Hadoop and Java Questions for interviews
Q1. What are the default configuration files that are used in Hadoop
As
of 0.20 release, Hadoop supported the following read-only default
configurations
- src/core/core-default.xml
- src/hdfs/hdfs-default.xml
- src/mapred/mapred-default.xml
Q2. How will you make changes to the default configuration files
Hadoop does not recommends changing the default configuration files, instead it recommends making all site specific changes in the following files
- conf/core-site.xml
- conf/hdfs-site.xml
- conf/mapred-site.xml
Unless explicitly turned off, Hadoop by default specifies two resources, loaded in-order from the classpath:
- core-default.xml : Read-only defaults for hadoop.
- core-site.xml: Site-specific configuration for a given hadoop installation.
Hence if same configuration is defined in file core-default.xml and src/core/core-default.xml then the values in file core-default.xml (same is true for other 2 file pairs) is used.
Q3. Consider case scenario where you have set property mapred.output.compress to true to ensure that all output files are compressed for efficient space usage on the cluster. If a cluster user does not want to compress data for a specific job then what will you recommend him to do ?
Ask him to create his own configuration file and specify configuration mapred.output.compress to false and load this file as a resource in his job.
Q4. In the above case scenario, how can ensure that user cannot override the configuration mapred.output.compress to false in any of his jobs
This can be done by setting the property final to true in the core-site.xml file
Q5. What of the following is the only required variable that needs to be set in file conf/hadoop-env.sh for hadoop to work
- HADOOP_LOG_DIR
- src/core/core-default.xml
- src/hdfs/hdfs-default.xml
- src/mapred/mapred-default.xml
Q2. How will you make changes to the default configuration files
Hadoop does not recommends changing the default configuration files, instead it recommends making all site specific changes in the following files
- conf/core-site.xml
- conf/hdfs-site.xml
- conf/mapred-site.xml
Unless explicitly turned off, Hadoop by default specifies two resources, loaded in-order from the classpath:
- core-default.xml : Read-only defaults for hadoop.
- core-site.xml: Site-specific configuration for a given hadoop installation.
Hence if same configuration is defined in file core-default.xml and src/core/core-default.xml then the values in file core-default.xml (same is true for other 2 file pairs) is used.
Q3. Consider case scenario where you have set property mapred.output.compress to true to ensure that all output files are compressed for efficient space usage on the cluster. If a cluster user does not want to compress data for a specific job then what will you recommend him to do ?
Ask him to create his own configuration file and specify configuration mapred.output.compress to false and load this file as a resource in his job.
Q4. In the above case scenario, how can ensure that user cannot override the configuration mapred.output.compress to false in any of his jobs
This can be done by setting the property final to true in the core-site.xml file
Q5. What of the following is the only required variable that needs to be set in file conf/hadoop-env.sh for hadoop to work
- HADOOP_LOG_DIR
-
JAVA_HOME
-
HADOOP_CLASSPATH
The only required variable to set is JAVA_HOME that needs to point to <java installation> directory
Q6. List all the daemons required to run the Hadoop cluster
- NameNode
- DataNode
- JobTracker
- TaskTracker
Q7. Whats the default port that jobtrackers listens to
50030
Q8. Whats the default port where the dfs namenode web ui will listen on
50070
The only required variable to set is JAVA_HOME that needs to point to <java installation> directory
Q6. List all the daemons required to run the Hadoop cluster
- NameNode
- DataNode
- JobTracker
- TaskTracker
Q7. Whats the default port that jobtrackers listens to
50030
Q8. Whats the default port where the dfs namenode web ui will listen on
50070
Q21.
Explain difference of Class Variable and Instance Variable and how
are they declared in Java
Class Variable is a variable which is declared with static modifier.
Instance variable is a variable in a class without static modifier.
The main difference between the class variable and Instance variable is, that first time, when class is loaded in to memory, then only memory is allocated for all class variables. That means, class variables do not depend on the Objets of that classes. What ever number of objects are there, only one copy is created at the time of class loding.Q22. Since an Abstract class in Java cannot be instantiated then how can you use its non static methods
By extending itQ23. How would you make a copy of an entire Java object with its state?
