Advance java interview question answers.
1. Is a class a subclass of itself?
Answer: A class is a subclass of itself.
2. What restrictions are placed on the values of each case of a switch statement?
Answer: During compilation, the values of each case of a switch statement must evaluate to a value that can be promoted to an int value.
3. What's new with the stop(), suspend() and resume() methods in JDK .?
Answer: The stop(), suspend() and resume() methods have been deprecated in JDK 1.2.
4. Name four Container classes.
Answer: Window, Frame, Dialog, FileDialog, Panel, Applet, or ScrollPane.
5. What is a layout manager?
Answer: A layout manager is an object that is used to organize components in a container.
6. How are the elements of a CardLayout organized?
Answer: The elements of a CardLayout are stacked, one on top of the other, like a deck of cards.
7. What interface must an object implement before it can be written to a stream as an object?
Answer: An object must implement the Serializable or Externalizable interface before it can be written to a stream as an object.
8. How is rounding performed under integer division?
Answer: The fractional part of the result is truncated. This is known as rounding toward zero.
9. What state is a thread in when it is executing?
Answer: An executing thread is in the running state.
10. What is the catch or declare rule for method declarations?
Answer: If a checked exception may be thrown within the body of a method, the method must either catch the exception or declare it in its throws clause.
11. What must a class do to implement an interface?
Answer: It must provide all of the methods in the interface and identify the interface in its implements clause.
12. Which Container method is used to cause a container to be laid out and redisplayed?
Answer: validate() method is used to cause a container to be laid out and redisplayed.
13. What happens if an exception is not caught?
Answer: 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.
14. What is the relationship between an event-listener interface and an event-adapter class?
Answer: An event-listener interface defines the methods that must be implemented by an event handler for a particular kind of event. An event adapter provides a default implementation of an event-listener interface.
15. Can try statements be nested?
Answer: Try statements may be tested.
16. What is clipping?
Answer: Clipping is the process of confining paint operations to a limited area or shape.
17. What is the purpose of the wait(), notify(), and notifyAll() methods?
Answer: The wait(), notify(), and notifyAll() methods are used to provide an efficient way for threads to wait for a shared resource. When a thread executes an object's wait() method, it enters the waiting state. It only enters the ready state after another thread invokes the object's notify() or notifyAll() methods..
18. What are synchronized methods and synchronized statements?
Answer: 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.
19. When is an object subject to garbage collection?
Answer: An object is subject to garbage collection when it becomes unreachable to the program in which it is used.
20. What is the purpose of the File class?
Answer: The File class is used to create objects that provide access to the files and directories of a local file system.
21. Which java.util classes and interfaces support event handling?
Answer: The EventObject class and the EventListener interface support event processing.
22. What is the highest-level event class of the event-delegation model?
Answer: The java.util.EventObject class is the highest-level class in the event-delegation class hierarchy.
23. What is the difference between preemptive scheduling and time slicing?
Answer: Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.
24. Which package is always imported by default?
Answer: The java.lang package is always imported by default.
25. What is the purpose of finalization?
Answer: The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup processing before the object is garbage collected.
26. How is it possible for two String objects with identical values not to be equal under the == operator?
Answer: The == operator compares two objects to determine if they are the same object in memory. It is possible for two String objects to have the same value, but located indifferent areas of memory.
27. Name the eight primitive Java types.
Answer: The eight primitive types are byte, char, short, int, long, float, double, and boolean.
28. Can an object be garbage collected while it is still reachable?
Answer: A reachable object cannot be garbage collected. Only unreachable objects may be garbage collected..
29. What are the problems faced by Java programmers who don't use layout managers?
Answer: 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.
30. What is the immediate superclass of the Applet class?
Answer: Panel.
31. What is the difference between yielding and sleeping?
Answer: When a task invokes its yield() method, it returns to the ready state. When a task invokes its sleep() method, it returns to the waiting state.
32. What classes of exceptions may be thrown by a throw statement?
Answer: A throw statement may throw any expression that may be assigned to the Throwable type.
33. What is your platform's default character encoding?
Answer: 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.
34. What is a native method?
Answer: A native method is a method that is implemented in a language other than Java.
35. How are commas used in the intialization and iteration parts of a for statement?
Answer: Commas are used to separate multiple statements within the initialization and iteration parts of a for statement.
36. What is the SimpleTimeZone class?
Answer: The SimpleTimeZone class provides support for a Gregorian calendar.
37. What is the Collections API?
Answer: The Collections API is a set of classes and interfaces that support operations on collections of objects.
38. What value does readLine() return when it has reached the end of a file?
Answer: The readLine() method returns null when it has reached the end of a file.
