Core Java Study guide and interview questions - Part 14


This is the 14th part of the 500+ Core JAVA interview questions and answers. These questions can also be used as a quick study guide for preparing for an interview or for any Core JAVA concepts revision. These questions cover all the topics in core JAVA from basics to advanced concepts.

51. What is the Set interface?
The Set interface provides methods for accessing the elements of a finite mathematical set. Sets do not allow duplicate elements.

52. What classes of exceptions may be thrown by a throw statement?
A throw statement may throw any expression that may be assigned to the Throwable type.

53. What are E and PI?
E is the base of the natural logarithm and PI is mathematical value pi.

54. Are true and false keywords?
The values true and false are not keywords.

55. What is a void return type?
A void return type indicates that a method does not return a value.

56. What is the purpose of the enableEvents() method?
The enableEvents() method is used to enable an event for a particular object. Normally, an event is enabled when a listener is added to an object for a particular event. The enableEvents() method is used by objects that handle events by overriding their event-dispatch methods.

57. What is the difference between the File and RandomAccessFile classes?
The File class encapsulates the files and directories of the local file system. The RandomAccessFile class provides the methods needed to directly access data contained in any part of a file.

58. What happens when you add a double value to a String?
The result is a String object.

59. What is your platform's default character encoding?
If you are running Java on English Windows platforms, it is probably Cp1252. If you are running Java on English Solaris platforms, it is most likely 8859_1..

60. Which package is always imported by default?
The java.lang package is always imported by default.

61. What interface must an object implement before it can be written to a stream as an object?
An object must implement the Serializable or Externalizable interface before it can be written to a stream as an object.

62. What is a layout manager?
A layout manager is an object that is used to organize components in a container.

63. What is a compilation unit?
A compilation unit is a Java source code file.

64. How are this and super used?
this is used to refer to the current object instance. super is used to refer to the variables and methods of the superclass of the current object instance.

65. What are three ways in which a thread can enter the waiting state?
A thread can enter the waiting state by invoking its sleep() method, by blocking on I/O, by unsuccessfully attempting to acquire an object's lock, or by invoking an object's wait() method. It can also enter the waiting state by invoking its  (deprecated) suspend() method.

66. How can a dead thread be restarted?
A dead thread cannot be restarted.

67. What is the ResourceBundle class?
The ResourceBundle class is used to store locale-specific resources that can be loaded by a program to tailor the program's appearance to the particular locale in which it is being run.

68. What interface is extended by AWT event listeners?
All AWT event listeners extend the java.util.EventListener interface.

69. What is the difference between a Scrollbar and a ScrollPane?
A Scrollbar is a Component, but not a Container. A ScrollPane is a Container. A ScrollPane handles its own events and performs its own scrolling.

70. Which arithmetic operations can result in the throwing of an ArithmeticException?
Integer / and % can result in the throwing of an Arithmetic Exception.

71. What happens if a try-catch-finally statement does not have a catch clause to handle an exception that is thrown within the body of the try statement?
The exception propagates up to the next higher level try-catch statement (if any) or results in the program's termination.

72. What is the purpose of garbage collection?
The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources may be reclaimed and reused.

73. Can an abstract class be final?
An abstract class may not be declared as final.

74. What is a Java package and how is it used?
A Java package is a naming context for classes and interfaces. A package is used to create a separate name space for groups of classes and interfaces. Packages are also used to organize related classes and interfaces into a single API unit and to control accessibility to these classes and interfaces.

75. What happens if an exception is not caught?
An uncaught exception results in the uncaughtException() method of the thread's ThreadGroup being invoked, which eventually results in the termination of the program in which it is thrown.

76. What methods are used to get and set the text label displayed by a Button object?
getLabel() and setLabel()

77. What is numeric promotion?
Numeric promotion is the conversion of a smaller numeric type to a larger numeric type, so that integer and floating-point operations may take place. In numerical promotion, byte, char, and short values are converted to int  values. The int values are also converted to long values, if necessary. The long and float values are converted to double values, as required.

78. What are the Object and Class classes used for?
The Object class is the highest-level class in the Java class hierarchy. The Class class is used to represent the classes and interfaces that are loaded by a Java program.

79. To what value is a variable of the boolean type automatically initialized?
The default value of the boolean type is false.

