Java Interview Questions -16

Java Interview Questions -16

1.When can an object reference be cast to an interface reference?

An object reference can be cast to an interface reference when the object implements the referenced interface.

2.What are Regular Inner Classes?

A Regular inner class is declared inside the curly braces of another class outside any method or other code block. This is the simplest form of inner classes.

3.What are Anonymous Inner Classes? Name the various forms of Anonymous Inner Classes.
Anonymous Inner Classes have no name, and their type must be either a subclass of the named type or an implementer of the named interface. The following are the different forms of inner classes:Anonymous subclass(i.e. extends a class)Anonymous implementer (i.e. implements an interface)Argument-Defined Anonymous Inner Classes

4.How many classes can an Anonymous Inner classes inherit from?


5.How many Interfaces can an Anonymous Inner classes implement?
One. Normal classes and other inner classes can implement more than one interface whereas anonymous inner classes can either implement a single interface or extend a single class.

6.Can a regular inner class access a private member of the enclosing class?
Yes. Since inner classes are treated as a member of the outer class they can access private members of the outer class.

7.What is a Marker Interface?
An Interface which doesn’t have any declaration inside but still enforces a mechanism.

8.What is the super class of all Inner Classes?
Inner class is just a concept and can be applied to any class, hence there is no common super class for inner classes.

9.How will you instantiate a regular inner class from outside the enclosing class?
Outer out=new Outer();Outer.Inner Inner();

10.What are Local Inner Classes or Method Local Inner Classes?
A method-local inner class is defined within a method of the enclosing class.

11. 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.

12.What is the List interface?

The List interface provides support for ordered collections of objects.

13.What are the constraints on Method Local Inner Classes?
The following are the restrictions for Method Inner Classes:Method Local Inner classes cannot acccess

local variables but can access final variables.Only abstract and final modifiers can be applied to

Method Local Inner classesMethod Local Inner classes can be instantiated only within the method in

which it is contained that too after the class definition.

14.Can a class be defined inside an Interface?
Yes it’s possible.

15.What are the disadvantages of Inner classes?

1. Inner classes are not reusable hence defeats one of the fundamental feature of Java.2. Highly

confusing and difficult syntax which leads poor code maintainability.

16.Name the different types of Inner Classes?
The following are the different types of Inner classes:Regular Inner ClassMethod Local Inner

ClassStatic Inner ClassAnonymous Inner Class

17.Can we define private and protected modifiers for variables in interfaces?

No. Always all variables declared inside a interface are of public access.

18.What are Inner Classes?

Inner classes are classes which are defined inside another class.

19.What are Nested Classes?
Static Inner classes are called sometimes referred as nested classes because these classes can exist

without any relationship with the containing class.

20.Can an Interface be defined inside a class?

Yes it’s possible.

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

22.How many class files are produced for source file having one Outer class and one Inner class?
Two class files will be produced as follows:Outer.classOuter$Inner.class

23.Explain about Java Collections API?

Java Collections Framework provides a set of interfaces and classes that support operations on a

collections of objects.

24.What is the difference between set and list?

Set stores elements in an unordered way but does not contain duplicate elements, where as List stores

elements in an ordered way but may contain duplicate elements.

25.What is map interface in a java?

Map is an object that maps keys to values. A map cannot contain duplicate keys: Each key can map to

at most one value. This Map permits null value

26.What is the difference between Map and Hashmap?
Map is Interface and Hashmap is class that implements that

27.Can you instantiate the static Inner Class without the existence of the outer class object?

If Yes, Write a sample statement. Yes. It can be instantiated as follows by referencing the Outer

class.Outer.Inner in = new Outer.Inner();

28.What are the constraints on Static Inner Classes?
It cannot access non-static members of the outer class.It cannot use this reference to the outer class.

29.What is an Iterator interface?
The Iterator is an interface, used to traverse through the elements of a Collection. It is not advisable

to modify the collection itself while traversing an Iterator.