39. What is an object's lock and which object's have locks?
Answer: An object's lock is a mechanism that is used by multiple threads to obtain synchronized access to the object. A thread may execute a synchronized method of an object only after it has acquired the object's lock. All objects and classes have locks. A class's lock is acquired on the class's Class object.
40. What is the purpose of the Runtime class?
Answer: The purpose of the Runtime class is to provide access to the Java runtime system.
41. How does Java handle integer overflows and underflows?
Answer: It uses those low order bytes of the result that can fit into the size of the type allowed by the operation.
42. What state does a thread enter when it terminates its processing?
Answer: When a thread terminates its processing, it enters the dead state.
43. What is a compilation unit?
Answer: A compilation unit is a Java source code file.
44. What is an Iterator interface?
Answer: The Iterator interface is used to step through the elements of a Collection.
45. How are the elements of a BorderLayout organized?
Answer: The elements of a BorderLayout are organized at the borders (North, South, East, and West) and the center of a container.
46. What are the two basic ways in which classes that can be run as threads may be defined?
Answer: A thread class may be declared as a subclass of Thread, or it may implement the Runnable interface.
47. What is the ResourceBundle class?
Answer: 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.
48. What invokes a thread's run() method?
Answer: After a thread is started, via its start() method or that of the Thread class, the JVM invokes the thread's run() method when the thread is initially executed.
49. Are true and false keywords?
Answer: The values true and false are not keywords.
50. What restrictions are placed on method overriding?
Answer: Overridden methods must have the same name, argument list, and return type. The overriding method may not limit the access of the method it overrides. The overriding method may not throw any exceptions that may not be thrown by the overridden method.
51. Can a for statement loop indefinitely?
Answer: Yes, a for statement can loop indefinitely. For example, consider the following: for(;;) ;
52. Can an exception be rethrown?
Answer: Yes, an exception can be rethrown.
53. What is the purpose of the enableEvents() method?
Answer: 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.
54. If a method is declared as protected, where may the method be accessed?
Answer: A protected method may only be accessed by classes or interfaces of the same package or by subclasses of the class in which it is declared.
55. What is the difference between a field variable and a local variable?
Answer: A field variable is a variable that is declared as a member of a class. A local variable is a variable that is declared local to a method.
56. Can a double value be cast to a byte?
Answer: Yes, a double value can be cast to a byte.
57. Which class is extended by all other classes?
Answer: The Object class is extended by all other classes.
58. What is the GregorianCalendar class?
Answer: The GregorianCalendar class provides support for traditional Western calendars.
59. If a variable is declared as private, where may the variable be accessed?
Answer: A private variable may only be accessed within the class in which it is declared.
60. What is the difference between the Font and FontMetrics classes?
Answer: The FontMetrics class is used to define implementation-specific properties, such as ascent and descent, of a Font object.
61. How can a GUI component handle its own events?
Answer: A component can handle its own events by implementing the required event-listener interface and adding itself as its own event listener.
62. What is the difference between a Scrollbar and a ScrollPane?
Answer: 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.
63. What is casting?
Answer: There are two types of casting, casting between primitive numeric types and casting between object references. Casting between numeric types is used to convert larger values, such as double values, to smaller values, such as byte values. Casting between object references is used to refer to an object by a compatible class, interface, or array type reference.
64. What is the difference between a Window and a Frame?
Answer: The Frame class extends Window to define a main application window that can have a menu bar.
65. What is a Java package and how is it used?
Answer: 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.
66. What interface is extended by AWT event listeners?
Answer: All AWT event listeners extend the java.util.EventListener interface.
67. How can the Checkbox class be used to create a radio button?
Answer: By associating Checkbox objects with a CheckboxGroup.
68. Which class is the immediate superclass of the Container class?
Answer: Component.
69. What is the difference between the File and RandomAccessFile classes?
Answer: 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.
70. What class of exceptions are generated by the Java run-time system?
Answer: The Java runtime system generates RuntimeException and Error exceptions.
71. What are wrapped classes?
Answer: Wrapped classes are classes that allow primitive types to be accessed as objects.
72. How are Observer and Observable used?
Answer: Objects that subclass the Observable class maintain a list of observers. When an Observable object is updated it invokes the update() method of each of its observers to notify the observers that it has changed state. The Observer interface is implemented by objects that observe Observable objects.
73. If a class is declared without any access modifiers, where may the class be accessed?
Answer: A class that is declared without any access modifiers is said to have package access. This means that the class can only be accessed by other classes and interfaces that are defined within the same package.
74. What is the preferred size of a component?
Answer: The preferred size of a component is the minimum component size that will allow the component to display normally.
75. What is the Set interface?
Answer: The Set interface provides methods for accessing the elements of a finite mathematical set. Sets do not allow duplicate elements.
