Frequently asked java interview questions - Part3

Frequently asked important java interview questions

Q:	How does a try statement determine which catch clause should be used to handle an exception?
A:	When an exception is thrown within the body of a try statement, the catch clauses of the try statement are examined in the order in which they appear. The first catch clause that is capable of handling the exceptionis executed. The remaining catch clauses are ignored.

Q:	Can an unreachable object become reachable again?
A:	An unreachable object may become reachable again. This can happen when the object's finalize() method is invoked and the object performs an operation which causes it to become accessible to reachable objects.

Q:	What method must be implemented by all threads?
A:	All tasks must implement the run() method, whether they are a subclass of Thread or implement the Runnable interface.

Q:	What are synchronized methods and synchronized statements?
A:	Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.

Q:	What is Externalizable?
A:	Externalizable is an Interface that extends Serializable Interface. And sends data into Streams in Compressed Format. It has two methods, writeExternal(ObjectOuput out) and readExternal(ObjectInput in)

Q:	What modifiers are allowed for methods in an Interface?
A:	Only public and abstract modifiers are allowed for methods in interfaces.

Q:	What are some alternatives to inheritance?
A:	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).

Q:	What does it mean that a method or field is "static"?
A:	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.

Q:	What is the difference between preemptive scheduling and time slicing?
A:	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.

Q:	What is the catch or declare rule for method declarations?
A:	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.

Q:	Is Empty .java file a valid source file?
A:	Yes, an empty .java file is a perfectly valid source file.

Q:	Can a .java file contain more than one java classes?
A:	Yes, a .java file contain more than one java classes, provided at the most one of them is a public class.

Q:	Is String a primitive data type in Java?
A:	No String is not a primitive data type in Java, even though it is one of the most extensively used object. Strings in Java are instances of String class defined in java.lang package.

Q:	Is main a keyword in Java?
A:	No, main is not a keyword in Java.

Q:	Is next a keyword in Java?
A:	No, next is not a keyword.

Q:	Is delete a keyword in Java?
A:	No, delete is not a keyword in Java. Java does not make use of explicit destructors the way C++ does.

Q:	Is exit a keyword in Java?
A:	No. To exit a program explicitly you use exit method in System object.