|
JAVA Interview Questions 16 - 20
|
Back to the list of all Java Interview Questions
|
|
Question 16: What is the difference between aggregation and composition?
|
| Aggregation
|
Composition
|
Aggregation is an association in which one class
belongs to a collection. This is a part of a whole
relationship where a part can exist without a whole.
For example a line item is a whole and product is a
part. If a line item is deleted then corresponding
product need not be deleted. So aggregation has a
weaker relationship.
|
Composition is an association in which one class belongs to a
collection. This is a part of a whole relationship where a part
cannot exist without a whole. If a whole is deleted then all parts are
deleted.
For example An order is a whole and line items are parts.
If an order is deleted then all corresponding line items for that
order should be deleted. So composition has a stronger
relationship.
|
|
|
Question 17: What is the difference between an abstract class and an interface and when should you use them?
|
In design, you want the base class to present only an interface for its derived classes. This means, you don’t want
anyone to actually instantiate an object of the base class. You only want to upcast to it (implicit upcasting, which
gives you polymorphic behavior), so that its interface can be used. This is accomplished by making that class
abstract using the abstract keyword. If anyone tries to make an object of an abstract class, the compiler prevents
it.
The interface keyword takes this concept of an abstract class a step further by preventing any method or function
implementation at all. You can only declare a method or function but not provide the implementation. The class,
which is implementing the interface, should provide the actual implementation. The interface is a very useful and
commonly used aspect in OO design, as it provides the separation of interface and implementation and
enables you to:
- Capture similarities among unrelated classes without artificially forcing a class relationship.
- Declare methods that one or more classes are expected to implement.
- Reveal an object's programming interface without revealing its actual implementation.
- Model multiple interface inheritance in Java, which provides some of the benefits of full on multiple
inheritances, a feature that some object-oriented languages support that allow a class to have more than one
superclass.
|
Abstract class
|
Interface
|
|
Have executable methods and abstract methods.
|
Have no implementation code. All methods are abstract.
|
|
Can only subclass one abstract class.
|
A class can implement any number of interfaces.
|
|
|
Question 18: When to use an abstract class?
|
In case where you want to use implementation inheritance then it is
usually provided by an abstract base class. Abstract classes are excellent candidates inside of application
frameworks. Abstract classes
let you define some default behavior and force subclasses to provide any specific
behavior. Care should be taken not to overuse implementation inheritance.
|
|
Question 19: When to use an interface?
|
For polymorphic interface inheritance, where the client wants to only deal with a
type and does not care about the actual implementation, use
interfaces. If you need to change your design
frequently, you should prefer using interface to abstract. Coding to an interface
reduces coupling and interface inheritance can achieve code reuse with the help of object composition.
For example: The Spring
framework’s dependency injection promotes code to an interface principle. Another justification for using interfaces
is that they solve the ‘diamond problem’ of traditional multiple inheritance as shown in the figure. Java does not
support multiple inheritance. Java only supports multiple interface inheritance. Interface will solve all the
ambiguities caused by this ‘diamond problem’.
Design pattern: Strategy design pattern lets you swap new algorithms and processes into your program without
altering the objects that use them.
|
|
Question 20: Why there are some interfaces with no defined methods (i.e. marker interfaces) in Java?
|
The interfaces with no defined methods act like markers.
They just tell the compiler that the objects of the classes
implementing the interfaces with no defined methods need to be treated differently.
Example java.io.Serializable, java.lang.Cloneable, java.util.EventListener etc.
Marker interfaces are also known as
“tag” interfaces since they tag all the derived classes into a category based on their purpose.
|
Back to the list of all Java Interview Questions
|
