Interview Questions and Answers for 'Java' | Search Interview Question

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Q1141. What is reflection ?

Ans. It is the process of examining / modifying the behaviour of an object at runtime.

Sample Code for reflection

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Q1142. Can we instantiate the object of derived class if parent constructor is protected ?

Core Java

Ans. Yes, as protected constructor is accessible in sub class.

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Q1143. Can we declare an abstract method private ?

Ans. No Abstract methods can only be declared protected or public.

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Q1144. What are the design considerations while making a choice between using interface and abstract class ?

Core Java

Ans. Keep it as a Abstract Class if its a "Is a" Relationsship and should do subset/all of the functionality. Keep it as Interface if its a "Should Do" relationship.

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Q1145. What is a config Object?

Java EE

Ans. The config object is an instantiation of javax.servlet.ServletConfig and is a direct wrapper around the ServletConfig object for the generated servlet. This object allows the JSP programmer access to the Servlet or JSP engine initialization parameters such as the paths or file location.

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Q1146. What is a pageContext Object?

Java EE

Ans. The pageContext object is an instance of a javax.servlet.jsp.PageContext object. The pageContext object is used to represent the entire JSP page. This object stores references to the request and response objects for each request. The application, config, session, and out objects are derived by accessing attributes of this object.The pageContext object also contains information about the directives issued to the JSP page, including the buffering information, the errorPageURL, and page scope.

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Q1147. What is suspend() method used for ?

Ans. suspend() method is used to suspend the execution of a thread for a period of time. We can then restart the thread by using resume() method.

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Q1148. Difference between suspend() and stop() ?

Ans. Suspend method is used to suspend thread which can be restarted by using resume() method. stop() is used to stop the thread, it cannot be restarted again.

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Q1149. What are the benefits of using Spring Framework ?

Spring

Ans. Spring enables developers to develop enterprise-class applications using POJOs. The benefit of using only POJOs is that you do not need an EJB container product.

Spring is organized in a modular fashion. Even though the number of packages and classes are substantial, you have to worry only about ones you need and ignore the rest.

Spring does not reinvent the wheel instead, it truly makes use of some of the existing technologies like several ORM frameworks, logging frameworks, JEE, Quartz and JDK timers, other view technologies.

Testing an application written with Spring is simple because environment-dependent code is moved into this framework. Furthermore, by using JavaBean-style POJOs, it becomes easier to use dependency injection for injecting test data.

Spring is web framework is a well-designed web MVC framework, which provides a great alternative to web frameworks such as Struts or other over engineered or less popular web frameworks.

Spring provides a convenient API to translate technology-specific exceptions (thrown by JDBC, Hibernate, or JDO, for example) into consistent, unchecked exceptions.

Lightweight IoC containers tend to be lightweight, especially when compared to EJB containers, for example. This is beneficial for developing and deploying applications on computers with limited memory and CPU resources.

Spring provides a consistent transaction management interface that can scale down to a local transaction

Sample Code for spring

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Q1150. what is the difference between collections class vs collections interface ?

Core Java

Ans. Collections class is a utility class having static methods for doing operations on objects of classes which implement the Collection interface. For example, Collections has methods for finding the max element in a Collection.

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Q1151. Will this code give error if i try to add two heterogeneous elements in the arraylist. ? and Why ?

List list1 = new ArrayList<>();
list1.add(5);
list1.add("5");

Ans. If we don't declare the list to be of specific type, it treats it as list of objects.

int 1 is auto boxed to Integer and "1" is String and hence both are objects.

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Q1152. Difference between Java beans and Spring Beans ?

Spring

Ans. Java Beans managed by Spring IoC are called Spring Beans.

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Q1153. What is the difference between System.console.write and System.out.println ?

Ans. System.console() returns null if your application is not run in a terminal (though you can handle this in your application)System.console() provides methods for reading password without echoing charactersSystem.out and System.err use the default platform encoding, while the Console class output methods use the console encoding

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Q1154. What are various types of Class loaders used by JVM ?

Core Java

Ans. Bootstrap - Loads JDK internal classes, java.* packages.

Extensions - Loads jar files from JDK extensions directory - usually lib/ext directory of the JRE

System - Loads classes from system classpath.

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Q1155. How are classes loaded by JVM ?

Core Java

Ans. Class loaders are hierarchical. The very first class is specially loaded with the help of static main() method declared in your class. All the subsequently loaded classes are loaded by the classes, which are already loaded and running.

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Q1156. Difference between C++ and Java ?

Core Java

Ans. Java does not support pointers.

Java does not support multiple inheritances.

Java does not support destructors but rather adds a finalize() method. Finalize methods are invoked by the garbage collector prior to reclaiming the memory occupied by the object, which has the finalize() method.

Java does not include structures or unions because the traditional data structures are implemented as an object oriented framework.

C++ compiles to machine language , when Java compiles to byte code .

In C++ the programmer needs to worry about freeing the allocated memory , where in Java the Garbage Collector takes care of the the unneeded / unused variables.

Java is platform independent language but c++ is depends upon operating system.

Java uses compiler and interpreter both and in c++ their is only compiler.

C++ supports operator overloading whereas Java doesn't.

Internet support is built-in Java but not in C++. However c++ has support for socket programming which can be used.

