inheritance

Multiple inheritance can be very useful in some cases.
Java uses multiple inheritance in a very limited way, only with interfaces.
Interfaces come with some serious limitations:
- Methods cannot have default implementations, thus forcing the programmer to implement each method again and again in each class that implements the interface.
- This often forces programmers to repeat code over and over again.

9. Java and Multiple Inheritance, Cont.

For example,
- You found a suitable interface in existing Java software: it already has 10 functions.
- Of these 10 functions you may need only 3. However, you are forced to implement all of the functions simply because Java forces you to do so.
- Solution 1: keep the interfaces small.
- Impractical solution 2: hope that one day Java will have default method implementation...