Monday, October 20, 2014

Java - Interview Questions and Answers - One Liners

Dear Job-Seekers, please find below some important one liner Java - Interview Questions and Answers. Please add on in comments more question, I'll append to it. It will help all. Please share at the max.

1. Java OOPS Concepts ?

Ans. Abstraction , Encapsulation , Inheritance and Polymorphism. 

2. Difference between Abstract and Concrete Classes ?

Ans. Abstract classes are only meant to be sub classed and not meant to be instantiated whereas concrete classes are meant to be instantiated.

3. Difference between Overloading and Overriding ?

Ans. Similar Signature but different definition , like function overloading. Overriding the definition of base class in the derived class , like Virtual functions.

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4. Types of Inner classes ?

Ans. Simple Inner Class , Local Inner Class , Anonymous Inner Class , Static Nested Inner Class.

5. Difference between TreeMap and HashMap ?

Ans. They are different the way they are stored in memory. 

6. Difference between List , Sets and Maps ?

Ans. Lists - Members are stored in sequence in memory and can be accessed through index. Sets - There is no relevance of sequence and index. Sets doesn't contain duplicates whereas multiset can have duplicates. Maps - Contains Key , Value pairs.

7. Difference between Private , Public and Protected ?

Ans. Private - Not accessible outside object scope. Public - Accessible outside object scope. Protected - Becomes private when inherited.
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8. What is role of Synchronization in Java ?

Ans. Managing exclusive access of the code block to single thread at a time to maintain its integrity.

9. Default Access ?

Ans. Accessible only by the objects in the same package.

10. Vector and ArrayList ?

Ans. Vectors are synchronized whereas Array lists are not.

11. Difference between class and objects ?

Ans. Class is a template using which objects are created in memory.

12. Garbage Collection in Java ?

Ans. Mechanism by which Java reclaims the memory inaccessible by the application.

13. Types of Polymorphism ?

Ans. Static ( function overloading ) and Run time ( Virtual functions ) 

14. Different ways of using threads ?

Ans. Extending Thread class and implementing runnable interface.

15. Composition and Inheritence ?

Ans. Has a relationship vs Is a relationship.

16. Static variables ?

Ans. Correspond to class and not objects. All objects share that variable.

17. Interfaces and Abstract classes ?

Ans. Abstract classes have a body ( member elements ) where as Interfaces just have method declarations. 
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18. Constructor ?

Ans. Its an operation that creates and initializes the object.

19. Dynamic Binding ?

Ans. Association of function call to function definition during run time.

20. Volatile ?

Ans. Volatile is a declaration that a variable can be accessed by multiple threads and hence shouldn't be cached.

21. Serialization ?

Ans. Storing the state of an object in a file or other medium.

22. Transient ?

Ans. keyword in Java is used to indicate that a field should not be serialized.

23. Static Methods ?

Ans. Can only operates on static variables.

25. Final variable ?

Ans. Variable constant. Variable value can't be changed after instantiation.

26. Final Methods ?
Ans. Can't be overridden

27. Final Classes ?

Ans. Can't be sub classed.

28 Immutable Object ?

Ans. Object that can't be changed after instantiation.

29. checked and unchecked exceptions ?

Ans. For checked exceptions compiler throws a errors if they are not checked whereas unchecked exceptions and caught during runtime only and hence can't be checked.

30 Example of checked and unchecked exception ?

Ans. ClassNotFoundException is checked exception whereas NoClassDefFoundError is a unchecked exception.

31. Singleton Class ?

Ans. Using which only 1 object can be created.
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32. Name few Java exceptions ?

Ans. IndexOutofBound , NoClassDefFound , OutOfMemory , IllegalArgument.

33. Name few Design Patterns used for designing Java applications ?

Ans. Singleton , Factory , Abstract Factory , Proxy , Command , Builder. 

34. Give an example of Runtime Polymorphism ?

Ans. Action Mapping in a web application , Generics.

35. Which of the following is tightly bound ? Inheritance or Composition ?

Ans. Composition.

36. Give an Example of Generics in Java 5 ?

Ans. List aList = new ArrayList < number> 

37. How can we make sure that a code segment gets executed even in case of uncatched exceptions ?

Ans. By putting it within finally.

38. Can we extend an interface ?

Ans. We implement the interface.

39. Does Java support multiple inheritence ?

Ans. We can't extend multiple classes but can implement multiple interfaces.

40. Can we use an interface reference for referring to an object ?

Ans Yes we can do it for object implementing that interface. 

41. What do you mean by return type Void ?

Ans. Returns Nothing.