Have this class implement Cloneable interface and call its method clone().Q24. Explain Encapsulation,Inheritance and Polymorphism
Encapsulation is a process of binding or wrapping the data and the codes that operates on the data into a single entity. This keeps the data safe from outside interface and misuse. One way to think about encapsulation is as a protective wrapper that prevents code and data from being arbitrarily accessed by other code defined outside the wrapper.
Inheritance is the process by which one object acquires the properties of another object.
The meaning of Polymorphism is something like one name many forms. Polymorphism enables one entity to be used as as general category for different types of actions. The specific action is determined by the exact nature of the situation. The concept of polymorphism can be explained as "one interface, multiple methods".Q25. Explain garbage collection?
Garbage collection is one of the most important feature of Java.
Garbage collection is also called automatic memory management as JVM automatically removes the unused variables/objects (value is null) from the memory. User program cann't directly free the object from memory, instead it is the job of the garbage collector to automatically free the objects that are no longer referenced by a program. Every class inherits finalize() method from java.lang.Object, the finalize() method is called by garbage collector when it determines no more references to the object exists. In Java, it is good idea to explicitly assign null into a variable when no more in use.
Q26. What is similarities/difference between an Abstract class and Interface?
Differences- Interfaces provide a form of multiple inheritance. A class can extend only one other class.
- Interfaces are limited to public methods and constants with no implementation. Abstract classes can have a partial implementation, protected parts, static methods, etc.
- A Class may implement several interfaces. But in case of abstract class, a class may extend only one abstract class.
- Interfaces are slow as it requires extra indirection to find corresponding method in in the actual class. Abstract classes are fast.Similarities
- Neither Abstract classes or Interface can be instantiated
Q27. What are different ways to make your class multithreaded in Java
There are two ways to create new kinds of threads:
- Define a new class that extends the Thread class
- Define a new class that implements the Runnable interface, and pass an object of that class to a Thread's constructor.
Q28. What do you understand by Synchronization? How do synchronize a method call in Java? How do you synchonize a block of code in java ?
Synchronization is a process of controlling the access of shared resources by the multiple threads in such a manner that only one thread can access one resource at a time. In non synchronized multithreaded application, it is possible for one thread to modify a shared object while another thread is in the process of using or updating the object's value. Synchronization prevents such type of data corruption.
- Synchronizing a method: Put keyword synchronized as part of the method declaration
- Synchronizing a block of code inside a method: Put block of code in synchronized (this) { Some Code }
Q29. What is transient variable?
Transient variable can't be serialize. For example if a variable is declared as transient in a Serializable class and the class is written to an ObjectStream, the value of the variable can't be written to the stream instead when the class is retrieved from the ObjectStreamthe value of the variable becomes null.
Q30. What is Properties class in Java. Which class does it extends?
The Properties class represents a persistent set of properties. The Properties can be saved to a stream or loaded from a stream. Each key and its corresponding value in the property list is a string
Q31. Explain the concept of shallow copy vs deep copy in Java
In case of shallow copy, the cloned object also refers to the same object to which the original object refers as only the object references gets copied and not the referred objects themselves.
In case deep copy, a clone of the class and all all objects referred by that class is made.
Q32. How can you make a shallow copy of an object in Java
Use clone() method inherited by Object class
Q33. How would you make a copy of an entire Java object (deep copy) with its state?
Have this class implement Cloneable interface and call its method clone().
Class Variable is a variable which is declared with static modifier.
Instance variable is a variable in a class without static modifier.
The main difference between the class variable and Instance variable is, that first time, when class is loaded in to memory, then only memory is allocated for all class variables. That means, class variables do not depend on the Objets of that classes. What ever number of objects are there, only one copy is created at the time of class loding.Q22. Since an Abstract class in Java cannot be instantiated then how can you use its non static methods
By extending itQ23. How would you make a copy of an entire Java object with its state?
Have this class implement Cloneable interface and call its method clone().Q24. Explain Encapsulation,Inheritance and Polymorphism
Encapsulation is a process of binding or wrapping the data and the codes that operates on the data into a single entity. This keeps the data safe from outside interface and misuse. One way to think about encapsulation is as a protective wrapper that prevents code and data from being arbitrarily accessed by other code defined outside the wrapper.
