Core Java Interview Questions- 10

Core Java Interview Questions- 10
1. What are wrapped classes?
Wrapped classes are classes that allow primitive types to be accessed as objects.

2. What is meant by pass by reference and pass by value in Java?
Pass by reference means, passing the address itself rather than passing the value. Pass by value means
passing a copy of the value.

3. If you’re overriding the method equals() of an object, which other method you might also consider?

hashCode()

4. What is Byte Code? What gives Java it’s “write once and run anywhere” nature?
All Java programs are compiled into class files that contain bytecodes. These byte codes can be run in any
platform and hence java is said to be platform independent.

5. Can there be an abstract class with no abstract methods in it?
Yes

6. Can an Interface have an inner class?
Yes

7. What are the different identifier states of a Thread?
The different identifiers of a Thread are: R - Running or runnable thread, S - Suspended thread, CW - Thread
waiting on a condition variable, MW - Thread waiting on a monitor lock, MS - Thread suspended waiting on a
monitor lock.

8. What if the main method is declared as private?

The program compiles properly but at runtime it will give "Main method not public." message.

9. Can an Interface be final?
No

10. What modifiers are allowed for methods in an Interface?
Only public and abstract modifiers are allowed for methods in interfaces.

11. Can we define private and protected modifiers for variables in interfaces?
No

12. What is Externalizable?
Externalizable is an Interface that extends Serializable Interface and sends data into Streams in Compressed
Format. It has two methods, writeExternal(ObjectOuput out) and readExternal(ObjectInput in)

13. What are some alternatives to inheritance?
Delegation is an alternative to inheritance. Delegation means that you include an instance of another class as
an instance variable, and forward messages to the instance. It is often safer than inheritance because it
forces you to think about each message you forward, because the instance is of a known class, rather than a
new class, and because it doesn’t force you to accept all the methods of the super class: you can provide
only the methods that really make sense. On the other hand, it makes you write more code, and it is harder to
re-use (because it is not a subclass).

14. What is a local, member and a class variable?

Variables declared within a method are “local” variables. Variables declared within the class i.e not within any
methods are “member” variables (global variables). Variables declared within the class i.e not within any
methods and are defined as “static” are class variables.

15. How do I convert a numeric IP address like 192.18.97.39 into a hostname like java.sun.com?
String hostname = InetAddress.getByName("192.18.97.39").getHostName();

16. What is the difference between JRE/JVM/JDK?
JDK is the Java Development Kit i.e. JDK is bundle of software that you can use to develop Java based
software. JRE is the Java Runtime Environment i.e. JRE is an implementation of the Java Virtual Machine
which actually executes Java programs. Typically, each JDK contains one (or more) JRE’s along with the
various development tools like the Java source compilers, bundling and deployment tools, debuggers,
development libraries, etc.

17. Why isn’t there operator overloading?
Because C++ has proven by example that operator overloading makes code almost impossible to maintain.
In fact there very nearly wasn’t even method overloading in Java, but it was thought that this was too useful
for some very basic methods like print(). Note that some of the classes like DataOutputStream have
unoverloaded methods like writeInt() and writeByte().

18. What does it mean that a method or field is “static”?
Static variables and methods are instantiated only once per class. In other words they are class variables, not
instance variables. If you change the value of a static variable in a particular object, the value of that variable
changes for all instances of that class. Static methods can be referenced with the name of the class rather
than the name of a particular object of the class (though that works too).

19. 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.

20. Is null a keyword?

The null value is not a keyword.

21. 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 errors.

22. 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.

23. 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.

24. 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.

25. 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 both.

26. What is a native method?
A native method is a method that is implemented in a language other than Java.

27. 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

28. 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.

29. 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.

30. 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 comments).

31.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.

