Java Interview Questions & Answers

1
Java Interview Q A

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2
Q1 To implement key Object in HashMap which two
methods are used?

• Answer
• In HashMap, to use any object as Key it must
  implement equals and HashCode method in Java.

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3
Q2What is an immutable object? Mention an
immutable object?

• Answer
• Objects that cannot be modified once created are
  immutable objects in JAVA. A new object can be
  resulted due to any modification in an immutable
  object. For example, in JAVA, String is
  immutable. In order to prevent sub class from
  overriding methods in Java which can compromise
  Immutability, immutable object are mostly final
  in JAVA. This same functionality can be achieved
  by making member as non final but private and not
  modifying them except in constructor.

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4
Q3WState the difference between creating String
as new() and literal?

• Answer
• When string is created with new() Operator,
  its created in a heap and not added into string
  pool whereas using literal String are created in
  String pool itself which exists in PermGen area
  of heap, Example String s new String("Test")
  It does not put the object in String pool , but
  needs to be called String.intern() method which
  is used to put them into String pool explicitly.
  Its only then thats it create String object as
  String literal. For e.g. String s "Test" Java
  automatically put that into String pool

5
Q4What is difference between StringBuffer and
StringBuilder in Java?

• Answer
• Classic Java questions can seem to be tricky
  and very easy. In Java 5, StringBuilder is
  introduced and the only difference between both
  of them is that StringBuffer methods are
  synchronized while StringBuilder is non-
  synchronized.

6
Q5 Overriding HashCode() method has any
difference in performance implication?

• Answer
• Yes .A poor HashCode function will result in
  frequent collision in HashMap which eventually
  increase time for adding an object into Hash Map.

7
Q6 While writing stored procedure or accessing
stored procedure from java how is the error
condition handled?

• Answer
• This is one of the tough questions normally
  asked in Java interview. The stored procedure
  should return error code if some operation fails
  but if stored procedure itself fails than
  catching SQLException is the only choice left .

8
Q7 At the compile time are the imports checked
for validity? That is, will the code containing
an import such as java.lang.ABCD compile up?

• Answer
• Yes. The imports are checked for the
  semantic validity at the compile time. The above
  line of import will not compile if it has the
  code, an error is shown saying, cannot resolve
  symbol.
• Symbol class ABCD
• Location package io
• Import java.io.ABCD

9
Q8 State difference between Executor.submit()
and Executer.execute() method?

• Answer
• When looking at exception handling, there is
  a difference. If the tasks throws an exception
  and if it was submitted with the execute then
  this exception will go to the uncaught exception
  handler (that is when one explicitly is not
  provided, the default one will just print the
  stack trace to System.err). If the task is
  submitted with any thrown exception or a checked
  exception or not, is then part of the task's
  return status. And for a task that was submitted
  with submit and that terminates with an
  exception, the future.get will re-throw this
  exception wrapped in an ExecutionException.

10
Q9 What is the difference between factory and
abstract factory pattern?

• Answer
• One more level of abstraction is provided by
  Abstract Factory .Different factories from each
  Abstract Factory responsible for the creation of
  different hierarchies of objects based on type of
  factory are considered. For example Abstract
  Factory extended by AutomobileFactory,
  UserFactory, RoleFactoryetc.For every object
  created in that genre, each individual factory
  would be responsible.

11
Q10 Define Singleton? Which is better to make
whole method synchronized or only critical
section synchronized?

• Answer
• Singleton in JAVA is a class with just one
  instance in the whole Java application. One such
  example is java.lang.Runtime is a Singleton class
  .With the introduction Enum by Java 5, creating a
  Singleton is easy rather than being tricky as
  earlier.

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