#Java Interview Questions and Answers

##Expectations

• Good Java Knowledge
• Source code

##Questions

###Java Platform

• 1.Why is Java so popular?
• 2.What is platform independence?
• 3.What is bytecode?
• 4.Compare JDK vs JVM vs JRE.
• 5.What are the important differences between C++ and Java?
• 6.What is the role for a classloader in Java?

###Wrapper Classes

• 7.What are Wrapper classes?
• 8.Why do we need Wrapper classes in Java?
• 9.What are the different ways of creating Wrapper class instances?
• 10.What are differences in the two ways of creating Wrapper classes?
• 11.What is auto boxing?
• 12.What are the advantages of auto boxing?
• 13.What is casting?
• 14.What is implicit casting?
• 15.What is explicit casting?

###Strings

• 16.Are all String’s immutable?
• 17.Where are String values stored in memory?
• 18.Why should you be careful about String concatenation(+) operator in loops?
• 19.How do you solve above problem?
• 20.What are differences between String and StringBuffer?
• 21.What are differences between StringBuilder and StringBuffer?
• 22.Can you give examples of different utility methods in String class?

###Object oriented programming basics

• 23.What is a class?
• 24.What is an object?
• 25.What is state of an object?
• 26.What is behavior of an object?
• 27.What is the super class of every class in Java?
• 28.Explain about toString method ?
• 29.What is the use of equals method in Java?
• 30.What are the important things to consider when implementing equals method?
• 31.What is the Hashcode method used for in Java?
• 32.Explain inheritance with examples.
• 33.What is method overloading?
• 34.What is method overriding?
• 35.Can super class reference variable can hold an object of sub class?
• 36.Is multiple inheritance allowed in Java?
• 37.What is an interface?
• 38.How do you define an interface?
• 39.How do you implement an interface?
• 40.Can you explain a few tricky things about interfaces?
• 41.Can you extend an interface?
• 42.Can a class extend multiple interfaces?
• 43.What is an abstract class?
• 44.When do you use an abstract class?
• 45.How do you define an abstract method?
• 46.Compare abstract class vs interface?
• 47.What is a constructor?
• 48.What is a default constructor?
• 49.Will this code compile?
• 50.How do you call a super class constructor from a constructor?
• 51.Will this code compile?
• 52.What is the use of this()?
• 53.Can a constructor be called directly from a method?
• 54.Is a super class constructor called even when there is no explicit call from a sub class constructor?

###Advanced object oriented concepts

• 55.What is polymorphism?
• 56.What is the use of instanceof operator in Java?
• 57.What is coupling?
• 58.What is cohesion?
• 59.What is encapsulation?
• 60.What is an inner class?
• 61.What is a static inner class?
• 62.Can you create an inner class inside a method?
• 63.What is an anonymous class?

###Modifiers

• 64.What is default class modifier?
• 65.What is private access modifier?
• 66.What is default or package access modifier?
• 67.What is protected access modifier?
• 68.What is public access modifier?
• 69.What access types of variables can be accessed from a class in same package?
• 70.What access types of variables can be accessed from a class in different package?
• 71.What access types of variables can be accessed from a sub class in same package?
• 72.What access types of variables can be accessed from a sub class in different package?
• 73.What is the use of a final modifier on a class?
• 74.What is the use of a final modifier on a method?
• 75.What is a final variable?
• 76.What is a final argument?
• 77.What happens when a variable is marked as volatile?
• 78.What is a static variable?

###conditions & loops

• 79.Why should you always use blocks around if statement?
• 80.Guess the output
• 81.Guess the output
• 82.Guess the output of this switch block.
• 83.Guess the output of this switch block?
• 84.Should default be the last case in a switch statement?
• 85.Can a switch statement be used around a String
• 86.Guess the output of this for loop
• 87.What is an enhanced for loop?
• 88.What is the output of the for loop below?
• 89.What is the output of the program below?
• 90.What is the output of the program below?

###Exception handling

• 91.Why is exception handling important?
• 92.What design pattern is used to implement exception handling features in most languages?
• 93.What is the need for finally block?
• 94.In what scenarios is code in finally not executed?
• 95.Will finally be executed in the program below?
• 96.Is try without a catch is allowed?
• 97.Is try without catch and finally allowed?
• 98.Can you explain the hierarchy of exception handling classes?
• 99.What is the difference between error and exception?
• 100.What is the difference between checked exceptions and unchecked exceptions?
• 101.How do you throw an exception from a method?
• 102.What happens when you throw a checked exception from a method?
• 103.What are the options you have to eliminate compilation errors when handling checked exceptions?
• 104.How do you create a custom exception?
• 105.How do you handle multiple exception types with same exception handling block?
• 106.Can you explain about try with resources?
• 107.How does try with resources work?
• 108.Can you explain a few exception handling best practices?

