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Q1291. What will be the crontab to run a job every last day of month ?

Ans. 59 23 28-31 * * [ "$(date +%d -d tomorrow)" = "01" ] && job_name

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Q1292. Write a unix command to find top 10 files by size ?

Ans. du -a /var | sort -n -r | head -n 10

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Q1293. What are fields that gets displayed when you execute "Top" command and briefly explain each field ?

Ans. PID - process identification number is an identification number that is automatically assigned to each process when it is created

USER - User Name

PR - PR is the process actual priority

NI is the nice value, which is a user-space concept.

VIRT -Virtual Image (kb). The total amount of virtual memory used by the task.

RES - Resident size (kb). The non-swapped physical memory a task has used.

SHR - Shared Mem size (kb). The amount of shared memory used by a task.

S - Process Status. The status of the task which can be one of:

D = uninterruptible sleep
R = running
S = sleeping
T = traced or stopped
Z = zombie

%CPU - % CPU usage

%MEM - % MEM Usage

TIME - Total CPU time the task has used since it started.

COMMAND - Command which was used to execute the process

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Q1294. Is null a dataype ?

Ans. It is special literal. It is neither keyword nor identifier.

Any reference in java that doesnt point to any object , gets assigned null i.e is a reference to null

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Q1295. Which method of String class is used to convert Boolean to String ?

Ans. toString() is an overloaded method of String class that is used to convert many data types to String, Boolean being one of them.

toString(Boolean bool)

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Q1296. What are Collection Classes ?

Ans. Collections in java is a framework of classes that provides an abstracted data structure to store and manipulate the group of objects. Each class is unique in the way it stores , search , sort and modify the elements.

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Q1297. Write a Program to print the positive and negative numbers separetly

Ans. int[] arr = {1,-1,2,-3,3,-4,4,5,6,-5,-6,-7,-8,8,9,-9};
List positiveNumbers = new ArrayList<>();
List negativeNumbers = new ArrayList<>();
for(int i = 0; i < arr.length(); i ){
if(I < 0){
negativeNumbers.add(i);
} else {
positiveNumbers.add(i);
}
}

System.out.println("Positive Numbers:" + positiveNumbers);
System.out.println("Negative Numbers:" + negativeNumbers);

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Q1298. Explain some of the http status codes you know ?

Ans. 500 is Internal Server Error

404 is resource not found

400 is Bad Request

403 is Forbidden

401 is Unauthorized

200 is OK

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Q1299. What is Spring Boot ?

Ans. Spring Boot is Springs convention-over-configuration solution for creating stand-alone, production-grade Spring-based Applications.

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Q1300. What is Maven ?

Ans. Maven is a build automation tool used primarily for Java projects.

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Q1301. Difference between Maven and Ant ?

Ans. Ant is procedural, we need to provide information about what to do and when to do through code.

Maven is declarative, everything is defined in the pom file.

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Q1302. What could be the problems due to the usage of static methods and elements ?

Ans. static methods and static elements are shared by all objects of a class and hence could create problem.

Static methods are not synchronized by default and hence could create problem when accessed through multiple threads. Static elements are shareable by class and hence state of one object could be altered by another object.

Some limitations with Unit testing as not all mocking framework facilitate mocking them. Power mock allows but Mockito doesn't

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Q1303. What is the advantage of using static final or constant variables in Java ?

Ans. Better Control - If the value is being used at multiple locations , that can be controlled better from single place. Any change would only require making single change.

Meaning , Aliasing and Better Readability - Sometimes its easy to read the value by its meaning or alias ( 0 as ZERO or 0 as NEUTRAL_VALUE ).

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Q1304. What is serialVersionUID ?

Ans. Everytime an object is serialized the java serialization mechanism automatically computes a hash value by passing the meta information for the class. This id is used during deserialization to verify that the sender and receiver of a serialized object have loaded classes for that object that are compatible with respect to serialization

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Q1305. With a Table employee having columns Id,Name,ManagerEmployeeId, Write a SQL to find the Manager with maximum number of employees ?

Ans. Select Name from EMPLOYEE where ID in (Select ManagerEmployeeId from EMPLOYEE
Group By ManagerEmployeeId
order by count(Id)
LIMIT 1)

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Q1306. How to make sure that only one instance is created in Singleton Pattern ?