Inheritance is the process by which one object acquires the properties of another object.
The meaning of Polymorphism is something like one name many forms. Polymorphism enables one entity to be used as as general category for different types of actions. The specific action is determined by the exact nature of the situation. The concept of polymorphism can be explained as "one interface, multiple methods".Q25. Explain garbage collection?
Garbage collection is one of the most important feature of Java.
Garbage collection is also called automatic memory management as JVM automatically removes the unused variables/objects (value is null) from the memory. User program cann't directly free the object from memory, instead it is the job of the garbage collector to automatically free the objects that are no longer referenced by a program. Every class inherits finalize() method from java.lang.Object, the finalize() method is called by garbage collector when it determines no more references to the object exists. In Java, it is good idea to explicitly assign null into a variable when no more in use.
Q26. What is similarities/difference between an Abstract class and Interface?
Differences- Interfaces provide a form of multiple inheritance. A class can extend only one other class.
- Interfaces are limited to public methods and constants with no implementation. Abstract classes can have a partial implementation, protected parts, static methods, etc.
- A Class may implement several interfaces. But in case of abstract class, a class may extend only one abstract class.
- Interfaces are slow as it requires extra indirection to find corresponding method in in the actual class. Abstract classes are fast.Similarities
- Neither Abstract classes or Interface can be instantiated
Q27. What are different ways to make your class multithreaded in Java
There are two ways to create new kinds of threads:
- Define a new class that extends the Thread class
- Define a new class that implements the Runnable interface, and pass an object of that class to a Thread's constructor.
Q28. What do you understand by Synchronization? How do synchronize a method call in Java? How do you synchonize a block of code in java ?
Synchronization is a process of controlling the access of shared resources by the multiple threads in such a manner that only one thread can access one resource at a time. In non synchronized multithreaded application, it is possible for one thread to modify a shared object while another thread is in the process of using or updating the object's value. Synchronization prevents such type of data corruption.
- Synchronizing a method: Put keyword synchronized as part of the method declaration
- Synchronizing a block of code inside a method: Put block of code in synchronized (this) { Some Code }
Q29. What is transient variable?
Transient variable can't be serialize. For example if a variable is declared as transient in a Serializable class and the class is written to an ObjectStream, the value of the variable can't be written to the stream instead when the class is retrieved from the ObjectStreamthe value of the variable becomes null.
Q30. What is Properties class in Java. Which class does it extends?
The Properties class represents a persistent set of properties. The Properties can be saved to a stream or loaded from a stream. Each key and its corresponding value in the property list is a string
Q31. Explain the concept of shallow copy vs deep copy in Java
In case of shallow copy, the cloned object also refers to the same object to which the original object refers as only the object references gets copied and not the referred objects themselves.
In case deep copy, a clone of the class and all all objects referred by that class is made.
Q32. How can you make a shallow copy of an object in Java
Use clone() method inherited by Object class
Q33. How would you make a copy of an entire Java object (deep copy) with its state?
Have this class implement Cloneable interface and call its method clone().
Q11. Which of the following object oriented principal is met with method overloading in java
- Inheritance
- Polymorphism
- Inheritance
Polymorphism
Q12. Which of the following object oriented principal is met with method overriding in java
- Inheritance
- Polymorphism
- Inheritance
Polymorphism
Q13. What is the name of collection interface used to maintain unique elements
MapQ
14. What access level do you need to specify in the class declaration to ensure that only classes from the same directory can access it? What keyword is used to define this specifier? It has to have default specifier.
You do not need to specify any access level, and Java will use a default package access level
Q15. What's the difference between a queue and a stack?
Stacks works by last-in-first-out rule (LIFO), while queues use the FIFO rule
Q16. How can you write user defined exceptions in Java
Make your class extend Exception Class
Q17. What is the difference between checked and Unchecked Exceptions in Java ? Give an example of each type
All predefined exceptions in Java are either a checked exception or an unchecked exception. Checked exceptions must be caught using try .. catch() block or we should throw the exception using throws clause. If you dont, compilation of program will fail.