32. What is the range of the char type?
The range of the char type is 0 to 2^16 - 1.

33. What is the difference between sleep() and wait() methods?
Difference between sleep() and wait() is that since sleep() is an static method of class Thread so whenever it
gets executed it puts the “thread of execution” i.e. currently executing thread to sleep and it will again goes to
runnable state once the specified time is finished whereas wait() is a method of class Object so whenver it
executes on an object it puts the thread who attains the object’s lock in waiting stage and once this thread get
notified (using notify()) by other thread or its wait time is elapsed, thread again comes to runnable state. I
hope this will help you to distinguish between sleep() and wait() methods.

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

35.What are the practical benefits, if any, of importing a specific class rather than an entire package (e.g. import java.net.* versus import java.net.Socket)?
It makes no difference in the generated class files since only the classes that are actually used are referenced by the generated class file. There is another practical benefit to importing single classes, and this arises when two (or more) packages have classes with the same name. Take java.util.Timer and javax.swing.Timer, for example. If I import java.util.* and javax.swing.* and then try to use "Timer", I get an error while compiling (the class name is ambiguous between both packages). Let's say what you really wanted was the javax.swing.Timer class, and the only classes you plan on using in java.util are Collection and HashMap. In this case, some people will prefer to import java.util.Collection and import java.util.HashMap instead of importing java.util.*. This will now allow them to use Timer, Collection, HashMap, and other java.swing classes without using fully qualified class names in.

36.Can a method be overloaded based on different return type but same argument type ?
No, because the methods can be called without using their return type in which case there is ambiquity for the compiler

37.What happens to a static var that is defined within a method of a class ?
Can't do it. You'll get a compilation error

38.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.

39. What is the use of anonymous class?
Annoymous classes are basically used where you want your class to do only one kind of job eg. catching the
user action (i.e. he clicked on some button or change the value of any UI component). It will help you to
maintain the modularity in the code whithin a single file.

40.How many static init can you have ?
As many as you want, but the static initializers and class variable initializers are executed in textual order and may not refer to class variables declared in the class whose declarations appear textually after the use, even though these class variables are in scope.

41.What is constructor chaining and how is it achieved in Java ?
A child object constructor always first needs to construct its parent (which in turn calls its parent constructor.). In Java it is done via an implicit call to the no-args constructor as the first statement.

42. What is the difference between Vector & ArrayList?
Both Vector and arraylist are same exept of one reason. Vector methods are synchronized(only one thread
can execute it) where as arraylist method are not sysnchronized. but u asked diff. between vector and array.
Vector is collection of objects and the space allocated for vector is in heap. Simple array is collection of
primitive values and space allocated is not heap.

43.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.

44.Is sizeof a keyword?
The sizeof operator is not a keyword.

45.Which Java operator is right associative?
The = operator is right associative.

46.What is the difference between method overriding and overloading?
Overriding is a method with the same name and arguments as in a parent, whereas overloading is the same method name but different arguments

47.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.

48.Can a double value be cast to a byte?

Yes, a double value can be cast to a byte.

49.What does it mean that a class or member is final?
A final class cannot be inherited. A final method cannot be overridden in a subclass. A final field cannot be changed after it's initialized, and it must include an initializer statement where it's declared.

50.What does it mean that a method or class is abstract?
An abstract class cannot be instantiated. Abstract methods may only be included in abstract classes. However, an abstract class is not required to have any abstract methods, though most of them do. Each subclass of an abstract class must override the abstract methods of its superclasses or it also should be declared abstract.

51.To what value is a variable of the String type automatically initialized?
The default value of an String type is null.
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.

52.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.

53.What is the difference between method overriding and overloading?
Overriding is a method with the same name and arguments as in a parent, whereas overloading is the same method name but different arguments

54.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.

55.Can a for statement loop indefinitely? Yes, a for statement can loop indefinitely. For example, consider the following: for(;;) ;

56.What is a transient variable?
transient variable is a variable that may not be serialized.

57.Why wait and notify is declared in Object class instead of Thread ?
Another tough java question, how can you answer this question if you are not designed Java programming language. anyway some common sense and deep knowledge of Java programming helps to answer such tough core java interview question. See this blog post to learn  Why wait and notify is declared in Object class and not in Thread.