30. Difference between Vector and ArrayList?
Vector & ArrayList both classes are implemented using dynamically resizable arrays, providing fast

random access and fast traversal. ArrayList and Vector class both implement the List interface.
1) Synchronization – ArrayList is not thread-safe whereas Vector is thread-safe. In Vector class each

method like add(), get(int i) is surrounded with a synchronized block and thus making Vector class

2) Data growth – Internally, both the ArrayList and Vector hold onto their contents using an Array.

When an element is inserted into an ArrayList or a Vector, the object will need to expand its internal

array if it runs out of room. A Vector defaults to doubling the size of its array, while the ArrayList

increases its array size by 50 percent.

31.What are Static Inner Classes?

Static Inner Classes are inner classes which marked with a static modifier. These classes need not

have any relationship with the outer class. These can be instantiated even without the existence of the

outer class object.

32.What is the difference between a HashMap and a Hashtable in Java?

Hashtable is synchronized, whereas HashMap is not. This makes HashMap better for non-threaded

applications, as unsynchronized Objects typically perform better than synchronized ones.
Hashtable does not allow null keys or values. HashMap allows one null key and any number of null

One of HashMap’s subclasses is LinkedHashMap, so in the event that you’d want predictable iteration

order (which is insertion order by default), you could easily swap out the HashMap for

aLinkedHashMap. This wouldn’t be as easy if you were using Hashtable.

33.What is a vector in Java?

Vector implements a dynamic array. It is similar to ArrayList, but with two differences:  Vector is

synchronized, and it contains many legacy methods that are not part of the collections framework.

34.What is ArrayList In java?

ArrayList is a part of the Collection Framework. We can store any type of objects, and we can deal

with only objects. It is growable.

35.What is the Properties class?

The properties class is a subclass of Hashtable that can be read from or written to a stream. It also

provides the capability to specify a set of default values to be used.

36.What is difference between HashMap and HashSet?

HashSet : HashSet does not allow duplicate values. It provides add method rather put method. You

also use its contain method to check whether the object is already available in HashSet. HashSet can

be used where you want to maintain a unique list.
HashMap : It allows null for both key and value. It is unsynchronized. So come up with better


37. Can I have multiple main() methods in the same class?

No the program fails to compile. The compiler says that the main() method is already defined in the


38. What environment variables do I need to set on my machine in order to be able to run Java


CLASSPATH and PATH are the two variables.

39. Can an application have multiple classes having main() method?

Yes it is possible. While starting the application we mention the class name to be run. The JVM will

look for the Main method only in the class whose name you have mentioned.
Hence there is not conflict amongst the multiple classes having main() method.

40. What is TreeSet ?

TreeSet – It is the implementation of SortedSet interface.This implementation provides guaranteed

log(n) time cost for the basic operations (add, remove and contains). The class is not synchronized.

41. what is Enumeration in java?

An enumeration is an object that generates elements one at a time, used for passing through a

collection, usually of unknown size. The traversing of elements can only be done once per creation.

42. Describe synchronization in respect to multithreading.

With respect to multithreading, synchronization is the capability to control the access of multiple

threads to shared resources.
Without synchonization, it is possible for one thread to modify a shared variable while another thread

is in the process of using or updating same shared variable. This usually leads to significant errors.

43.What is difference between Iterator and Enumeration?

Both Iterator and Enumeration are used to traverse Collection objects, in a sequential fashion.

Enumeration can be applied to Vector and HashTable. Iterator can be used with most of the Collection

The main difference between the two is that Iterator is fail-safe. i.e,  If you are using an iterator to go

through a collection you can be sure of no concurrent modifications in the underlying collection which

may happen in multi-threaded environments.

44. What is the Difference between the Iterator  and ListIterator?

Iterator : Iterator takes the place of Enumeration in the Java collections framework. One can traverse

throughr the the collection with the help of iterator in forward direction only and Iterators allow the

caller to remove elements from the underlying collection during the iteration with well-defined

ListIterator: An iterator for lists that allows one to traverse the list in either direction.modify the list

during iteration, and obtain the iterator’s current position in the list. A ListIterator has no current

element. its cursor position always lies between the element that would be returned by a call to

previous() and the element that would be returned by a call to next(). In a list of length n, there are

n+1 valid index values, from 0 to n, inclusive.

