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Core Java Interview Questions and Answers

Ques 246. What is the difference between Serializalble and Externalizable interface?

Ans. When you use Serializable interface, your class is serialized automatically by default. But you can override writeObject() and readObject() two methods to control more complex object serailization process. When you use Externalizable interface, you have a complete control over your class's serialization process.

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Ques 247. Which containers use a border layout as their default layout?
Ans. The Window, Frame and Dialog classes use a border layout as their default layout.
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Ques 248. When should I use abstract methods?
Ans. Abstract methods are usually declared where two or more subclasses are expected to fulfil a similar role in different ways. Often the subclasses are required to the fulfil an interface, so the abstract superclass might provide several of the interface methods, but leave the subclasses to implement their own variations of the abstract methods. Abstract classes can be thought of as part-complete templates that make it easier to write a series of subclasses.

For example, if you were developing an application for working with different types of documents, you might have a Document interface that each document must fulfil. You might then create an AbstractDocument that provides concrete openFile() and closeFile() methods, but declares an abstract displayDocument(JPanel) method. Then separate LetterDocument, StatementDocument or InvoiceDocument types would only have to implement their own version of displayDocument(JPanel) to fulfil the Document interface.
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Ques 249. Why use an abstract class instead of an interface?
Ans. The main reason to use an abstract class rather than an interface is because abstract classes can carry a functional "payload" that numerous subclasses can inherit and use directly. Interfaces can define the same abstract methods as an abstract class but cannot include any concrete methods.

In a real program it is not a question of whether abstract classes or interfaces are better, because both have features that are useful. It is common to use a mixture of interface and abstract classes to create a flexible and efficient class hierarchy that introduces concrete methods in layers. In practical terms it is more a question of the appropriate point in the hierarchy to define "empty" abstract methods, concrete methods and combine them through the extends and implements keywords.

The example below compares a "Spectrum" type defined by an interface and an abstract class and shows how the abstract class can provide protected methods that minimise the implementation requirements in its subclasses.
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Ques 250. In which case, do we need abstract classes with no abstract methods?
Ans. An abstract class without any abstract methods should be a rare thing and you should always question your application design if this case arises. Normally you should refactor to use a concrete superclass in this scenario.

One specific case where abstract class may justifiably have no abstract methods is where it partially implements an interface, with the intention that its subclasses must complete the interface. 
Example: To take a slightly contrived motoring analogy, a Chassis class may partially implement a Vehicle interface and provide a set of core methods from which a range of concrete Vehicle types are extended. Chassis is not a viable implementation of a Vehicle in its own right, so a concrete Car subclass would have to implement interface methods for functional wheels, engine and bodywork.
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