Ans. public class SingleTon {
private SingleTon() {
if (singleTon != null) {
throw new RuntimeException("cant not create the object");
}
}
public Object clone() throws CloneNotSupportedException {
throw new CloneNotSupportedException("can not be create");
}

static private volatile SingleTon singleTon;
public static SingleTon getInstance() {
SingleTon singleTon = this.sample;
if (singleTon == null) {
synchronized (this) {
singleTon = this.singleTon;
if (singleTon == null) {
singleTon = this.singleton = new SingleTon();
}
}
}
return singleTon;
}
}

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Q1307. Can we have an entity in hibernate without primary Key ?

Ans. No, Every entity in hibernate needs to have a key, either primary or composite.

If we dont have a primary key on table, there are various ways this problem can be countered.

1. By using composite key on entity ( make sure that the appropriate unique constraint in defined on columns in Database )

2. By mapping Id in entity to ROWID of table.

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Q1308. Provide an implementation for Producer / Consumer using threads.

Ans. https://www.tutorialspoint.com/javaexamples/thread_procon.htm

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Q1309. When should we use Abstract classes ?

Ans. Abstract classes provide a mechanism of interfacing ( using abstract method ) as well as inheritance ( extending abstract class ). So they should be used in place of interfaces in case there is some code ( methods ) or object body ( member elements ) that can be reused with inheritance.

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Q1310. What is the difference between following code segments

1.

try {
for(int i=0;i<100;i++){
calculate(i);
}
} catch (Exception ex) {
System.out.println("Error while processing")
}

and

2.

for(int i=0;i<100;i++){
try {
calculate(i);
} catch (Exception ex) {
System.out.println("Error while processing")
}
}

Ans. In first case the whole loop will terminate as soon as any exception happens in the method calculate ( assuming calculate method is not handling its exception and those are thrown back at the calling method )

In Second case exception will be caught for individual iteration and hence it wont break the loop and will continue for the next iteration.

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Q1311. What will be the output of following code

public class BuggyBread {
   static class A {
      A() {
         f();
      }

      public void f() {
         System.out.println("A ctor");
      }
   }

   static class B extends A {
      B() {
         f();
      }

      public void f() {
         System.out.println("B ctor");
      }
   }

   public static void main(String[] args) {
      B b = new B();
      b.f();
      A a = new A();
      a.f();
   }
}

Ans. B ctor
B ctor
B ctor
A ctor
A ctor

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Q1312. What is the difference between ServletConfig and ServletContext ?

Ans. http://www.java4s.com/java-servlet-tutorials/difference-between-servletconfig-and-servletcontext-in-java/

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Q1313. Explain some of the features of Java ?

Ans. Object Oriented - Java is object oriented but isn't purely object oriented as we have primitives along with objects.

Platform Independent - As JVM provides the translation to the Machine code as per the underlying Operating System.

Interpreted as well as compiled - Java files are compiled as class files and then class files are interpreted by JVM.

Runs on a Virtual Machine - Which is JVM that acts as an independent sub environment.

Multi-threaded - As applications can run on single thread as well as multi thread.

Modularity - Through usage of Classes , methods and Interfaces.

Robust

Usage in variety of Application types - Web , Gaming, BigData.

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Q1314. Name some of the Java Keywords ?

Ans. Static , Final , Synchronized, private , public , protected, volatile, transient, super, this,import , abstract,native,default (effective java 8), new

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Q1315. Which operators in Java have highest precedence ?

Ans. Braces, i.e () and [] have the highest precedence

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Q1316. Which class is the root class of all Exceptions in Java ?

Ans. Throwable

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Q1317. What is MAC Address ?

Ans. A media access control address is a unique identifier assigned to network interfaces for communications at the data link layer of a network segment. MAC addresses are used as a network address for most IEEE 802 network technologies, including Ethernet and Wi-Fi.

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Q1318. What is an exception and exception handling in Java ?

Ans. An Exception in java is the occurrence during computation that is anomalous and is not expected.

Exception handling is the mechanism which is used to handle such situations.

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Q1319. What technical stuff you like to do in your free time ?

Ans. [Open Ended Answer]

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Q1320. What was the most difficult task in your last job ?

Ans. [Open Ended Answer]

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