Core Java Interview Questions -8
1. How does multithreading take place on a computer with a single CPU?
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.
2. What is the relationship between an event-listener interface and an event-adapter class?
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.
3. How are the elements of a CardLayout organized?
The elements of a CardLayout are stacked, one on top of the other, like a deck of cards.
4.Is &&= a valid Java operator?
No, it is not.
5. What restrictions are placed on the values of each case of a switch statement?
During compilation, the values of each case of a switch statement must evaluate to a value that can be promoted to an int value.
6. How are the elements of a GridBagLayout organized?
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.
7. Which TextComponent method is used to set a TextComponent to the read-only state?
8. What is the relationship between clipping and repainting?
When a window is repainted by the AWT painting thread, it sets the clipping regions to the area of the window that requires repainting.
9. Name the eight primitive Java types.
The eight primitive types are byte, char, short, int, long, float, double, and boolean.
10. Is “abc” a primitive value?
The String literal “abc” is not a primitive value. It is a String object.
11. What event results from the clicking of a button?
The ActionEvent event is generated as the result of the clicking of a button.
12. Which class should you use to obtain design information about an object?
The Class class is used to obtain information about an object’s design.
13. What is the difference between a while statement and a do statement?
A while statement checks at the beginning of a loop to see whether the next loop iteration should occur. A do statement checks at the end of a loop to see whether the next iteration of a loop should occur. The do statement will always execute the body of a loop at least once.
14. Which class is the immediate superclass of the Container class?
15. What is the highest-level event class of the event-delegation model?
The java.util.EventObject class is the highest-level class in the event-delegation class hierarchy.
16. What modifiers may be used with an interface declaration?
An interface may be declared as public or abstract.
17. Which Math method is used to calculate the absolute value of a number?
The abs() method is used to calculate absolute values.
18. How can a GUI component handle its own events?
A component can handle its own events by implementing the required event-listener interface and adding
itself as its own event listener.
19. Is a class a subclass of itself?
A class is a subclass of itself.
20. What advantage do Java’s layout managers provide over traditional windowing systems?
Java uses layout managers to lay out components in a consistent manner across all windowing platforms. Since Java’s layout managers aren’t tied to absolute sizing and positioning, they are able to accommodate platform-specific differences among windowing systems
21. When is the finally clause of a try-catch-finally statement executed?
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.
22. What modifiers can be used with a local inner class?
A local inner class may be final or abstract.
23. What is the difference between static and non-static variables?
A static variable is associated with the class as a whole rather than with specific instances of a class. Nonstatic variables take on unique values with each object instance.
24. How can the Checkbox class be used to create a radio button?
By associating Checkbox objects with a CheckboxGroup
25. What is the purpose of the File class?
The File class is used to create objects that provide access to the files and directories of a local file system.124. Can an exception be rethrown?
Yes, an exception can be rethrown.
26. What is the Collection interface?
The Collection interface provides support for the implementation of a mathematical bag – an unordered collection of objects that may contain duplicates.
27. What is the difference between the paint() and repaint() methods?
The paint() method supports painting via a Graphics object. The repaint() method is used to cause paint() to be invoked by the AWT painting thread.
28. If a method is declared as protected, where may the method be accessed?
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.
29. When does the compiler supply a default constructor for a class?
The compiler supplies a default constructor for a class if no other constructors are provided.
30. Which non-Unicode letter characters may be used as the first character of an identifier?
The non-Unicode letter characters $ and _ may appear as the first character of an identifier
31. What restrictions are placed on method overloading?
Two methods may not have the same name and argument list but different return types.
32. What happens when you invoke a thread’s interrupt method while it is sleeping or waiting?
When a task’s interrupt() method is executed, the task enters the ready state. The next time the task enters
the running state, an InterruptedException is thrown.
33. What is casting?
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.
34. What is the return type of a program’s main() method?
A program’s main() method has a void return type.
35. Name four Container classes.
Window, Frame, Dialog, FileDialog, Panel, Applet, or ScrollPane
36. What is the difference between a Choice and a List?
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.
37. What class of exceptions are generated by the Java run-time system?
The Java runtime system generates RuntimeException and Error exceptions.140. What class allows you to read objects directly from a stream?
The ObjectInputStream class supports the reading of objects from input streams.
38. What is the difference between a field variable and a local variable?
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.
39. Under what conditions is an object’s finalize() method invoked by the garbage collector?
The garbage collector invokes an object’s finalize() method when it detects that the object has become
40. How are this() and super() used with constructors?
this() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor.
41. What is the relationship between a method’s throws clause and the exceptions that can be thrown during the method’s execution?
A method’s throws clause must declare any checked exceptions that are not caught within the body of the method.
42. What is the difference between the JDK 1.02 event model and the event-delegation model introduced with JDK 1.1?
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
43. How is it possible for two String objects with identical values not to be equal under the ==operator?
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.
44. Why are the methods of the Math class static?
So they can be invoked as if they are a mathematical code library.
45. What Checkbox method allows you to tell if a Checkbox is checked?
46. What state is a thread in when it is executing?
An executing thread is in the running state.
47. What are the legal operands of the instanceof operator?
The left operand is an object reference or null value and the right operand is a class, interface, or array type.
48. How are the elements of a GridLayout organized?
The elements of a GridBad layout are of equal size and are laid out using the squares of a grid.
49. What an I/O filter?
An I/O filter is an object that reads from one stream and writes to another, usually altering the data in some way as it is passed from one stream to another.
50. If an object is garbage collected, can it become reachable again?
Once an object is garbage collected, it ceases to exist. It can no longer become reachable again.
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.169. What restrictions are placed on method overriding?
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.
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
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
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?
89. Can try statements be nested?
Try statements may be tested.183. 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
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.
98) 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.