- Example checked Exception: ParseTextException
- Example unchecked exception: ArrayIndexOutOfBounds
Q18. We know that FileNotFoundExceptionis inherited from IOExceptionthen does it matter in what order catch statements for FileNotFoundExceptionand IOExceptipon are written?
Yes, it does. The FileNoFoundExceptionis inherited from the IOException. Exception's subclasses have to be caught first.
Q19. How do we find if two string are same or not in Java. If answer is equals() then why do we have to use equals, why cant we compare string like integers
We use method equals() to compare the values of the Strings. We can't use == like we do for primitive types like int because == checks if two variables point at the same instance of a String object.
Q20. What is "package" keyword
This is a way to organize files when a project consists of multiple modules. It also helps resolve naming conflicts when different packages have classes with the same names. Packages access level also allows you to protect data from being used by the non-authorized classes .
Since
Hadoop and all its eco-system is built in java hence when hiring for
a hadoop developer it makes sense to test the core java skills of the
interviewee as well. Following are some questions that I have
compiled that test the basic java understanding of the candidate. I
would expect any decent candidate to answer 90% of these questions
Q1. What is mutable object and immutable object
If a object value is changeable then we can call it as Mutable object. (Ex., StringBuffer) If you are not allowed to change the value of an object, it is immutable object. (Ex., String, Integer, Float)
Q2. What are wrapped classes in Java. Why do they exist. Give examples
Wrapped classes are classes that allow primitive types to be accessed as objects, e.g. Integer, Float etc
Q3. Even though garbage collection cleans memory, why can't it guarantee that a program will run out of memory? Give an example of a case when garbage collection will run out ot memory
Because it is possible for programs to use up memory resources faster than they are garbage collected. It is also possible for programs to create objects that are not subject to garbage collection. Once example can be if yuo try to load a very big file into an array.
Q4. What is the difference between Process and Thread?
A process can contain multiple threads. In most multithreading operating systems, a process gets its own memory address space; a thread doesn't. Threads typically share the heap belonging to their parent process. For instance, a JVM runs in a single process in the host O/S. Threads in the JVM share the heap belonging to that process; that's why several threads may access the same object. Typically, even though they share a common heap, threads have their own stack space. This is how one thread's invocation of a method is kept separate from another's
Q5. How can you write a indefinate loop in java
while(true) {
}
ORfor ( ; ; ){
}
Q6. How can you create singleton class in Java
Make the constructor of the class private and provide a static method to get instance of the classQ7. What do keywords "this" and "super" do in Java
"this" is used to refer to current object. "super" is used to refer to the class extended by the current class
Q8. What are access specifiers in java. List all of them. Access specifiers are used to define score of variables in Java. There are four levels of access specifiers in java- public
- private
- protected
- default
Q9. Which of the following 3 object oriented principals does access specifiers implement in java
- Encapsulation
- Polymorphism
- Intheritance
Encapsulation
Q10. What is method overriding and method overloading
With overriding, you change the method behavior for a subclass class. Overloading involves having a method with the same name within the class with different signature
Q1. What is mutable object and immutable object
If a object value is changeable then we can call it as Mutable object. (Ex., StringBuffer) If you are not allowed to change the value of an object, it is immutable object. (Ex., String, Integer, Float)
Q2. What are wrapped classes in Java. Why do they exist. Give examples
Wrapped classes are classes that allow primitive types to be accessed as objects, e.g. Integer, Float etc
Q3. Even though garbage collection cleans memory, why can't it guarantee that a program will run out of memory? Give an example of a case when garbage collection will run out ot memory
Because it is possible for programs to use up memory resources faster than they are garbage collected. It is also possible for programs to create objects that are not subject to garbage collection. Once example can be if yuo try to load a very big file into an array.
Q4. What is the difference between Process and Thread?
A process can contain multiple threads. In most multithreading operating systems, a process gets its own memory address space; a thread doesn't. Threads typically share the heap belonging to their parent process. For instance, a JVM runs in a single process in the host O/S. Threads in the JVM share the heap belonging to that process; that's why several threads may access the same object. Typically, even though they share a common heap, threads have their own stack space. This is how one thread's invocation of a method is kept separate from another's
Q5. How can you write a indefinate loop in java
while(true) {
}
ORfor ( ; ; ){
}
Q6. How can you create singleton class in Java
Make the constructor of the class private and provide a static method to get instance of the classQ7. What do keywords "this" and "super" do in Java
"this" is used to refer to current object. "super" is used to refer to the class extended by the current class
Q8. What are access specifiers in java. List all of them. Access specifiers are used to define score of variables in Java. There are four levels of access specifiers in java- public
- private
- protected
- default
Q9. Which of the following 3 object oriented principals does access specifiers implement in java
- Encapsulation
- Polymorphism
- Intheritance
Encapsulation
Q10. What is method overriding and method overloading
With overriding, you change the method behavior for a subclass class. Overloading involves having a method with the same name within the class with different signature
Q2. What does the statement "HDFS is block structured file system" means It means that in HDFS individual files are broken into blocks of a fixed size. These blocks are stored across a cluster of one or more machines with data storage capacity
Q3. What does the term "Replication factor" mean ?
Replication factor is the number of times a file needs to be replicated in HDFS
Q4. What is the default replication factor in HDFS? 3
Q5. What is the typical block size of an HDFS block? 64Mb to 128Mb
Q6. What is the benefit of having such big block size (when compared to block size of linux file system like ext)?
It allows HDFS to decrease the amount of metadata storage required per file (the list of blocks per file will be smaller as the size of individual blocks increases). Furthermore, it allows for fast streaming reads of data, by keeping large amounts of data sequentially laid out on the disk
Q7. Why is it recommended to have few very large files instead of a lot of small files in HDFS?
This is because the Name node contains the meta data of each and every file in HDFS and more files means more metadata and since namenode loads all the metadata in memory for speed hence having a lot of files may make the metadata information big enough to exceed the size of the memory on the Name node
Q8. True/false question. What is the lowest granularity at which you can apply replication factor in HDSF
- You can choose replication factor per directory
- You can choose replication factor per file in a directory
- You can choose replication factor per block of a file
- True
- True
- False
Q9. What is a datanode in HDFS?
Individual machines in the HDFS cluster that hold blocks of data are called datanodes
Q10. What is a Namenode in HDSF?
The Namenode stores all the metadata for the file system
Q11. What alternate way does HDFS provides to recover data in case a Namenode, without backup, fails and cannot be recovered
There is no way. If Namenode dies and there is no backup then there is no way to recover data
Q12. Describe how a HDFS client will read a file in HDFS, like will it talk to data node or namenode ... how will data flow etc
To open a file, a client contacts the Name Node and retrieves a list of locations for the blocks that comprise the file. These locations identify the Data Nodes which hold each block. Clients then read file data directly from the Data Node servers, possibly in parallel. The Name Node is not directly involved in this bulk data transfer, keeping its overhead to a minimum.
Q13. Using linux command line. how will you
- List the the number of files in a HDFS directory
- Create a directory in HDFS
- Copy file from your local directory to HDSF
hadoop fs -ls
hadoop fs -mkdir
hadoop fs -put localfile hdfsfile
Q31.
How will you write a custom partitioner for a Hadoop job
To have hadoop use a custom partitioner you will have to do minimum the following three
- Create a new class that extends Partitioner class
- Override method getPartition
- In the wrapper that runs the Map Reducer, either
- add the custom partitioner to the job programtically using method setPartitionerClass or
- add the custom partitioner to the job as a config file (if your wrapper reads from config file or oozie)
Q32. How did you debug your Hadoop code
There can be several ways of doing this but most common ways are
- By using counters
- The web interface provided by Hadoop framework
Q33. Did you ever built a production process in Hadoop ? If yes then what was the process when your hadoop job fails due to any reason
Its an open ended question but most candidates, if they have written a production job, should talk about some type of alert mechanisn like email is sent or there monitoring system sends an alert. Since Hadoop works on unstructured data, its very important to have a good alerting system for errors since unexpected data can very easily break the job.