Java does not support header file, include library files just like C++ .Java use import to include different Classes and methods.

There is no goto statement in Java.

There is no scope resolution operator :: in Java. It has . using which we can qualify classes with the namespace they came from.

Java is pass by value whereas C++ is both pass by value and pass by reference.

Java Enums are objects instead of int values in C++

C++ programs runs as native executable machine code for the target and hence more near to hardware whereas Java program runs in a virtual machine.

C++ was designed mainly for systems programming, extending the C programming language whereas Java was created initially to support network computing.

C++ allows low-level addressing of data. You can manipulate machine addresses to look at anything you want. Java access is controlled.

C++ has several addressing operators . * & -> where Java has only one: the .

We can create our own package in Java(set of classes) but not in c and c++.

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Q1157. Difference between static vs. dynamic class loading?

Core Java

Ans. static loading - Classes are statically loaded with Java new operator.

dynamic class loading - Dynamic loading is a technique for programmatically invoking the functions of a class loader at run time.

Class.forName (Test className);

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Q1158. Tell something about BufferedWriter ? What are flush() and close() used for ?

Ans. BufferedWriter is temporary source for data storage.BufferedWriter is used to write character data to the file.Flush() is method available in B.W which ensures that all data items are written to file including last character.Close() is used to closes the character output stream.

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Q1159. What is Scanner class used for ? when was it introduced in Java ?

Core Java

Ans. Scanner class introduced in Java 1.5 for reading Data Stream from the imput device. Previously we used to write code to read a input using DataInputStream. After reading the stream , we can convert into respective data type using in.next() as String ,in.nextInt() as integer, in.nextDouble() as Double etc

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Q1160. Why Struts 1 Classes are not Thread Safe whereas Struts 2 classes are thread safe ?

Ans. Struts 1 actions are singleton. So all threads operates on the single action object and hence makes it thread unsafe.

Struts 2 actions are not singleton and a new action object copy is created each time a new action request is made and hence its thread safe.

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Q1161. What are some Java related technologies used for distributed computing ?

Ans. sockets, RMI. EJB

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Q1162. Whats the purpose of marker interfaces ?

Ans. They just tell the compiler that the objects of the classes implementing the interfaces with no defined methods need to be treated differently.

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Very frequently asked. Favorite question in Walk in Drive of many Indian service companies. Frequently asked in HCL Technologies, TCS and Accenture.

Q1163. What is the difference between final, finally and finalize() ?

Core Java

Ans. final - constant variable, objects cannot be de-referenced, restricting method overriding, restricting class sub classing.

finally - handles exception. The finally block is optional and provides a mechanism to clean up regardless of what happens within the try block. Use the finally block to close files or to release other system resources like database connections, statements etc.

finalize() - method helps in garbage collection. A method that is invoked before an object is discarded by the garbage collector, allowing it to clean up its state.

Sample Code for final

Sample Code for finally

Sample Code for finalize

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Q1164. When do you get ClassCastException?

or

What is ClassCastException ?

Ans. As we only downcast class in the hierarchy, The ClassCastException is thrown to indicate that code has attempted to cast an object to a subclass of which it is not an instance.

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Frequently asked to fresh graduates.

Q1165. Explain Thread States ?

Operating System

Ans. Runnable - waiting for its turn to be picked for execution by the thread schedular based on thread priorities.

Running - The processor is actively executing the thread code. It runs until it becomes blocked, or voluntarily gives up its turn.

Waiting: A thread is in a blocked state while it waits for some external processing such as file I/O to finish.

Sleeping - Java threads are forcibly put to sleep (suspended) with Thread.sleep. they can resume using Thread.resume method.

Blocked on I/O - Will move to runnable after I/O condition like reading bytes of data etc changes.

Blocked on synchronization - Will move to Runnable when a lock is acquired.

Dead - The thread is finished working.

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Q1166. What are strong, soft, weak and phantom references in Java ?

Core Java

Ans. Garbage Collector wont remove a strong reference.

A soft reference will only get removed if memory is low.

A weak reference will get removed on the next garbage collection cycle.

A phantom reference will be finalized but the memory will not be reclaimed. Can be useful when you want to be notified that an object is about to be collected.

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Q1167. Difference between yield() and sleeping()?

Ans. When a task invokes yield(), it changes from running state to runnable state. When a task invokes sleep(), it changes from running state to waiting/sleeping state.

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Q1168. What is a daemon thread? Give an Example ?

Ans. These are threads that normally run at a low priority and provide a basic service to a program or programs when activity on a machine is reduced. garbage collector thread is daemon thread.

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Q1169. What is the difference between AWT and Swing?

Ans. Swing provides both additional components like JTable, JTree etc and added functionality to AWT-replacement components.

Swing components can change their appearance based on the current â€œlook and feelâ€ library thatâ€™s being used.

Swing components follow the MVC paradigm, and thus can provide a much more flexible UI.

Swing provides extras for components, such as icons on many components, decorative borders for components, tool tips for components etc.

Swing components are lightweight than AWT.

Swing provides built-in double buffering ,which means an off-screen buffer is used during drawing and then the resulting bits are copied onto the screen.

Swing provides paint debugging support for when you build your own component.

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Q1170. What is the order of method invocation in an applet?

Ans. public void init()
public void start()
public void stop()
public void destroy()

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