42. Is Java Compiled or an Interpreted language ?

Ans Both

43. What is a class file ?

Ans. Compiled Java files.

44. Why Java is a called a platform independent language ?

Ans. Java creates the pre compiled files ( class files ) which can then be interpreted on multiple platforms.

45. Native ?

Ans. Used in method declarations to specify that the method is not implemented in the same Java source file, but rather in another language

46. super ?

Ans. Used to access members of a class inherited by the class in which it appears.

47. What is "this" keyword used for ?

Ans. Used to represent an instance of the class in which it appears
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48. Transient ?

Ans. Declares that an instance field is not part of the default serialized form of an object

49. What will happen if we declare a class abstract as well as final ?

Ans. Compile time error.

50 Encapsulation ?

Ans. Encapsulation is the mechanism that binds together code and data it manipulates and keeps both safe from outside interference and misuse

51. Inheritence ?

Ans. Inheritance is the process by which one object acquires the properties of another object.

52. Polymorphism ?

Ans. Polymorphism is the feature that allows one interface to be used for general class actions

53. Difference between assignment and initialization ?

Ans. Assignment can be done anytime but initialization can be done only once during object instantiation.

54. difference between boolean and Boolean ?

Ans. boolean is a primitive type whereas Boolean is a class.

55. Immutable ?

Ans. Objects that can't be changed after initialization ?

56. Give an example of immutable class ?

Ans. String

57. Casting ?

Ans. Process to convert one data type to another.

58. Finalize () ?

Ans. finalize() method is used just before an object is destroyed.

59. Marker Interfaces ?

Ans. These are the interfaces which have no declared methods.

60. Name few Java marker interfaces ?

Ans. Serializable and cloneable.

61. Is runnable a Marker interface ?

Ans. No , it has run method declared.

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62 String and StringBuffer ?

Ans. String is an immutable class where StringBuffer is not.

63. Process and Thread ?

Ans. Process is a program in execution whereas thread is a separate path of execution in a program.

64. Thread states ?

Ans. ready, running, waiting and dead.

65. Deadlock ?

Ans. When two threads are waiting each other and can’t precede the program is said to be deadlock.

66. Serialization and Deserialization ?

Ans. Serialization is the process of writing the state of an object to a byte stream. Deserialization is the process of restoring these objects.

67. Difference between Java 1.4 and Java 5 ?

Ans. Generics , Autoboxing , Enum and Static Imports.

68. Autoboxing ?

Ans. Autoboxing is the automatic conversion that the Java compiler makes between the primitive types and their corresponding object wrapper classes

69. Enum ?

Ans. An enum type is a special data type that enables for a variable to be a set of predefined constants
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70. Wrapper Classes ?

Ans. A wrapper class is any class which "wraps" or "encapsulates" the functionality of another class or component.

71. Primitive Wrapper Classes ?

Ans. A Wrapper Class that wraps or encapsulates the primitive data type.

72. What is automatic conversion of primitive data type to its Wrapper class called ?

Ans. Autoboxing.

73. What Design pattern Wrapper Classes implement ?

Ans. Adapter.

74. Import ?

Ans. Enables the programmer to abbreviate the names of classes defined in a package.

75. Servlet Chaining ?

Ans. Servlet chaining is a technique in which two or more servlets can cooperate in servicing a single request.

76. What is the difference between set and list?

Ans. Set stores elements in an unordered way but does not contain duplicate elements, whereas list stores elements in an ordered way but may contain duplicate elements.

77. Difference between HashMap and HashTable ?

Ans. The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls.

78. Iterator ?

Ans. Some of the collection classes provide traversal of their contents via a java.util.Iterator interface. This interface allows you to walk through a collection of objects, operating on each object in turn.

79. JVM ?

Ans. Java Virtual Machine or JVM is an abstract which provides the runtime environment for the Java byte code to run.

80. JDK ?

Ans. Java Development Kit or JDK is physical entity that comprises of JRE and Development Tools. 

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81. JRE ?

Ans. Java Run Time Environment is the implementation of JVM.

82. different types of memory used by JVM ?

Ans. Class , Heap , Stack , Register , Native Method Stack.

83. Class Loader ?

Ans. Part of JVM which is used to load classes and interfaces.

84. Does Static Public Void instead of Public static Void gives compilation error ?

Ans. No

85. Does constructor returns any value ?

Ans. Yes , the current instance or object of the class.

86. Can we make constructor final ?

Ans. No

87. Why main method is static ?

Ans. Object needs to be created for calling non static methods.

88. Static Block ?

Ans. It is executed before main method at the time of class loading.

89. Can we execute program without mail method ?

Ans. Yes , one way is through static block.