45. What is an abstract class?

Abstract class must be extended/subclassed (to be useful). It serves as a template. A class that is

abstract may not be instantiated (ie. you may not call its constructor), abstract class may contain

static data.
Any class with an abstract method is automatically abstract itself, and must be declared as such. A

class may be declared abstract even if it has no abstract methods. This prevents it from being


46.What is the difference between an Interface and an Abstract class?

An abstract class can have instance methods that implement a default behavior. An Interface can only

declare constants and instance methods, but cannot implement default behavior and all methods are

implicitly abstract.
An interface has all public members and no implementation. An abstract class is a class which may

have the usual flavors of class members (private, protected, etc.), but has some abstract methods.

47.Explain different way of using thread?

The thread could be implemented by using runnable interface or by inheriting from the Thread class.

The former is more advantageous, ’cause when you are going for multiple inheritance, the only

interface can help.

48. Can I import same package/class twice? Will the JVM load the package twice at runtime?

One can import the same package or same class multiple times. Neither compiler nor JVM complains

about it. And the JVM will internally load the class only once no matter how many times you import

the same class.

49. What are Checked and UnChecked Exception?

A checked exception is some subclass of Exception (or Exception itself), excluding class

RuntimeException and its subclasses. Making an exception checked forces client programmers to deal

with the possibility that the exception will be thrown.
Example: IOException thrown by’s read() method·
Unchecked exceptions are RuntimeException and any of its subclasses. Class Error and its subclasses

also are unchecked. With an unchecked exception, however, the compiler doesn’t force client

programmers either to catch the exception or declare it in a throws clause. In fact, client programmers

may not even know that the exception could be thrown.
Example: StringIndexOutOfBoundsException thrown by String’s charAt() method· Checked exceptions

must be caught at compile time. Runtime exceptions do not need to be. Errors often cannot be.

50. What is the difference between a constructor and a method?

A constructor is a member function of a class that is used to create objects of that class. It has the

same name as the class itself, has no return type, and is invoked using the new operator.
A method is an ordinary member function of a class. It has its own name, a return type (which may be

void), and is invoked using the dot operator.

51. What is the purpose of garbage collection in Java, and when is it used?

The purpose of garbage collection is to identify and discard objects that are no longer needed by a

program so that their resources can be reclaimed and reused.
A Java object is subject to garbage collection when it becomes unreachable to the program in which it

is used.

52. What is final class?

A final class can’t be extended ie., final class may not be subclassed. A final method can’t be

overridden when its class is inherited. You can’t change value of a final variable (is a constant).

53. What if the main() method is declared as private?

The program compiles properly but at runtime it will give “main() method not public.” message.

54. What if the static modifier is removed from the signature of the main() method?

Program compiles. But at runtime throws an error “NoSuchMethodError”.

55. What if I write static public void instead of public static void?

Program compiles and runs properly.

56. What if I do not provide the String array as the argument to the method?

Program compiles but throws a runtime error “NoSuchMethodError”.

57. Do I need to import java.lang package any time? Why ?

No. It is by default loaded internally by the JVM.

58. What is the first argument of the String array in main() method?

The String array is empty. It does not have any element. This is unlike C/C++ where the first element

by default is the program name.

59. If I do not provide any arguments on the command line, then the String array of main() method

will be empty or null?
It is empty. But not null.

60.What is an Iterator?

Some of the collection classes provide traversal of their contents via a java.util.Iterator interface. This

interface allows you to walk through a collection of objects, operating on each object in turn.
Remember when using Iterators that they contain a snapshot of the collection at the time the Iterator

was obtained; generally it is not advisable to modify the collection itself while traversing an Iterator.

