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Q1981. Difference between log4j and slf4j ?

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Q1982. How will you design a Log4j kind of framework.

Log4j

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Q1983. What are the ways in which an application can maintain config variables ?

Ans. 1. Property files
2. config xml
3. Database

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Q1984. What is the use of Property files ?

Ans. Property files are used to maintain configurable values as key value pairs.

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Q1985. What does the property maxBackupIndex in Log4j means ?

Log4j

Ans. It means that the app will only maintain these many backed up log files. Older backed up logs will be deleted.

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Q1986. How to change the backup log directory using log4j ?

Log4j

Ans. There is a property called "filePattern" within appender that regulates where the backed up files will be created for rolling logs. It actually contains the absolute path which can be modified to have backed up logs within some other directory.

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Q1987. how many reducer task runs on a hadoop cluster ?

BigData

Ans. Generally one reducer runs for all mappers, but it can be increased as per requirements.

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Q1988. What is the difference between DEBUG and TRACE logging levels ?

Log4j

Ans. TRACE designates finer grained informational events than the DEBUG.TRACE is level lower than DEBUG.

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Q1989. What is the difference between INFO and DEBUG logging levels ?

Log4j

Ans. Info Messages are something which we would like to see even if the application is in best of state. DEBUG messages are usually something that we would like to see while debugging some problem. DEBUG is lower level than Info. Any message logged with info gets printed if the Root level set is DEBUG.

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Q1990. What should be the logging level while logging exceptions ?

Log4j

Ans. It depends on how severe the exception is. If the exception is completely unexpected and breaks the request, It should be logged as ERROR or FATAL. If it's not expected but we can live with it and the application request continue in-spite of it, It should be WARN. If it's expected , it can be just INFO.

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Q1991. What are the application Logging levels specified in your applications ?

Log4j

Ans. In our Organization, the policy is to keep it WARN in production so as to reduce logging Noise. We use Info in Test environments.

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Q1992. Can you organize following Log4j levels in hierarchy ?

WARN, DEBUG,INFO, ERROR

Log4j

Ans. DEBUG
INFO
WARN
ERROR

Root Log level of DEBUG will print all
Root Log Level of INFO will print INFO, WARN and ERROR
Root Log level of WARN will print WARN and ERROR
Root Log level of ERROR will print ERROR alone

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Q1993. What are some of the Log4j appenders ?

Log4j

Ans. https://logging.apache.org/log4j/2.x/manual/appenders.html

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Q1994. In the following log4j config, What if we don't need rolling logs

<Configuration status="DEBUG">
<Appenders>
<RollingFile name="File" fileName="logs/xyz.log" filePattern="logs/xyz-%d{MM-dd-yyyy}.log.gz">
<TimeBasedTriggeringPolicy />
</RollingFile>
</Appenders>
<Loggers>
<Root level="DEBUG">
<AppenderRef ref="File" />
</Root>
</Loggers>
</Configuration>

Log4j

Ans. We can change the appender from RollingFile to File or we can specify a new File appender and set that within AppenderRef

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Q1995. in the following log4j config file, How should we change the file name and format

<Configuration status="DEBUG">
<Appenders>
<RollingFile name="File" fileName="logs/xyz.log" filePattern="logs/xyz-%d{MM-dd-yyyy}.log.gz">
<TimeBasedTriggeringPolicy />
</RollingFile>
</Appenders>
<Loggers>
<Root level="DEBUG">
<AppenderRef ref="File" />
</Root>
</Loggers>
</Configuration>

Log4j

Ans. We can specify the file name with fileName property within RollingFile. File name pattern for the rolled logs can be specified using filePattern property.

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Q1996. In following log4j configuration file, How should we change the logging level to WARN

<Configuration status="DEBUG">
<Appenders>
<RollingFile name="File" fileName="logs/xyz.log" filePattern="logs/xyz-%d{MM-dd-yyyy}.log.gz">
<TimeBasedTriggeringPolicy />
</RollingFile>
</Appenders>
<Loggers>
<Root level="DEBUG">
<AppenderRef ref="File" />
</Root>
</Loggers>
</Configuration>

Log4j

Ans. We should change the root level from DEBUG to WARN

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Q1997. What is the default interval for TimeBasedTriggeringPolicy, if specified none like following

<Appenders>
<RollingFile name="RollingFile" fileName="logs/xyz.log" filePattern="logs/xyz-%d{MM-dd-yyyy}.log.gz"
ignoreExceptions="false">
<TimeBasedTriggeringPolicy />
</RollingFile>
</Appenders>

Log4j

Ans. 1 day

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Q1998. How can we change the Log level for the application ?

Log4j

Ans. By changing the Root Level within Log4j config.

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Q1999. What is the difference between RollingFile and RollingRandomAccessFile in log4j ?

Log4j

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Q2000. What is the difference between TimeBasedRollingPolicy and SizeBasedTriggeringPolicy within Log4j ? Can we use both together and What would that mean ?

Log4j

Ans. TimeBasedRollingPolicy enables rolling of logs based on the time / days whereas SizeBasedTriggeringPolicy enables rolling of logs based on size cap.

Yes we can use both together. In that case Logs will be rolled in case any of the condition is met i.e after the interval or if the size is reached.

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Q2001. Can we override an abstract method with abstract method in the derived class ?

Core Java

Ans. Yes, but in that case the derived class itself should be abstract. We cannot have an object of a class without definition for the method which has been declared abstract in the parent.

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Q2002. What is ACL ?

Operating System

Ans. Access Control List or ACL is the list of permissions attached to an object in the File System.

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Q2003. Can we override abstract methods ?

Core Java

Ans. Abstract methods are the methods that need to be overridden in the derived class ( either as implementing method or again abstract method ) so it's not only allowed but its required to override abstract method in the derived class.

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Q2004. Can we overload abstract methods ?

Core Java

Ans. Yes

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Q2005. What is the precedence of operators in Java ?

Core Java

Ans. http://introcs.cs.princeton.edu/java/11precedence/

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Q2006. What is Substitutability in Java ?

Core Java

Ans. Substitutability means that the type of the variable does not have to exactly match with the type of the value assigned to that variable. For example - Object reference of Parent object can hold derived object.

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Q2007. How do threads share information ?

Core Java

Ans. Through common memory area.

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Q2008. Can the foreign key column have null values ?

Database

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Q2009. write program to do matrix multiplication

Core Java

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Q2010. Write program to create a linked list and perform different operations on it.

Algorithm

Ans. import java.util.*;
class LinkedListSolution{
protected LinkedList list;
public LinkedListSolution(){
list = new LinkedList();
}
public Object pop() throws NoSuchElementException{
if(list.isEmpty())
throw new NoSuchElementException();
else
return list.removeFirst();
}
public void push(Object obj){
list.addFirst(obj);
}
public Object peek() throws NoSuchElementException{
if(list.isEmpty())
throw new NoSuchElementException();
else
return list.getFirst();
}
public boolean isEmpty(){
return list.isEmpty();
}
public String toString(){
return list.toString();
}
}
class TestStack{
public static void main(String args[]){
LinkedListSolution s = new LinkedListSolution();
s.push("First");
s.push("Second");
s.push("Third");
System.out.println("Top: " s.peek());
s.push("Fourth");
while(!(s.isEmpty()))
System.out.println(s.pop());
}
}

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