58.Why multiple inheritance is not supported in Java ?
I found this core Java question really tough to answer because your answer may not satisfy Interviewer, in most cases Interviewer is looking for specific points and if you can bring them, they would be happy. Key to answer this kind of tough question in Java is to prepare topic well to accommodate any follow-ups. See Why multiple inheritance is not supported in Java for answer of this tough Java question.

59.Why Java does not support operator overloading ?
One more similar category of  tough Java question. C++ supports operator overloading than why not Java? this is the argument Interviewer will give to you and some time even say that + operator is overloaded in Java for String concatenation, Don't be fooled with such arguments. See  Why support operator overloading is not supported in Java for detailed answer of this tricky Java question.

60.Why String is immutable in Java?
My favorite Java interview question, this is tough, tricky but same time very useful as well. Some interviewer also ask this question as Why String is final in Java. look at this post for some points which make sense on Why String is final or immutable in Java

61.Why char array is preferred to store password than String in Java?
Another tricky Java question which is based on String and believe me there are only few Java programmer which can answer this question correctly. This is a real tough core Java interview question and again solid knowledge of String is required to answer this. see Why char array is better than String for storing password in Java to find out answer of this tough Java question.

62.How to create thread-safe singleton in Java using double checked locking?
This Java question is also asked as What is thread-safe singleton  and how to do you write it. Well Singleton created with double checked locking before Java 5 was broker and its possible to have multiple instance of Singleton if multiple thread try to create instance of Singleton at same time. from Java 5 its easy to create thread safe Singleton using Enum. but if interviewer persist with double checked locking then you have to write that code for them. remember to use volatile variable.  See 10 Java singleton interview question for more details on this topic.

63. 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 normally.

64. What is the Collections API?

The Collections API is a set of classes and interfaces that support operations on collections of objects.

65. Which containers use a FlowLayout as their default layout?

The Panel and Applet classes use the FlowLayout as their default layout.

66.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.

67. What state does a thread enter when it terminates its processing?

When a thread terminates its processing, it enters the dead state.

68.Is null a keyword?

The null value is not a keyword.

69. 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.

70. What method is used to specify a container's layout?

The setLayout() method is used to specify a container's layout.

71. 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.

72. 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.

73. 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.

74. What is an Iterator interface?
The Iterator interface is used to step through the elements of a Collection.

75. 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.

76. Which method of the Component class is used to set the position and size of a component?

setBounds()

77. Which java.util classes and interfaces support event handling?

The EventObject class and the EventListener interface support event processing.

78. What is the List interface?

The List interface provides support for ordered collections of objects.

79.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.

80. What is the Vector class?

The Vector class provides the capability to implement a growable array of objects

81. Is sizeof a keyword?

The sizeof operator is not a keyword.

82. 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.

83. 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.

84. 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.

85. What is a native method?

A native method is a method that is implemented in a language other than Java.

86. What are wrapped classes?

Wrapped classes are classes that allow primitive types to be accessed as objects.

87. What is the immediate superclass of the Dialog class?
Window

88. Can a for statement loop indefinitely?
Yes, a for statement can loop indefinitely. For example, consider the following: for(;;) ;

89. What is clipping?
Clipping is the process of confining paint operations to a limited area or shape.

90. 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.

91. 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

92.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 comments).

93. Name three Component subclasses that support painting.

The Canvas, Frame, Panel, and Applet classes support painting.

94. 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.

95. What is the range of the char type?

The range of the char type is 0 to 2^16 - 1. 

96. 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.

97. 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.

98. What is the immediate superclass of Menu?

MenuItem

99. 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.

100. Which class is the immediate superclass of the MenuComponent class.
Object

101. Name three subclasses of the Component class.

Box.Filler, Button, Canvas, Checkbox, Choice, Container, Label, List, Scrollbar, or TextComponent

102. What is the immediate superclass of the Applet class?
Panel 

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