Q34. Did you ever ran into a lop sided job that resulted in out of memory error, if yes then how did you handled it
This is an open ended question but a candidate who claims to be an intermediate developer and has worked on large data set (10-20GB min) should have run into this problem. There can be many ways to handle this problem but most common way is to alter your algorithm and break down the job into more map reduce phase or use a combiner if possible.
To have hadoop use a custom partitioner you will have to do minimum the following three
- Create a new class that extends Partitioner class
- Override method getPartition
- In the wrapper that runs the Map Reducer, either
- add the custom partitioner to the job programtically using method setPartitionerClass or
- add the custom partitioner to the job as a config file (if your wrapper reads from config file or oozie)
Q32. How did you debug your Hadoop code
There can be several ways of doing this but most common ways are
- By using counters
- The web interface provided by Hadoop framework
Q33. Did you ever built a production process in Hadoop ? If yes then what was the process when your hadoop job fails due to any reason
Its an open ended question but most candidates, if they have written a production job, should talk about some type of alert mechanisn like email is sent or there monitoring system sends an alert. Since Hadoop works on unstructured data, its very important to have a good alerting system for errors since unexpected data can very easily break the job.
Q34. Did you ever ran into a lop sided job that resulted in out of memory error, if yes then how did you handled it
This is an open ended question but a candidate who claims to be an intermediate developer and has worked on large data set (10-20GB min) should have run into this problem. There can be many ways to handle this problem but most common way is to alter your algorithm and break down the job into more map reduce phase or use a combiner if possible.
Hadoop Interview Questions Part 3
Q22.
Whats is Distributed Cache in Hadoop
Distributed Cache is a facility provided by the Map/Reduce framework to cache files (text, archives, jars and so on) needed by applications during execution of the job. The framework will copy the necessary files to the slave node before any tasks for the job are executed on that node.
Q23. What is the benifit of Distributed cache, why can we just have the file in HDFS and have the application read it
This is because distributed cache is much faster. It copies the file to all trackers at the start of the job. Now if the task tracker runs 10 or 100 mappers or reducer, it will use the same copy of distributed cache. On the other hand, if you put code in file to read it from HDFS in the MR job then every mapper will try to access it from HDFS hence if a task tracker run 100 map jobs then it will try to read this file 100 times from HDFS. Also HDFS is not very efficient when used like this.
Q.24 What mechanism does Hadoop framework provides to synchronize changes made in Distribution Cache during runtime of the application
This is a trick questions. There is no such mechanism. Distributed Cache by design is read only during the time of Job execution
Q25. Have you ever used Counters in Hadoop. Give us an example scenario
Anybody who claims to have worked on a Hadoop project is expected to use counters
Q26. Is it possible to provide multiple input to Hadoop? If yes then how can you give multiple directories as input to the Hadoop job
Yes, The input format class provides methods to add multiple directories as input to a Hadoop job
Q27. Is it possible to have Hadoop job output in multiple directories. If yes then how
Yes, by using Multiple Outputs class
Q28. What will a hadoop job do if you try to run it with an output directory that is already present? Will it
- overwrite it
- warn you and continue
- throw an exception and exit
The hadoop job will throw an exception and exit.
Q29. How can you set an arbitary number of mappers to be created for a job in Hadoop
This is a trick question. You cannot set it
Q30. How can you set an arbitary number of reducers to be created for a job in Hadoop
You can either do it progamatically by using method setNumReduceTasksin the JobConfclass or set it up as a configuration setting.
Distributed Cache is a facility provided by the Map/Reduce framework to cache files (text, archives, jars and so on) needed by applications during execution of the job. The framework will copy the necessary files to the slave node before any tasks for the job are executed on that node.
Q23. What is the benifit of Distributed cache, why can we just have the file in HDFS and have the application read it
This is because distributed cache is much faster. It copies the file to all trackers at the start of the job. Now if the task tracker runs 10 or 100 mappers or reducer, it will use the same copy of distributed cache. On the other hand, if you put code in file to read it from HDFS in the MR job then every mapper will try to access it from HDFS hence if a task tracker run 100 map jobs then it will try to read this file 100 times from HDFS. Also HDFS is not very efficient when used like this.
