Java-Interview-Questions-&-Answers by BestOnlineTrainers

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Java Interview Q A

• By www.bestonlinetrainers.com

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Q11 Differentiate between a constructor and a
method?

• Answer
• A member function of a class that is used to
  create objects of that class is called a
  Constructor is. Its name is same as the class
  itself, has no return type. It is invoked using
  the new operator. An ordinary member function of
  a class is called Method. It has its own name and
  a return type (which may be void).It is invoked
  using the dot operator.

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3
Q12 State the purpose of garbage collection in
Java, and also when is it used?

• Answer
• Identifying and discarding objects that are no
  longer needed by a program so that their
  resources can be reclaimed and reused is the
  purpose of garbage collection. When a Java
  object is subjected becomes unreachable to the
  program in which it is used it is subjected to
  garbage collection.

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4
Q13When is HashCode and equals() overridded ?

• Answer
• To do equality check or use object as a key
  in HashMap, HashCode and equals() are overridden.

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Q14What is difference between StringBuffer and
StringBuilder in Java?

• Answer
• If the HashCode() is not overridded , the
  object will not be recovered from the HashMap if
  used as key.

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Q15 State which is better to synchronize
critical section of getInstance() method or whole
getInstance() method ?

• Answer
• Here the answer is a bit critical as if we
  lock the whole method than every time then this
  method will be called and would have to wait even
  though any object is not being created.

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Q16 When String gets created using literal or
new() operator , what difference is seen?

• Answer
• When a string is created with new() its
  created in heap and not added into string pool
  while using literal , String can be created in
  String pool itself which exists in Perm area of
  heap.

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Q17 Does overriding HashCode() method have any
performance implication or not ?

• Answer
• Does overriding HashCode() method have any
  performance implication or not ?

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Q18 In multithreaded environment whats wrong in
using HashMap? When does get() method go to
infinite loop ?

• Answer
• This happens during concurrent access and
  re-sizing.

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Q19 What is thread-safety? Why is it required?
And how to achieve thread-safety in Java
Applications?

• Answer
• The legal interaction of threads with the
  memory in a real computer system defines Java
  Memory Model. It also describes what behaviors
  are legal in a multi-threaded code. It can also
  determine when a Thread can reliably see writes
  to variables made by other threads. It also
  defines semantics for volatile, final and
  synchronize, that makes guarantee of visibility
  of memory operations across the Threads. In a
  Memory Barrier which there are two type of memory
  barrier instructions in JMM - read barriers and
  write barrier.
• To make the changes made by other threads
  visible to the current Thread, the read barrier
  invalidates the local memory (cache, registers,
  etc) and then reads the contents from the main
  memory .To make the changes made by current
  Thread visible to other Thread, a Write barrier
  flushes out the contents of the processors local
  memory to the main memory.

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Q20 What happens if you call return statement or
System.exit on try or catch block? Will it
finally block execute?

• Answer
• This is a very popular tricky Java question
  because many programmers think that finally the
  block always gets executed. This question
  challenges the concept by putting return
  statement in try or catch block or calling
  System.exit from try or catch block. In Java,
  this finally block will execute even if you put
  return statement in the try block or catch block
  finally block won't run even if you call
  System.exit form to try or catch.

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