61. State the significance of public, private, protected, default modifiers both singly and in

combination and state the effect of package relationships on declared items qualified by these

public: Public class is visible in other packages, field is visible everywhere (class must be public too)
private : Private variables or methods may be used only by an instance of the same class that declares

the variable or method, A private feature may only be accessed by the class that owns the feature.
protected : Is available to all classes in the same package and also available to all subclasses of the

class that owns the protected feature. This access is provided even to subclasses that reside in a

different package from the class that owns the protected feature.
What you get by default ie, without any access modifier (ie, public private or protected). It means that

it is visible to all within a particular package.

62. What is static in java?

Static means one per class, not one for each object no matter how many instance of a class might

exist. This means that you can use them without creating an instance of a class.Static methods are

implicitly final, because overriding is done based on the type of the object, and static methods are

attached to a class, not an object.
A static method in a superclass can be shadowed by another static method in a subclass, as long as

the original method was not declared final. However, you can’t override a static method with a

nonstatic method. In other words, you can’t change a static method into an instance method in a


63. How can one prove that the array is not null but empty using one line of code?

Print args.length. It will print 0. That means it is empty. But if it would have been null then it would

have thrown a NullPointerException on attempting to print args.length.

64. What is Overriding?

When a class defines a method using the same name, return type, and arguments as a method in its

superclass, the method in the class overrides the method in the superclass.
When the method is invoked for an object of the class, it is the new definition of the method that is

called, and not the method definition from superclass. Methods may be overridden to be more public,

not more private.

65. Are the imports checked for validity at compile time? Example: will the code containing an import

such as java.lang.ABCD compile?
Yes the imports are checked for the semantic validity at compile time. The code containing above line

of import will not compile. It will throw an error saying, can not resolve symbol
symbol : class ABCD
location: package io

66. Does importing a package imports the subpackages as well? Example: Does importing

com.MyTest.* also import com.MyTest.UnitTests.*?
No you will have to import the subpackages explicitly. Importing com.MyTest.* will import classes in

the package MyTest only. It will not import any class in any of it’s subpackage.

67. What is the difference between declaring a variable and defining a variable?

In declaration we just mention the type of the variable and it’s name. We do not initialize it. But

defining means declaration + initialization.
Example: String s; is just a declaration while String s = new String (“abcd”); Or String s = “abcd”;

are both definitions.

68. What is the default value of an object reference declared as an instance variable?

The default value will be null unless we define it explicitly.

69. What is serialization?

Serialization is a mechanism by which you can save the state of an object by converting it to a byte


70. What is the common usage of serialization?

Whenever an object is to be sent over the network, objects need to be serialized. Moreover if the state

of an object is to be saved, objects need to be serilazed.

71. Can a top level class be private or protected?

No. A top level class cannot be private or protected. It can have either “public” or no modifier. If it

does not have a modifier it is supposed to have a default access.
If a top level class is declared as private the compiler will complain that the “modifier private is not

allowed here”. This means that a top level class can not be private. Same is the case with protected.

72. What type of parameter passing does Java support?

In Java the arguments are always passed by value.

73. Primitive data types are passed by reference or pass by value?

Primitive data types are passed by value.

74. Objects are passed by value or by reference?

Java only supports pass by value. With objects, the object reference itself is passed by value and so

both the original reference and parameter copy both refer to the same object.

75. What is Externalizable interface?

Externalizable is an interface which contains two methods readExternal and writeExternal. These

methods give you a control over the serialization mechanism.
Thus if your class implements this interface, you can customize the serialization process by

implementing these methods.

76. When you serialize an object, what happens to the object references included in the object?

The serialization mechanism generates an object graph for serialization. Thus it determines whether

the included object references are serializable or not. This is a recursive process.
Thus when an object is serialized, all the included objects are also serialized alongwith the original


77. What one should take care of while serializing the object?

One should make sure that all the included objects are also serializable. If any of the objects is not

serializable then it throws a NotSerializableException.

78. What happens to the static fields of a class during serialization?

There are three exceptions in which serialization doesnot necessarily read and write to the stream.

These are
1. Serialization ignores static fields, because they are not part of ay particular state state.
2. Base class fields are only hendled if the base class itself is serializable.
3. Transient fields.

