Java Interview Questions                                 i
                       Java Interview Questions
                       Java Interview Questions                                                                                                                                                                                    ii
                       Contents
                       1     Object Oriented Programming (OOP)                                                                                                                                                                    1
                             1.1      Encapsulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                                                     1
                             1.2      Polymorphism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                                                      1
                             1.3      Inheritance           .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . . . . . . . . . . . . . . . . . . . .                         1
                             1.4      Abstraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                                                     2
                             1.5      Differences between Abstraction and Encapsulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                                                         2
                       2     General Questions about Java                                                                                                                                                                         3
                             2.1      WhatisJVM?WhyisJavacalledthePlatformIndependentProgrammingLanguage? . . . . . . . . . . . . . .                                                                                             3
                             2.2      WhatistheDifference between JDK and JRE ? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                                                             3
                             2.3      Whatdoesthe“static” keyword mean ? Can you override private or static method in Java ? . . . . . . . . . . . .                                                                              3
                             2.4      Canyouaccessnonstatic variable in static context ?                                      .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . . . . . . . . . . . . . . .                    3
                             2.5      WhataretheDataTypessupportedbyJava? WhatisAutoboxingandUnboxing? . . . . . . . . . . . . . . . .                                                                                            4
                             2.6      WhatisFunction Overriding and Overloading in Java ? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                                                           4
                             2.7      WhatisaConstructor, Constructor Overloading in Java and Copy-Constructor . . . . . . . . . . . . . . . . . . .                                                                              4
                             2.8      DoesJavasupport multiple inheritance ? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                                                        4
                             2.9      Whatisthedifference between an Interface and an Abstract class ? . . . . . . . . . . . . . . . . . . . . . . . . .                                                                          4
                             2.10 What are pass by reference and pass by value ?                                     .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . . . . . . . . . . . . . . . .                     5
                       3     Java Threads                                                                                                                                                                                         6
                             3.1      Whatisthedifference between processes and threads ? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                                                           6
                             3.2      Explain different ways of creating a thread. Which one would you prefer and why ? . . . . . . . . . . . . . . . .                                                                           6
                             3.3      Explain the available thread states in a high-level. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                                                    6
                             3.4      Whatisthedifference between a synchronized method and a synchronized block ?                                                            .  .  .  . . . . . . . . . . . . .                  7
                             3.5      Howdoesthreadsynchronization occurs inside a monitor ? What levels of synchronization can you apply ? . . .                                                                                 7
                             3.6      What’s a deadlock ? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                                                     7
                             3.7      HowdoyouensurethatNthreadscanaccessNresourceswithoutdeadlock? . . . . . . . . . . . . . . . . . . .                                                                                         7
                       Java Interview Questions                                                                                                                                                                                   iii
                       4     Java Collections                                                                                                                                                                                     8
                             4.1      Whatarethebasic interfaces of Java Collections Framework ? . . . . . . . . . . . . . . . . . . . . . . . . . . .                                                                            8
                             4.2      WhyCollection doesn’t extend Cloneable and Serializable interfaces ? . . . . . . . . . . . . . . . . . . . . . . .                                                                          8
                             4.3      WhatisanIterator ? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                                                      8
                             4.4      Whatdifferences exist between Iterator and ListIterator ? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                                                       8
                             4.5      Whatisdifference between fail-fast and fail-safe ? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                                                      9
                             4.6      HowHashMapworksinJava? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                                                              9
                             4.7      Whatistheimportance of hashCode() and equals() methods ?                                               .  .  .  .  .  .  .  .  .  .  .  .  . . . . . . . . . . . . . . .                    9
                             4.8      Whatdifferences exist between HashMap and Hashtable ?                                            .  .  .  .  .  .  .  .  .  .  .  .  .  .  . . . . . . . . . . . . . . .                    9
                             4.9      Whatisdifference between Array and ArrayList ? When will you use Array over ArrayList ? . . . . . . . . . . .                                                                               9
                             4.10 What is difference between ArrayList and LinkedList ? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                                                           10
                             4.11 What is Comparable and Comparator interface ? List their differences. . . . . . . . . . . . . . . . . . . . . . . .                                                                           10
                             4.12 What is Java Priority Queue ? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                                                       10
                             4.13 What do you know about the big-O notation and can you give some examples with respect to different data
                                      structures ?          .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . . . . . . . . . . . . . . . . . . . .                       10
                             4.14 What is the tradeoff between using an unordered array versus an ordered array ? . . . . . . . . . . . . . . . . . .                                                                           10
                             4.15 What are some of the best practices relating to the Java Collection framework ? . . . . . . . . . . . . . . . . . .                                                                           11
                             4.16 What’s the difference between Enumeration and Iterator interfaces ? . . . . . . . . . . . . . . . . . . . . . . . .                                                                           11
                             4.17 What is the difference between HashSet and TreeSet ?                                           .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . . . . . . . . . . . . . . .                  11
                       5     GarbageCollectors                                                                                                                                                                                  12
                             5.1      Whatisthepurpose of garbage collection in Java, and when is it used ?                                                 .  .  .  .  .  .  .  .  . . . . . . . . . . . . . .                 12
                             5.2      WhatdoesSystem.gc() and Runtime.gc() methods do ? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                                                           12
                             5.3      Whenisthefinalize() called ? What is the purpose of finalization ? . . . . . . . . . . . . . . . . . . . . . . . . .                                                                        12
                             5.4      If an object reference is set to null, will the Garbage Collector immediately free the memory held by that object ?                                                                       12
                             5.5      Whatisstructure of Java Heap ? What is Perm Gen space in Heap ? . . . . . . . . . . . . . . . . . . . . . . . .                                                                           12
                             5.6      Whatisthedifference between Serial and Throughput Garbage collector ? . . . . . . . . . . . . . . . . . . . . .                                                                           13
                             5.7      WhendoesanObjectbecomeseligibleforGarbagecollection in Java ? . . . . . . . . . . . . . . . . . . . . . .                                                                                 13
                             5.8      DoesGarbagecollection occur in permanent generation space in JVM ? . . . . . . . . . . . . . . . . . . . . . .                                                                            13
                       6     Exception Handling                                                                                                                                                                                 14
                             6.1      WhatarethetwotypesofExceptions in Java ? Which are the differences between them ? . . . . . . . . . . . . .                                                                               14
                             6.2      Whatisthedifference between Exception and Error in java ? . . . . . . . . . . . . . . . . . . . . . . . . . . . .                                                                         14
                             6.3      Whatisthedifference between throw and throws ?                                       .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . . . . . . . . . . . . . . . .                   14
                             6.4      Whatistheimportance of finally block in exception handling ? . . . . . . . . . . . . . . . . . . . . . . . . . . .                                                                         14
                             6.5      Whatwill happen to the Exception object after exception handling ? . . . . . . . . . . . . . . . . . . . . . . . .                                                                        14
                             6.6      Howdoesfinallyblockdiffer from finalize() method ?                                           .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . . . . . . . . . . . . . . . .                   15