Core Java Interview Questions -7
1.Can a lock be acquired on a class?
Yes, a lock can be acquired on a class. This lock is acquired on the class’s Class object.
2. What method is used to specify a container’s layout?
The setLayout() method is used to specify a container’s layout.
3. How are Observer and Observable used?
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.
4.what is a transient variable?
A transient variable is a variable that may not be serialized.
5.Why do threads block on I/O?
Threads block on i/o (that is enters the waiting state) so that other threads may execute while the i/o
Operation is performed.
6. Which containers use a FlowLayout as their default layout?
The Panel and Applet classes use the FlowLayout as their default layout.
7.which containers use a border Layout as their default layout?
The window, Frame and Dialog classes use a border layout as their default layout.
8.What’s new with the stop(), suspend() and resume() methods in JDK 1.2?
The stop(), suspend() and resume() methods have been deprecated in JDK 1.2.
9. Is null a keyword?
The null value is not a keyword.
10. What is the Collections API?
The Collections API is a set of classes and interfaces that support operations on collections of objects.
11. What is the Vector class?
The Vector class provides the capability to implement a growable array of objects
12. Which characters may be used as the second character of an identifier, but not as the first
character of an identifier?
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.
13. What state does a thread enter when it terminates its processing?
When a thread terminates its processing, it enters the dead state.
14. What restrictions are placed on the location of a package statement within a source code file?
A package statement must appear as the first line in a source code file (excluding blank lines and
15. How does Java handle integer overflows and underflows?
It uses those low order bytes of the result that can fit into the size of the type allowed by the operation.
16. Can an object’s finalize() method be invoked while it is reachable?An object’s finalize() method cannot be invoked by the garbage collector while the object is still reachable.
However, an object’s finalize() method may be invoked by other objects.
17. What modifiers may be used with an inner class that is a member of an outer class?
A (non-local) inner class may be declared as public, protected, private, static, final, or abstract.
18.What is the difference between yielding and sleeping?
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.
19. Does garbage collection guarantee that a program will not run out of memory?
Garbage collection does not guarantee that a program will not run out of memory. It is possible for
programs to use up memory resources faster than they are garbage collected. It is also possible for
programs to create objects that are not subject to garbage collection
20. What is the immediate superclass of the Applet class?
21. What is the List interface?
The List interface provides support for ordered collections of objects.
22. What is the difference between the >> and >>> operators?
The >> operator carries the sign bit when shifting right. The >>> zero-fills bits that have been shifted out.
23.What is synchronization and why is it important?
With respect to multithreading, synchronization is the capability to control the access of multiple threads to
shared resources. Without synchronization, it is possible for one thread to modify a shared object while
another thread is in the process of using or updating that object’s value. This often leads to significant
24. What are wrapped classes?
Wrapped classes are classes that allow primitive types to be accessed as objects.
25. Which method of the Component class is used to set the position and size of a component?
26. Which java.util classes and interfaces support event handling?
The EventObject class and the EventListener interface support event processing.
27. Is sizeof a keyword?
The sizeof operator is not a keyword.
28. What is an Iterator interface?
The Iterator interface is used to step through the elements of a Collection.
29. How many bits are used to represent Unicode, ASCII, UTF-16, and UTF-8 characters?
Unicode requires 16 bits and ASCII require 7 bits. Although the ASCII character set uses only 7 bits, it is
usually represented as 8 bits. UTF-8 represents characters using 8, 16, and 18 bit patterns. UTF-16 uses 16-
bit and larger bit patterns.
30.What is the preferred size of a component?
The preferred size of a component is the minimum component size that will allow the component to display
31. What are order of precedence and associativity, and how are they used?
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
32. What value does readLine() return when it has reached the end of a file?
The readLine() method returns null when it has reached the end of a file.
33. What is a native method?
A native method is a method that is implemented in a language other than Java.