90. Cloning ?

Ans. Keyword to create the exact copy of the object.

91. Can we overload main method ?

Ans. Yes.

92. Can we declare main method as final ?

Ans. Yes.

93. static binding ?

Ans. Associating function call and its definition during compile time.

94. Dynamic Binding ?

Ans. Associating function call and its definition during runtime.

95. Can we declare an interface method static ?

Ans. No

96. Can an interface be final ?

Ans. No

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97. Can we define private , protected modifiers with variables in interface ?

Ans. No, they are implicitely public.

98. Static import ?

Ans. By static import , we can access the static members of a class directly without prefixing it with the class name.

99. Base class for Error and Exceptions ?

Ans. Throwable

100. StringBuffer and StringBuilder ?

Ans. StringBuffer is synchronized whereas String Builder is not.

101. Default Package for Collection classes ?

Ans. Java.Util

102. Difference between List and Queue ?

Ans. In queue access can only happen at the ends whereas we can access any element of the list.

103. Difference between Vector and ArrayList ?

Ans. Vectors are synchronized whereas ArrayList are not.

104. Which is the base interface for all collection classes ?

Ans. Collection interface.

105. How can we make HashMap synchronized ?

Ans. Map m = Collections.synchronizedMap(hashMap);

106. Difference between Map and HashMap ?

Ans. Map is an interface where HashMap is the concrete class.

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107. Property Class ?

Ans. The properties class is a subclass of Hashtable that can be read from or written to a stream.

108 Why Java is a preferred technology for Web Applications ?

Ans. Web application involves dealing with multiple requests and thread and hence deals with lot of memory allocation and de allocation. Hence Java with its intrinsic capability of Garbage collection offer a perfect platform.

109. What are concepts introduced with Java 5 ?

Ans. Generics , Enums , Autoboxing , Annotations and Static Import.

110. Difference between get and post ?

Ans. Request parameters in get are passed within url whereas with post these are passed within post body as object.

111. Difference between C++ and Java ?

Ans. We have pointers in C++ , We have garbage collection in Java.

112. Name few Design Patterns ?

Ans. Singleton , Adapter , Factory , Abstract Factory , Builder , Facade , Observer , Proxy , Prototype.

113. Give an Example of Factory Design Pattern ?

Ans. Action Mapping within Web Application. 

114. Give an example of Adapter design pattern ?

Ans. Wrapper Classes.

115. Can we override static methods ?

Ans. Yes , static methods can be overridden by static methods in sub class.

116. What precaution should be used while dealing with static variables ?

Ans. Static variables are shared by all objects of the class and hence each instance may update it with its local value.

117. Can we override main method ?

Ans. No

118. Difference between server side and client side validation ?

Ans. Client side validation is done using scripting technologies like javascript before the form is submitted to server whereas server side validation is done after the form is submitted to server.

119. Difference between sleep and wait ? 

Ans. Sleep puts thread aside for exactly the time specified. Wait causes a wait of up to time specified. A thread could stop waiting earlier if it receives the notify() or notifyAll() call.

120. Name Wrapper classes available for primitive types ?


boolean - java.lang.Boolean
byte - java.lang.Byte
char - java.lang.Character
double - java.lang.Double
float - java.lang.Float
int - java.lang.Integer
long - java.lang.Long
short - java.lang.Short
void - java.lang.Void

121. Explain System.out.println ?

Ans. System is the class having static variable out that defaults to system console and println is the overloaded method.

122. Explain public static void main() ?

Ans. public means it has public access, static means that the method can be invoked without instantiating the object, void means it returns nothing. 

123. What is Java Byte Code ?

Ans. Java bytecode is the form of instructions that the Java virtual machine executes.

124. What is the relation between class file and byte code ?

Ans. Class files holds the instructions in byte code.

125. How can we make the object immutable ?

Ans. By making it a final class and making all members as private with those being initialized through constructor.

126. What is the purpose of making an object immutable ?

Ans. Since they cannot change state, they cannot be corrupted by thread interference or observed in an inconsistent state.

127. Explain the scenerios to choose between String , StringBuilder and StringBuffer ?

If the Object value will not change in a scenario use String Class because a String object is immutable.
If the Object value can change and will only be modified from a single thread, use a StringBuilder because StringBuilder is unsynchronized(means faster).
If the Object value may change, and can be modified by multiple threads, use a StringBuffer because StringBuffer is thread safe(synchronized).
128. Explain java.lang.OutOfMemoryError ?

Ans. This Error is thrown when the Java Virtual Machine cannot allocate an object because it is out of memory, and no more memory could be made available by the garbage collector.

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