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Java Interview 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 the 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 compare and swap 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?
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