Q.24 What mechanism does Hadoop framework provides to synchronize changes made in Distribution Cache during runtime of the application
This is a trick questions. There is no such mechanism. Distributed Cache by design is read only during the time of Job execution
Q25. Have you ever used Counters in Hadoop. Give us an example scenario
Anybody who claims to have worked on a Hadoop project is expected to use counters
Q26. Is it possible to provide multiple input to Hadoop? If yes then how can you give multiple directories as input to the Hadoop job
Yes, The input format class provides methods to add multiple directories as input to a Hadoop job
Q27. Is it possible to have Hadoop job output in multiple directories. If yes then how
Yes, by using Multiple Outputs class
Q28. What will a hadoop job do if you try to run it with an output directory that is already present? Will it
- overwrite it
- warn you and continue
- throw an exception and exit
The hadoop job will throw an exception and exit.
Q29. How can you set an arbitary number of mappers to be created for a job in Hadoop
This is a trick question. You cannot set it
Q30. How can you set an arbitary number of reducers to be created for a job in Hadoop
You can either do it progamatically by using method setNumReduceTasksin the JobConfclass or set it up as a configuration setting.
Q21.
Explain difference of Class Variable and Instance Variable and how
are they declared in Java
Class Variable is a variable which is declared with static modifier.
Instance variable is a variable in a class without static modifier.
The main difference between the class variable and Instance variable is, that first time, when class is loaded in to memory, then only memory is allocated for all class variables. That means, class variables do not depend on the Objets of that classes. What ever number of objects are there, only one copy is created at the time of class loding.
Q22. Since an Abstract class in Java cannot be instantiated then how can you use its non static methods
By extending it
Q23. How would you make a copy of an entire Java object with its state?
Have this class implement Cloneable interface and call its method clone().
Q24. Explain Encapsulation,Inheritance and Polymorphism
Encapsulation is a process of binding or wrapping the data and the codes that operates on the data into a single entity. This keeps the data safe from outside interface and misuse. One way to think about encapsulation is as a protective wrapper that prevents code and data from being arbitrarily accessed by other code defined outside the wrapper.
Inheritance is the process by which one object acquires the properties of another object.
The meaning of Polymorphism is something like one name many forms. Polymorphism enables one entity to be used as as general category for different types of actions. The specific action is determined by the exact nature of the situation. The concept of polymorphism can be explained as "one interface, multiple methods".
Q25. Explain garbage collection?
Garbage collection is one of the most important feature of Java.
Garbage collection is also called automatic memory management as JVM automatically removes the unused variables/objects (value is null) from the memory. User program cann't directly free the object from memory, instead it is the job of the garbage collector to automatically free the objects that are no longer referenced by a program. Every class inherits finalize() method from java.lang.Object, the finalize() method is called by garbage collector when it determines no more references to the object exists. In Java, it is good idea to explicitly assign null into a variable when no more in us
Q26. What is similarities/difference between an Abstract class and Interface?
Differences- Interfaces provide a form of multiple inheritance. A class can extend only one other class.
- Interfaces are limited to public methods and constants with no implementation. Abstract classes can have a partial implementation, protected parts, static methods, etc.
- A Class may implement several interfaces. But in case of abstract class, a class may extend only one abstract class.
- Interfaces are slow as it requires extra indirection to find corresponding method in in the actual class. Abstract classes are fast.
Similarities
- Neither Abstract classes or Interface can be instantiated
Q27. What are different ways to make your class multithreaded in Java
There are two ways to create new kinds of threads:
- Define a new class that extends the Thread class
- Define a new class that implements the Runnable interface, and pass an object of that class to a Thread's constructor.
Q28. What do you understand by Synchronization? How do synchronize a method call in Java? How do you synchonize a block of code in java ?
Synchronization is a process of controlling the access of shared resources by the multiple threads in such a manner that only one thread can access one resource at a time. In non synchronized multithreaded application, it is possible for one thread to modify a shared object while another thread is in the process of using or updating the object's value. Synchronization prevents such type of data corruption.