79. How do I serialize an object to a file?

The class whose instances are to be serialized should implement an interface Serializable. Then you

pass the instance to the ObjectOutputStream which is connected to a fileoutputstream. This will save

the object to a file.

80. Which methods of Serializable interface should I implement?

The serializable interface is an empty interface, it does not contain any methods. So we do not

implement any methods.

81. How can I customize the seralization process? i.e. how can one have a control over the

serialization process?

Yes it is possible to have control over serialization process. The class should implement Externalizable

interface. This interface contains two methods namely readExternal and writeExternal.
You should implement these methods and write the logic for customizing the serialization process.

82. Why do we need wrapper classes?

It is sometimes easier to deal with primitives as objects. Moreover most of the collection classes store

objects and not primitive data types. And also the wrapper classes provide many utility methods also.
Because of these resons we need wrapper classes. And since we create instances of these classes we

can store them in any of the collection classes and pass them around as a collection. Also we can pass

them around as method parameters where a method expects an object.

82. What are checked exceptions?

Checked exception are those which the Java compiler forces you to catch.
Example: IOException are checked exceptions.

83. Is it necessary that each try block must be followed by a catch block?

It is not necessary that each try block must be followed by a catch block. It should be followed by

either a catch block or a finally block. And whatever exceptions are likely to be thrown should be

declared in the throws clause of the method.

84. If I write return at the end of the try block, will the finally block still execute?

Yes even if you write return as the last statement in the try block and no exception occurs, the finally

block will execute. The finally block will execute and then the control return.

85. What are runtime exceptions?

Runtime exceptions are those exceptions that are thrown at runtime because of either wrong input

data or because of wrong business logic etc. These are not checked by the compiler at compile time.

86. Does Java provide any construct to find out the size of an object?

No, there is not sizeof operator in Java. So there is not direct way to determine the size of an object

directly in Java.

87. What are wrapper classes?

Java provides specialized classes corresponding to each of the primitive data types. These are called

wrapper classes.
They are example: Integer, Character, Double etc.

88. What is the difference between error and an exception?

An error is an irrecoverable condition occurring at runtime. Such as OutOfMemory error.
These JVM errors and you can not repair them at runtime. While exceptions are conditions that occur

because of bad input etc. Example: FileNotFoundException will be thrown if the specified file does not

exist. Or a NullPointerException will take place if you try using a null reference.
In most of the cases it is possible to recover from an exception (probably by giving user a feedback

for entering proper values etc.).

89. How does an exception permeate through the code?

An unhandled exception moves up the method stack in search of a matching When an exception is

thrown from a code which is wrapped in a try block followed by one or more catch blocks, a search is

made for matching catch block. If a matching type is found then that block will be invoked. If a

matching type is not found then the exception moves up the method stack and reaches the caller

Same procedure is repeated if the caller method is included in a try catch block. This process

continues until a catch block handling the appropriate type of exception is found. If it does not find

such a block then finally the program terminates.

90. What are the different ways to handle exceptions?

There are two ways to handle exceptions,
1. By wrapping the desired code in a try block followed by a catch block to catch the exceptions. and
2. List the desired exceptions in the throws clause of the method and let the caller of the method

hadle those exceptions.

91. How to create custom exceptions?

Your class should extend class Exception, or some more specific type thereof.

92. If I want an object of my class to be thrown as an exception object, what should I do?

The class should extend from Exception class. Or you can extend your class from some more precise

exception type also.

93. If my class already extends from some other class what should I do if I want an instance of my

class to be thrown as an exception object?
One can not do anytihng in this scenarion. Because Java does not allow multiple inheritance and does

not provide any exception interface as well.

94. If I write System.exit(0); at the end of the try block, will the finally block still execute?

No. In this case the finally block will not execute because when you say System.exit(0); the control

immediately goes out of the program, and thus finally never executes.

95. 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.

96. 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.

97. 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.

98. What is the Locale class?

The Locale class is used to tailor program output to the conventions of a particular geographic,

political, or cultural region.

99. 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

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.

100. 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.

101. 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.

Non-static variables take on unique values with each object instance.

102. 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


103. 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.

104. 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


105. 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


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