76. What is the relationship between clipping and repainting?
Answer: When a window is repainted by the AWT painting thread, it sets the clipping regions to the area of the window that requires repainting.
77. When is the finally clause of a try-catch-finally statement executed?
Answer: The finally clause of the try-catch-finally statement is always executed unless the thread of execution terminates or an exception occurs within the execution of the finally clause.
78. How can a dead thread be restarted?
Answer: A dead thread cannot be restarted.
79. Can a Byte object be cast to a double value?
Answer: No. An object cannot be cast to a primitive value.
80. What is numeric promotion?
Answer: 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.
81. What is the difference between the Reader/Writer class hierarchy and the InputStream/OutputStream class hierarchy?
Answer: The Reader/Writer class hierarchy is character-oriented, and the InputStream/OutputStream class hierarchy is byte-oriented.
82. What class allows you to read objects directly from a stream?
Answer: The ObjectInputStream class supports the reading of objects from input streams.
83. What are the legal operands of the instanceof operator?
Answer: The left operand is an object reference or null value and the right operand is a class, interface, or array type.
84. What is the purpose of a statement block?
Answer: A statement block is used to organize a sequence of statements as a single statement group.
85. Is sizeof a keyword?
Answer: The sizeof operator is not a keyword.
86. What restrictions are placed on method overloading?
Answer: Two methods may not have the same name and argument list but different return types.
87. What is the difference between the String and StringBuffer classes?
Answer: String objects are constants. StringBuffer objects are not constants.
88. What are the Object and Class classes used for?
Answer: 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..
89. What is the difference between the JDK . event model and the event-delegation model introduced with JDK .?
Answer: The JDK 1.02 event model uses an event inheritance or bubbling approach. In this model, components are required to handle their own events. If they do not handle a particular event, the event is inherited by (or bubbled up to) the component's container. The container then either handles the event or it is bubbled up to its container and so on, until the highest-level container has been tried. In the event-delegation model, specific objects are designated as event handlers for GUI components. These objects implement event-listener interfaces. The event-delegation model is more efficient than the event-inheritance model because it eliminates the processing required to support the bubbling of unhandled events.
90. When can an object reference be cast to an interface reference?
Answer: An object reference be cast to an interface reference when the object implements the referenced interface.
91. What is the % operator?
Answer: It is referred to as the modulo or remainder operator. It returns the remainder of dividing the first operand by the second operand.
92. What Checkbox method allows you to tell if a Checkbox is checked?
Answer: getState().
93. What are order of precedence and associativity, and how are they used?
Answer: Order of precedence determines the order in which operators are evaluated in expressions. Associatity determines whether an expression is evaluated left-to-right or right-to-left.
94. When a thread blocks on I/O, what state does it enter?
Answer: A thread enters the waiting state when it blocks on I/O.
95. Which class should you use to obtain design information about an object?
Answer: The Class class is used to obtain information about an object's design.
96. Can a lock be acquired on a class?
Answer: Yes, a lock can be acquired on a class. This lock is acquired on the class's Class object..
97. How are the elements of a GridBagLayout organized?
Answer: The elements of a GridBagLayout are organized according to a grid. However, the elements are of different sizes and may occupy more than one row or column of the grid. In addition, the rows and columns may have different sizes.
98. What is the difference between a Choice and a List?
Answer: A Choice is displayed in a compact form that requires you to pull it down to see the list of available choices. Only one item may be selected from a Choice. A List may be displayed in such a way that several List items are visible. A List supports the selection of one or more List items.
99. How does multithreading take place on a computer with a single CPU?
Answer: The operating system's task scheduler allocates execution time to multiple tasks. By quickly switching between executing tasks, it creates the impression that tasks execute sequentially.
100. Is "abc" a primitive value?
Answer: The String literal "abc" is not a primitive value. It is a String object.
101. What methods are used to get and set the text label displayed by a Button object?
Answer: getLabel() and setLabel().
102. What modifiers may be used with an inner class that is a member of an outer class?
Answer: A (non-local) inner class may be declared as public, protected, private, static, final, or abstract.
103. Which containers may have a MenuBar?
Answer: Frame.
104. What happens when a thread cannot acquire a lock on an object?
Answer: If a thread attempts to execute a synchronized method or synchronized statement and is unable to acquire an object's lock, it enters the waiting state until the lock becomes available.
105. What is the difference between the prefix and postfix forms of the ++ operator?
Answer: 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.
106. How are Java source code files named?
Answer: A Java source code file takes the name of a public class or interface that is defined within the file. A source code file may contain at most one public class or interface. If a public class or interface is defined within a source code file, then the source code file must take the name of the public class or interface. If no public class or interface is defined within a source code file, then the file must take on a name that is different than its classes and interfaces. Source code files use the .java extension.