###Miscellaneous topics

• 109.What are the default values in an array?
• 110.How do you loop around an array using enhanced for loop?
• 111.How do you print the content of an array?
• 112.How do you compare two arrays?
• 113.What is an enum?
• 114.Can you use a switch statement around an enum?
• 115.What are variable arguments or varargs?
• 116.What are asserts used for?
• 117.When should asserts be used?
• 118.What is garbage collection?
• 119.Can you explain garbage collection with an example?
• 120.When is garbage collection run?
• 121.What are best practices on garbage collection?
• 122.What are initialization blocks?
• 123.What is a static initializer?
• 124.What is an instance initializer block?
• 125.What is tokenizing?
• 126.Can you give an example of tokenizing?
• 127.What is serialization?
• 128.How do you serialize an object using serializable interface?
• 129.How do you de-serialize in Java?
• 130.What do you do if only parts of the object have to be serialized?
• 131.How do you serialize a hierarchy of objects?
• 132.Are the constructors in an object invoked when it is de-serialized?
• 133.Are the values of static variables stored when an object is serialized?

###Collections

• 134.Why do we need collections in Java?
• 135.What are the important interfaces in the collection hierarchy?
• 136.What are the important methods that are declared in the collection interface?
• 137.Can you explain briefly about the List interface?
• 138.Explain about ArrayList with an example?
• 139.Can an ArrayList have duplicate elements?
• 140.How do you iterate around an ArrayList using iterator?
• 141.How do you sort an ArrayList?
• 142.How do you sort elements in an ArrayList using comparable interface?
• 143.How do you sort elements in an ArrayList using comparator interface?
• 144.What is vector class? How is it different from an ArrayList?
• 145.What is linkedList? What interfaces does it implement? How is it different from an ArrayList?
• 146.Can you briefly explain about the Set interface?
• 147.What are the important interfaces related to the Set interface?
• 148.What is the difference between Set and sortedSet interfaces?
• 149.Can you give examples of classes that implement the Set interface?
• 150.What is a HashSet?
• 151.What is a linkedHashSet? How is different from a HashSet?
• 152.What is a TreeSet? How is different from a HashSet?
• 153.Can you give examples of implementations of navigableSet?
• 154.Explain briefly about Queue interface?
• 155.What are the important interfaces related to the Queue interface?
• 156.Explain about the Deque interface?
• 157.Explain the BlockingQueue interface?
• 158.What is a priorityQueue?
• 159.Can you give example implementations of the BlockingQueue interface?
• 160.Can you briefly explain about the Map interface?
• 161.What is difference between Map and sortedMap?
• 162.What is a HashMap?
• 163.What are the different methods in a Hash Map?
• 164.What is a TreeMap? How is different from a HashMap?
• 165.Can you give an example of implementation of navigableMap interface?
• 166.What are the static methods present in the collections class?

###Advanced collections

• 167.What is the difference between synchronized and concurrent collections in Java?
• 168.Explain about the new concurrent collections in Java?
• 169.Explain about copyonwrite concurrent collections approach?
• 170.What is compareandswap approach?
• 171.What is a lock? How is it different from using synchronized approach?
• 172.What is initial capacity of a Java collection?
• 173.What is load factor?
• 174.When does a Java collection throw UnsupportedOperationException?
• 175.What is difference between fail-safe and fail-fast iterators?
• 176.What are atomic operations in Java?
• 177.What is BlockingQueue in Java?

###Generics

• 178.What are Generics?
• 179.Why do we need Generics? Can you give an example of how Generics make a program more flexible?
• 180.How do you declare a generic class?
• 181.What are the restrictions in using generic type that is declared in a class declaration?
• 182.How can we restrict Generics to a subclass of particular class?
• 183.How can we restrict Generics to a super class of particular class?
• 184.Can you give an example of a generic method?

###Multi threading

• 185.What is the need for threads in Java?
• 186.How do you create a thread?
• 187.How do you create a thread by extending thread class?
• 188.How do you create a thread by implementing runnable interface?
• 189.How do you run a thread in Java?
• 190.What are the different states of a thread?
• 191.What is priority of a thread? How do you change the priority of a thread?
• 192.What is executorservice?
• 193.Can you give an example for executorservice?
• 194.Explain different ways of creating executor services.
• 195.How do you check whether an executionservice task executed successfully?
• 196.What is callable? How do you execute a callable from executionservice?
• 197.What is synchronization of threads?
• 198.Can you give an example of a synchronized block?
• 199.Can a static method be synchronized?
• 200.What is the use of join method in threads?
• 201.Describe a few other important methods in threads?
• 202.What is a deadlock?
• 203.What are the important methods in Java for inter-thread communication?
• 204.What is the use of wait method?
• 205.What is the use of notify method?
• 206.What is the use of notifyall method?
• 207.Can you write a synchronized program with wait and notify methods?

###Functional Programming - Lamdba expressions and Streams

• 208.What is functional programming?
• 209.Can you give an example of functional programming?
• 210.What is a stream?
• 211.Explain about streams with an example?
• what are intermediate operations in streams?
• 212.What are terminal operations in streams?
• 213.What are method references?
• 214.What are lambda expressions?
• 215.Can you give an example of lambda expression?
• 216.Can you explain the relationship between lambda expression and functional interfaces?
• 217.What is a predicate?
• 218.What is the functional interface - function?
• 219.What is a consumer?
• 220.Can you give examples of functional interfaces with multiple arguments?

New Features

• 221.What are the new features in Java 5?
• 222.What are the new features in Java 6?
• 223.What are the new features in Java 7?
• 224.What are the new features in Java 8?