80. How does a try statement determine which catch clause should be used to handle an exception?
When an exception is thrown within the body of a try statement, the catch clauses of the try statement are examined in the order in which they appear. The first catch clause that is capable of handling the exception is executed.  The remaining catch clauses are ignored.

81. Can an unreachable object become reachable again?
An unreachable object may become reachable again. This can happen when the object's finalize() method is invoked and the object performs an operation which causes it to become accessible to reachable objects.

82. What are the two basic ways in which classes that can be run as threads may be defined?
A thread class may be declared as a subclass of Thread, or it may implement the Runnable interface.

83. What is the purpose of a statement block?
A statement block is used to organize a sequence of statements as a single statement group.

84. What method must be implemented by all threads?
All tasks must implement the run() method, whether they are a subclass of  Thread or implement the Runnable interface.

85. What is the difference between a public and a non-public class?
A public class may be accessed outside of its package. A non-public class may not be accessed outside of its package.

86. What are synchronized methods and synchronized statements?
Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.

87. What modifiers may be used with a top-level class?
A top-level class may be public, abstract, or final.

88. Which Component subclass is used for drawing and painting?
Canvas

89. What is the difference between the prefix and postfix forms of the ++ operator?
The prefix form performs the increment operation and returns the value of  the increment operation. The postfix form returns the current value all of the expression and then performs the increment operation on that value.

90. When is an object subject to garbage collection?
An object is subject to garbage collection when it becomes unreachable to the program in which it is used.

91. What are the problems faced by Java programmers who don't use layout managers?
Without layout managers, Java programmers are faced with determining how their GUI will be displayed across multiple windowing systems and finding a common sizing  and positioning that will work within the constraints imposed by each windowing system.

92. What is the difference between an if statement and a switch statement?
The if statement is used to select among two alternatives. It uses a boolean expression to decide which alternative should be executed. The switch statement is used to select among multiple alternatives. It uses an int expression to determine which alternative should be executed.

93. What is the List interface?
The List interface provides support for ordered collections of objects.

94)What is it object serialization ?
Serialization is a way to convert objects (including complex data structures such as lists and trees) into a stream of bytes.

95)How to make application thread-safe ?
You should use the word synchronized to mark the critical section of code. You may also use other methods of thread synchronization (see wait(), notify(), notifyAll() etc.

96)what is it reflection (introspection) ? Why is reflection possible in the Java language?
Reflection (introspection) is querying a class about its properties, and operating on methods and fields by the name for a given object instance. Reflection is possible in the Java language because of late binding.

97) What is the difference between an Applet and an Application?
1. Applets can be embedded in HTML pages and downloaded over the Internet whereas Applications have no special support in HTML for embedding or downloading.
2. Applets can only be executed inside a java compatible container, such as a browser or appletviewer whereas Applications are executed at command line by java.exe or    jview.exe.
3. Applets execute under strict security limitations that disallow certain operations(sandbox model security) whereas Applications have no inherent security restrictions.
4. Applets don't have the main() method as in applications. Instead they operate on an entirely different mechanism where they are initialized by init(),started by start(),stopped by stop() or destroyed by destroy().

98)Why are Java ARchive (JAR) files important?
JAR files bundle .class files and optimize applet downloads.

99)  How can you force all derived classes to implement a method present in the base class?
Creating and implementing an interface would be the best way for this situation. Just create an interface with empty methods which forces a programmer to implement all the methods present under it. Another way of achieving this task is to declare a class as abstract with all its methods abstract.

100) Java says "write once, run anywhere". What are some ways this isn't quite true?
Any time you use system calls specific to one operating system and do not create alternative calls for another operating system, your program will not function correctly. Solaris systems and Intel systems order the bits of an integer differently. (You may have heard of little endian vs. big endian)
If your code uses bit shifting, or other binary operators, they will not work on systems that have opposite endianism.

101) Describe java's security model.
Java's security model is one of the most interesting and unique aspects of the language. For the most part it's broken into two pieces: the user adjustable security manager that checks various API operations like file access, and the byte code verifier that asserts the validity of compiled byte code. public abstract class SecurityManager  java.lang.SecurityManager is an abstract class which different applications subclass to implement a particular security policy. It allows an application to determine whether or not a particular operation will generate a security exception.

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