34. What is the immediate superclass of the Dialog class?
35. What is the difference between preemptive scheduling and time slicing?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.
36. When a thread blocks on I/O, what state does it enter?
A thread enters the waiting state when it blocks on I/O.
37. To what value is a variable of the String type automatically initialized?
The default value of an String type is null.
38. What is the catch or declare rule for method declarations?
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.
39. What is clipping?
Clipping is the process of confining paint operations to a limited area or shape.
40. Name three Component subclasses that support painting.
The Canvas, Frame, Panel, and Applet classes support painting.
41. What is the purpose of the finally clause of a try-catch-finally statement?
The finally clause is used to provide the capability to execute code no matter whether or not an exception is
thrown or caught.
42. Can a for statement loop indefinitely?
Yes, a for statement can loop indefinitely. For example, consider the following:
43. What is a task’s priority and how is it used in scheduling?
A task’s priority is an integer value that identifies the relative order in which it should be executed with
respect to other tasks. The scheduler attempts to schedule higher priority tasks before lower priority tasks.
44. What class is the top of the AWT event hierarchy?
The java.awt.AWTEvent class is the highest-level class in the AWT event-class hierarchy.
45. In which package are most of the AWT events that support the event-delegation model defined?
Most of the AWT-related events of the event-delegation model are defined in the java.awt.event package.
The AWTEvent class is defined in the java.awt package.
46. When a thread is created and started, what is its initial state?
A thread is in the ready state after it has been created and started.
47. Can an anonymous class be declared as implementing an interface and extending a class?
An anonymous class may implement an interface or extend a superclass, but may not be declared to do
48. What is the range of the short type?
The range of the short type is -(2^15) to 2^15 – 1.
49. What is the range of the char type?
The range of the char type is 0 to 2^16 – 1.
50. Which Container method is used to cause a container to be laid out and redisplayed?
51.What is the immediate superclass of Menu?
52. What is the purpose of the Runtime class?
The purpose of the Runtime class is to provide access to the Java runtime system.
53. How many times may an object’s finalize() method be invoked by the garbage collector?
An object’s finalize() method may only be invoked once by the garbage collector.
54. What is the purpose of finalization?
The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup
processing before the object is garbage collected.
55. Which class is the immediate superclass of the MenuComponent class.
56. Name three subclasses of the Component class.
Box.Filler, Button, Canvas, Checkbox, Choice, Container, Label, List, Scrollbar, or TextComponent
57. What invokes a thread’s run() method?
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.
58. What is the difference between the Boolean & operator and the && operator?
If an expression involving the Boolean & operator is evaluated, both operands are evaluated. Then the &
operator is applied to the operand. When an expression involving the && operator is evaluated, the first
operand is evaluated. If the first operand returns a value of true then the second operand is evaluated. The
&& operator is then applied to the first and second operands. If the first operand evaluates to false, the
evaluation of the second operand is skipped.
59. What is the difference between a MenuItem and a CheckboxMenuItem?
The CheckboxMenuItem class extends the MenuItem class to support a menu item that may be checked or
60. What is the GregorianCalendar class?
The GregorianCalendar provides support for traditional Western calendars.
61. What must a class do to implement an interface?
It must provide all of the methods in the interface and identify the interface in its implements clause.
62. What is the difference between a break statement and a continue statement?
A break statement results in the termination of the statement to which it applies (switch, for, do, or while).
A continue statement is used to end the current loop iteration and return control to the loop statement.
63. What is the argument type of a program’s main() method?
A program’s main() method takes an argument of the String type.
64. What method is invoked to cause an object to begin executing as a separate thread?
The start() method of the Thread class is invoked to cause an object to begin executing as a separate thread.
65. Name two subclasses of the TextComponent class.
TextField and TextArea
66. Which Java operator is right associative?
The = operator is right associative.63. What is the Locale class?
