150+java interview questions and answers

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?