Question 1

What is the SimpleTimeZone class?

Accepted Answer

SimpleTimeZone is a concrete subclass of TimeZone class The TimeZone class represents a time zone, that is to be used with Gregorian calendar The SimpleTimeZone is created by using the base time zone offset from GMT time zone ID and rules, for starting and ending the time of daylight

Question 2

What are Native methods in Java?

Accepted Answer

Java applications can call code written in C, C++, or assembler This is sometimes done for performance and sometimes to access the underlying host operating system or GUI API using the JNI The steps for doing that are: First write the Java code and compile it Then create a C header file Create C stubs file Write the C code Create shared code library (or DLL) Run application

Question 3

What is Auto boxing and unboxing?

Accepted Answer

Autoboxing is the process of converting a primitive type data into its corresponding wrapper class object instance Example: Integer number = new Integer (100); // number is now refers to the object 100 Unboxing is the process of converting a wrapper instance into a primitive type Example: Integer number = new Integer (100); int num = number;// without type casting number would be changed into int type

Question 4

How does thread synchronization occurs inside a monitor?

Accepted Answer

A Monitor defines a lock and condition variables for managing concurrent access to shared data The monitor uses the lock to ensure that only a single thread inactive in the monitor code at any time A monitor allows only one thread to lock an object at once

Question 5

Why do we need wrapper classes in Java?

Accepted Answer

Dealing with primitives as objects is easier at times Most of the objects collection store objects and not primitive types Many utility methods are provided by wrapper classes To get these advantages we need to use wrapper classes As they are objects, they can be stored in any of the collection and pass this collection as parameters to the methods Features of the Java wrapper Classes: - Wrapper classes convert numeric strings into numeric values - The way to store primitive data in an object - The valueOf() method is available in all wrapper classes except Character - All wrapper classes have typeValue() method This method returns the value of the object as its primitive type

Question 6

What is Shallow and deep cloning in Java?

Accepted Answer

Cloning refers to creating duplicate copies of objects in java Shallow Cloning: Shallow cloning is a bitwise copy of an object New object is created which is an exact copy that of the original one In case any objects are referring the fields of these objects, just the references are copied Deep Cloning: In deep cloning, complete duplicate copy of the original copy is created Deep cloning creates not only the primitive values of the original objects but also copies all its sub objects as well Clonable interface is used to perform cloning in java

Question 7

What is the similarity between Dynamic Binding and linking?

Accepted Answer

Dynamic binding is orthogonal to dynamic linking Binding refers to the linking of a procedure call to the code to be executed in response to the call Dynamic binding It is associated with polymorphism and inheritance, it(also known as late binding) means that the code associated with a given procedure call is not known until the time of the call at run-time

Question 8

What is dynamic method dispatch?

Accepted Answer

Dynamic method dispatch which is also known as runtime polymorphism is a process in which a call to an overridden method is resolved at runtime rather than at compile-time In this process, an overridden method is called through the reference variable of a superclass The determination of the method to be called is based on the object being referred to by the reference variable

Question 9

What are pass by reference and pass by value ?

Accepted Answer

When an object is passed by value, this means that a copy of the object is passed Thus, even if changes are made to that object, it doesn’t affect the original value When an object is passed by reference, this means that the actual object is not passed, rather a reference of the object is passed Thus, any changes made by the external method, are also reflected in all places

Question 10

Explain different ways of creating a thread. Which one would you prefer and why ?

Accepted Answer

There are three ways that can be used in order for a Thread to be created: A class may extend the Thread class A class may implement the Runnable interface An application can use the Executor framework, in order to create a thread pool The Runnable interface is preferred, as it does not require an object to inherit the Thread class In case your application design requires multiple inheritance, only interfaces can help you Also, the thread pool is very efficient and can be implemented and used very easily

Question 11

What is the difference between processes and threads ?

Accepted Answer

- A process is an execution of a program, while a Thread is a single execution sequence within a process - A process can contain multiple threads A Thread is sometimes called a lightweight process

Question 12

What are the basic interfaces of Java Collections Framework ?

Accepted Answer

Java Collections Framework provides a well designed set of interfaces and classes that support operations on a collections of objects The most basic interfaces that reside in the Java Collections Framework are: Collection, which represents a group of objects known as its elements Set, which is a collection that cannot contain duplicate elements List, which is an ordered collection and can contain duplicate elements Map, which is an object that maps keys to values and cannot contain duplicate keys

Question 13

What do you know about the big-O notation and can you give some examples with respect to different data structures ?

Accepted Answer

The Big-O notation simply describes how well an algorithm scales or performs in the worst case scenario as the number of elements in a data structure increases The Big-O notation can also be used to describe other behavior such as memory consumption Since the collection classes are actually data structures, we usually use the Big-O notation to chose the best implementation to use, based on time, memory and performance Big-O notation can give a good indication about performance for large amounts of data

Question 14

What’s the difference between Enumeration and Iterator interfaces ?

Accepted Answer

Enumeration is twice as fast as compared to an Iterator and uses very less memory However, the Iterator is much safer compared to Enumeration, because other threads are not able to modify the collection object that is currently traversed by the iterator Also, Iteratorsallow the caller to remove elements from the underlying collection, something which is not possible with Enumerations

Question 15

What is the output of this program ? class main_arguments { public static void main(String [ ] args) { String [][] argument = new String[2][2]; int x; argument[0] = args; x = argument[0].length; for (int y = 0; y < x; y++) System.out.print(" " + arg…

Accepted Answer

In argument[0] = args;, the reference variable arg[0], which was referring to an array with two elements, is reassigned to an array (args) with three elements. Output: $ javac main_arguments.java $ java main_arguments 1 2 3

Question 16

What is autoboxing and unboxing ?

Accepted Answer

Autoboxing is the automatic conversion that the Java compiler makes between the primitive types and their corresponding object wrapper classes For example, converting an int to an Integer, a double to a Double, and so on If the conversion goes the other way, this is called unboxing

Question 17

Can you access non static variable in static context ?

Accepted Answer

A static variable in Java belongs to its class and its value remains the same for all its instances A static variable is initialized when the class is loaded by the JVM If your code tries to access a non-static variable, without any instance, the compiler will complain, because those variables are not created yet and they are not associated with any instance

Question 18

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

Accepted Answer

1Main difference is methods of a Java interface are implicitly abstract and cannot have implementations A Java abstract class can have instance methods that implements a default behavior 2Variables declared in a Java interface is by default final An abstract class may contain non-final variables 3Members of a Java interface are public by default A Java abstract class can have the usual flavors of class members like private, protected, etc 4Java interface should be implemented using keyword “implements”; A Java abstract class should be extended using keyword “extends” 5An interface can extend another Java interface only, an abstract class can extend another Java class and implement multiple Java interfaces 6A Java class can implement multiple interfaces but it can extend only one abstract class 7Interface is absolutely abstract and cannot be instantiated; A Java abstract class also cannot be instantiated, but can be invoked if a main() exists 8In comparison with java abstract classes, java interfaces are slow as it requires extra indirection

Question 19

Which of the following statements are equal for a variable declared in the interface ? 1. int X=10 2. public int X=10 3. public static final int X=10

Accepted Answer

All the statements are equal because inside an interface a variable declared will be public static final by default

Question 20

Inside an interface when should the variables be initialized?

Accepted Answer

They should be initialized during the time of declaration itself otherwise it is a compilation eror

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