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Question 26 :

What are some alternatives to inheritance?

Answer :

Delegation is an alternative to inheritance. Delegation means that you include an instance of another class as an instance variable, and forward messages to the instance. It is often safer than inheritance because it forces you to think about each message you forward, because the instance is of a known class, rather than a new class, and because it doesn't force you to accept all the methods of the super class: you can provide only the methods that really make sense. On the other hand, it makes you write more code, and it is harder to re-use (because it is not a subclass).


Question 27 :

What does it mean that a method or field is "static"?

Answer :

Static variables and methods are instantiated only once per class. In other words they are class variables, not instance variables. If you change the value of a static variable in a particular object, the value of that variable changes for all instances of that class.
Static methods can be referenced with the name of the class rather than the name of a particular object of the class (though that works too). That's how library methods like System.out.println() work out is a static field in the java.lang.System class.


Question 28 :

What is the difference between preemptive scheduling and time slicing?

Answer :

Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.


Question 29 :

What is the catch or declare rule for method declarations?

Answer :

If a checked exception may be thrown within the body of a method, the method must either catch the exception or declare it in its throws clause.


Question 30 :

Is Empty .java file a valid source file?

Answer :

Yes, an empty .java file is a perfectly valid source file.