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Frequently asked.

Q541. What is an Iterator?

Core Java

Ans. Iterator is an interface that provides methods to iterate over any Collection.

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Q542. Which interface provides the capability to store objects using a key-value pair?

Core Java

Ans. java.util.map

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Q543. Does java allow overriding static methods ?

Core Java

Ans. No. Static methods belong to the class and not the objects. They belong to the class and hence doesn't fit properly for the polymorphic behavior.

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Q544. Difference between long.Class and Long.TYPE ?

Ans. They both represent the long primitive type. They are exactly the same.

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Q545. Does Java provides default copy constructor ?

Core Java

Ans. No

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Frequently asked to fresh graduates.

Q546. What are the difference between Threads and Processes ?

Operating System

Ans. 1. when an OS wants to start running program it creates new process means a process is a program that is currently executing and every process has at least one thread running within it.
2). A thread is a path of code execution in the program, which has its own local variables, program counter(pointer to current execution being executed) and lifetime.
3. When the JavaVirtual Machine (JavaVM, or just VM) is started by the operating system, a new process is created. Within that process, many threads can be created.
4. Consider an example : when you open Microsoft word in your OS and you check your task manger then you can see this running program as a process. now when you write something in opened word document, then it performs more than one work at same time like it checks for the correct spelling, it formats the word you enter , so within that process ( word) , due to different path execution(thread) all different works are done at same time.
5. Within a process , every thread has independent path of execution but there may be situation where two threads can interfere with each other then concurrency and deadlock come is picture.
6. like two process can communicate ( ex:u open an word document and file explorer and on word document you drag and drop another another file from file explorer), same way two threads can also communicate with each other and communication with two threads is relatively low.
7. Every thread in java is created and controlled by unique object of java.lang.Thread class.
8. prior to jdk 1.5, there were lack in support of asynchronous programming in java, so in that case it was considered that thread makes the runtime environment asynchronous and allow different task to perform concurrently.

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Q547. Why java doesn't support multiple Inheritence ?

Core Java

Ans. class A {
void test() {
System.out.println("test() method");
}
}

class B {
void test() {
System.out.println("test() method");
}
}

Suppose if Java allows multiple inheritance like this,

class C extends A, B {
}

A and B test() methods are inheriting to C class.

So which test() method C class will take? As A & B class test() methods are different , So here we would Facing Ambiguity.

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Q548. Why threads block or enters to waiting state on I/O?

Operating System

Ans. Threads enters to waiting state or block on I/O because other threads can execute while the I/O operations are performed.

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Q549. What are transient variables in java?

Core Java

Ans. Transient variables are variable that cannot be serialized.

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Q550. What is the difference between yield() and sleep()?

Operating System

Ans. When a object invokes yield() it returns to ready state. But when an object invokes sleep() method enters to not ready state.

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Q551. What is the initial state of a thread when it is created and started?

Ans. Ready state.

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Q552. Which memory segment loads the java code ?

Core Java

Ans. Code segment.

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Q553. Can a lock be acquired on a class ?

Ans. Yes, a lock can be acquired on a class. This lock is acquired on the class's Class object.

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Q554. What state does a thread enter when it terminates its processing?

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

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Q555. What restrictions are placed on method overriding?

Ans. Overridden methods must have the same name, argument list, and return type. The overriding method may not limit the access of the method it overrides. The overriding method may not throw any exceptions that may not be thrown by the overridden method.

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Q556. How does a try statement determine which catch clause should be used to handle an exception?

Core Java

Ans. When an exception is thrown within the body of a try statement, the catch clauses of the try statement are examined in the order in which they appear. The first catch clause that is capable of handling the exception is executed. The remaining catch clauses are ignored.

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Q557. What is RMI ?

Ans. RMI stands for Remote Method Invocation. Traditional approaches to executing code on other machines across a network have been confusing as well as tedious and error-prone to implement. The nicest way to think about this problem is that some object happens to live on another machine, and that you can send a message to the remote object and get a result as if the object lived on your local machine. This simplification is exactly what Java Remote Method Invocation (RMI) allows you to do.

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Q558. Explain JMS ( Java Messaging Services ) ?

JMS

Ans. JMS Provides high-performance asynchronous messaging. It enables Java EE applications to communicate with non-Java systems on top of various transports.

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Q559. Which MVC is struts2 based on ?

Ans. MVC2

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Q560. Explain features of struts2 ?

Ans. 1) It is an Action based MVC based framework which has adopt mvc2.

2) Struts2 is a pull-MVC (or MVC2) framework where action takes the role of the model rather than the controller. The â€œpullâ€ concepts means views ability to pull data from an action, rather than having a separate model object available.

3) The Model View-Controller pattern in Struts2 is implemented with five core components â€“ actions, interceptors, value stack / OGNL, result types and results / view technologies.

4) XML configuration as well as Annotation option available.

5) POJO based action available so we can write test cases easily.

6) Integration with Spring, tiles and OGNL based expression langugae.

7) Theme based tag libraries integrated with struts tag as well as support of Ajax tag.

8) Can have various view options like jsp, velocity, freemarker etc.

9) We can embed plugin through which we can modify and extend framework features.

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Q561. what is content negotiation?

Ans. Suppose we want to visit a site for any information, information can be represented in different languages like English,German or may be other and their format for presentation can also differ from HTML to PDF or may be Plain text. In this case when an client makes an HTTP request to a server, client can also specify the media types here. Client can specify what it can accept back from host and on the basis of availability the host will return to the client. This is called content negotiation because client and server negotiated on the language and format of the content to be shared.

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Q562. Explain static blocks in Java ?

Core Java

Ans. A static initialization block is a normal block of code enclosed in braces, { }, and preceded by the static keyword. Here is an example:

static {
// whatever code is needed for initialization goes here
}

A class can have any number of static initialization blocks, and they can appear anywhere in the class body. The runtime system guarantees that static initialization blocks are called in the order that they appear in the source code.

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Q563. Explain use of nested or inner classes ?

Core Java

Ans. Sometime we just need classes or class objects just to be used as part of a particular class or objects. Making them non nested won't make any difference as far as functionality is concerner but making them Nested provide a level of convenience and protection fro, being used anywhere else. Moreover it helps reducing the Code.

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Q564. Explain Annotations ?

Ans. Annotations, a form of metadata, provide data about a program that is not part of the program itself. Annotations have no direct effect on the operation of the code they annotate. Annotations have a number of uses, among them:
â€¢ Information for the compiler â€” Annotations can be used by the compiler to detect errors or suppress warnings.
â€¢ Compile-time and deployment-time processing â€” Software tools can process annotation information to generate code, XML files, and so forth.
â€¢ Runtime processing â€” Some annotations are available to be examined at runtime.

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Q565. Give an Example of checked and unchecked exception ?

Core Java

Ans. ClassNotFoundException is checked exception whereas NoClassDefFoundError is a unchecked exception.

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Q566. Explain the use of "Native" keyword ?

Core Java

Ans. Used in method declarations to specify that the method is not implemented in the same Java source file, but rather in another language

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Q567. What is a finalize method ?

Core Java

Ans. finalize() method is called just before an object is destroyed.

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Q568. Is runnable a Marker interface ?

Core Java

Ans. No , it has run method declared.

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Q569. Difference between implicit and explicit type casting ?

Ans. An explicit conversion is where you use some syntax to tell the program to do a conversion whereas in case of implicit type casting you need not provide the data type.

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Q570. What is assert keyword used for ?

Ans. The assert keyword is used to make an assertionâ€”a statement which the programmer believes is always true at that point in the program. This keyword is intended to aid in testing and debugging.

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