The Locale class is used to tailor program output to the conventions of a particular geographic, political, or
67. What is the advantage of the event-delegation model over the earlier event-inheritance model?
The event-delegation model has two advantages over the event-inheritance model. First, it enables event
handling to be handled by objects other than the ones that generate the events (or their containers). This
allows a clean separation between a component’s design and its use. The other advantage of the eventdelegation model is that it performs much better in applications where many events are generated. This
performance improvement is due to the fact that the event-delegation model does not have to repeatedly
process unhandled events, as is the case of the event-inheritance model.
68. Can a double value be cast to a byte?
Yes, a double value can be cast to a byte.
69. What is the relationship between the Canvas class and the Graphics class?
A Canvas object provides access to a Graphics object via its paint() method.
70. How are commas used in the initialization and iteration parts of a for statement?
Commas are used to separate multiple statements within the initialization and iteration parts of a for
71. What is the purpose of the wait(), notify(), and notifyAll() methods?
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.
72. What is the difference between a static and a non-static inner class?
A non-static inner class may have object instances that are associated with instances of the class’s outer
class. A static inner class does not have any object instances.
73. Which containers may have a MenuBar?
74. What is the difference between the String and StringBuffer classes?
String objects are constants. StringBuffer objects are not.
75. How are Java source code files named?
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.
76. What is an object’s lock and which object’s have locks?
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.
77. What is an abstract method?
An abstract method is a method whose implementation is deferred to a subclass.
78. Can a Byte object be cast to a double value?
No, an object cannot be cast to a primitive value.
79. When can an object reference be cast to an interface reference?
An object reference be cast to an interface reference when the object implements the referenced interface.
80. What are the high-level thread states?
The high-level thread states are ready, running, waiting, and dead.
81. If a variable is declared as private, where may the variable be accessed?
A private variable may only be accessed within the class in which it is declared.
82. What is the difference between a Window and a Frame?
The Frame class extends Window to define a main application window that can have a menu bar.
83. What is the SimpleTimeZone class?
The SimpleTimeZone class provides support for a Gregorian calendar.
84. For which statements does it make sense to use a label?
The only statements for which it makes sense to use a label are those statements that can enclose a break or
85. What value does read() return when it has reached the end of a file?
The read() method returns -1 when it has reached the end of a file.
86. Is the ternary operator written x : y ? z or x ? y : z ?
It is written x ? y : z.
87. What is the difference between the Reader/Writer class hierarchy and the InputStream/
OutputStream class hierarchy?
The Reader/Writer class hierarchy is character-oriented, and the InputStream/OutputStream class hierarchy
88. Does a class inherit the constructors of its superclass?
A class does not inherit constructors from any of its superclasses.
89. What is the % operator?
It is referred to as the modulo or remainder operator. It returns the remainder of dividing the first operand
by the second operand.
90. If a class is declared without any access modifiers, where may the class be accessed?
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.
91. Which class is extended by all other classes?
The Object class is extended by all other classes.
92. Can an object be garbage collected while it is still reachable?
A reachable object cannot be garbage collected. Only unreachable objects may be garbage collected..
93. What is the Dictionary class?
The Dictionary class provides the capability to store key-value pairs.
94. What is the difference between the Font and FontMetrics classes?
The FontMetrics class is used to define implementation-specific properties, such as ascent and descent, of a
95. What is the Map interface?
The Map interface replaces the JDK 1.1 Dictionary class and is used associate keys with values.
96. How are the elements of a BorderLayout organized?
The elements of a BorderLayout are organized at the borders (North, South, East, and West) and the center
of a container.
97. What happens when a thread cannot acquire a lock on an object?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.
98. What classes of exceptions may be caught by a catch clause?
A catch clause can catch any exception that may be assigned to the Throwable type. This includes the Error
and Exception types.
99. How is rounding performed under integer division?
The fractional part of the result is truncated. This is known as rounding toward zero.
100. What is the purpose of the System class?
The purpose of the System class is to provide access to system resources