107. Which Java operator is right associative?
Answer: The = operator is right associative.
108. What is the difference between a public and a non-public class?
Answer: A public class may be accessed outside of its package. A non-public class may not be accessed outside of its package.
109. How are this and super used?
Answer: 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.
110. Why are the methods of the Math class static?
Answer: So they can be invoked as if they are a mathematical code library.
111. When a thread is created and started, what is its initial state?
Answer: A thread is in the ready state after it has been created and started.
112. Name three Component subclasses that support painting.
Answer: The Canvas, Frame, Panel, and Applet classes support painting.
113. Does a class inherit the constructors of its superclass?
Answer: A class does not inherit constructors from any of its superclasses.
114. What is the return type of a program's main() method?
Answer: A program's main() method has a void return type.
115. What is a transient variable?
Answer: A transient variable is a variable that may not be serialized.
116. What is the difference between the >> and >>> operators?
Answer: The >> operator carries the sign bit when shifting right. The >>> zero-fills bits that have been shifted out.
117. What method is invoked to cause an object to begin executing as a separate thread?
Answer: The start() method of the Thread class is invoked to cause an object to begin executing as a separate thread.
118. What classes of exceptions may be caught by a catch clause?
Answer: A catch clause can catch any exception that may be assigned to the Throwable type. This includes the Error and Exception types.
119. What modifiers may be used with a top-level class?
Answer: A top-level class may be public, abstract, or final.
120. What is the relationship between a method's throws clause and the exceptions that can be thrown during the method's execution?
Answer: A method's throws clause must declare any checked exceptions that are not caught within the body of the method.
121. What is an abstract method?
Answer: An abstract method is a method whose implementation is deferred to a subclass.
122. What value does read() return when it has reached the end of a file?
Answer: The read() method returns -1 when it has reached the end of a file.
123. What is the immediate superclass of the Dialog class?
Answer: Window.
124. Is the ternary operator written x : y ? z or x ? y : z ?
Answer: It is written x ? y : z.
125. Why do threads block on I/O?
Answer: Threads block on I/O (that is enters the waiting state) so that other threads may execute while the I/O Operation is performed.
126. If an object is garbage collected, can it become reachable again?
Answer: Once an object is garbage collected, it ceases to exist. It can no longer become reachable again.
127. What is the immediate superclass of Menu?
Answer: MenuItem.
128. What is the purpose of the finally clause of a try-catch-finally statement?
Answer: The finally clause is used to provide the capability to execute code no matter whether or not an exception is thrown or caught.
129. What is a void return type?
Answer: A void return type indicates that a method does not return a value.
130. What is the argument type of a program's main() method?
Answer: A program's main() method takes an argument of the String[] type.
131. When does the compiler supply a default constructor for a class?
Answer: The compiler supplies a default constructor for a class if no other constructors are provided.
132. What happens when you add a double value to a String?
Answer: The result is a String object.
133. What method is used to specify a container's layout?
Answer: The setLayout() method is used to specify a container's layout.
134. Name three subclasses of the Component class.
Answer: Box.Filler, Button, Canvas, Checkbox, Choice, Container, Label, List, Scrollbar, or TextComponent.
135. Is &&= a valid Java operator?
Answer: No. It is not a valid java operator.
136. Which characters may be used as the second character of an identifier, but not as the first character of an identifier?
Answer: The digits 0 through 9 may not be used as the first character of an identifier but they may be used after the first character of an identifier.
137. Which containers use a border Layout as their default layout?
Answer: The Window, Frame and Dialog classes use a border layout as their default layout.
138. What class is the top of the AWT event hierarchy?
Answer: The java.awt.AWTEvent class is the highest-level class in the AWT event-class hierarchy.
139. What is the Vector class?
Answer: The Vector class provides the capability to implement a growable array of objects
140. What method must be implemented by all threads?
Answer: All tasks must implement the run() method, whether they are a subclass of Thread or implement the Runnable interface.
141. Can an anonymous class be declared as implementing an interface and extending a class?
Answer: An anonymous class may implement an interface or extend a superclass, but may not be declared to do both.
142. For which statements does it make sense to use a label?
Answer: The only statements for which it makes sense to use a label are those statements that can enclose a break or continue statement.
143. Which TextComponent method is used to set a TextComponent to the read-only state?
Answer: setEditable().
144. Which arithmetic operations can result in the throwing of an ArithmeticException?
Answer: Integer / and % can result in the throwing of an ArithmeticException.
145. Name two subclasses of the TextComponent class.
Answer: TextField and TextArea.
146. What are three ways in which a thread can enter the waiting state?
Answer: 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 in