- Synchronizing a method: Put keyword synchronized as part of the method declaration
- Synchronizing a block of code inside a method: Put block of code in synchronized (this) { Some Code }
Q29. What is transient variable?
Transient variable can't be serialize. For example if a variable is declared as transient in a Serializable class and the class is written to an ObjectStream, the value of the variable can't be written to the stream instead when the class is retrieved from the ObjectStreamthe value of the variable becomes null.
Q30. What is Properties class in Java. Which class does it extends?
The Properties class represents a persistent set of properties. The Properties can be saved to a stream or loaded from a stream. Each key and its corresponding value in the property list is a string
Q31. Explain the concept of shallow copy vs deep copy in Java
In case of shallow copy, the cloned object also refers to the same object to which the original object refers as only the object references gets copied and not the referred objects themselves.
In case deep copy, a clone of the class and all all objects referred by that class is made.
Q32. How can you make a shallow copy of an object in Java
Use clone() method inherited by Object class
Q33. How would you make a copy of an entire Java object (deep copy) with its state?
Have this class implement Cloneable interface and call its method clone().
Class Variable is a variable which is declared with static modifier.
Instance variable is a variable in a class without static modifier.
The main difference between the class variable and Instance variable is, that first time, when class is loaded in to memory, then only memory is allocated for all class variables. That means, class variables do not depend on the Objets of that classes. What ever number of objects are there, only one copy is created at the time of class loding.
Q22. Since an Abstract class in Java cannot be instantiated then how can you use its non static methods
By extending it
Q23. How would you make a copy of an entire Java object with its state?
Have this class implement Cloneable interface and call its method clone().
Q24. Explain Encapsulation,Inheritance and Polymorphism
Encapsulation is a process of binding or wrapping the data and the codes that operates on the data into a single entity. This keeps the data safe from outside interface and misuse. One way to think about encapsulation is as a protective wrapper that prevents code and data from being arbitrarily accessed by other code defined outside the wrapper.
Inheritance is the process by which one object acquires the properties of another object.
The meaning of Polymorphism is something like one name many forms. Polymorphism enables one entity to be used as as general category for different types of actions. The specific action is determined by the exact nature of the situation. The concept of polymorphism can be explained as "one interface, multiple methods".
Q25. Explain garbage collection?
Garbage collection is one of the most important feature of Java.
Garbage collection is also called automatic memory management as JVM automatically removes the unused variables/objects (value is null) from the memory. User program cann't directly free the object from memory, instead it is the job of the garbage collector to automatically free the objects that are no longer referenced by a program. Every class inherits finalize() method from java.lang.Object, the finalize() method is called by garbage collector when it determines no more references to the object exists. In Java, it is good idea to explicitly assign null into a variable when no more in us
Q26. What is similarities/difference between an Abstract class and Interface?
Differences- Interfaces provide a form of multiple inheritance. A class can extend only one other class.
- Interfaces are limited to public methods and constants with no implementation. Abstract classes can have a partial implementation, protected parts, static methods, etc.
- A Class may implement several interfaces. But in case of abstract class, a class may extend only one abstract class.
- Interfaces are slow as it requires extra indirection to find corresponding method in in the actual class. Abstract classes are fast.
Similarities
- Neither Abstract classes or Interface can be instantiated
Q27. What are different ways to make your class multithreaded in Java
There are two ways to create new kinds of threads:
- Define a new class that extends the Thread class
- Define a new class that implements the Runnable interface, and pass an object of that class to a Thread's constructor.
Q28. What do you understand by Synchronization? How do synchronize a method call in Java? How do you synchonize a block of code in java ?
Synchronization is a process of controlling the access of shared resources by the multiple threads in such a manner that only one thread can access one resource at a time. In non synchronized multithreaded application, it is possible for one thread to modify a shared object while another thread is in the process of using or updating the object's value. Synchronization prevents such type of data corruption.
- Synchronizing a method: Put keyword synchronized as part of the method declaration
- Synchronizing a block of code inside a method: Put block of code in synchronized (this) { Some Code }
Q29. What is transient variable?
Transient variable can't be serialize. For example if a variable is declared as transient in a Serializable class and the class is written to an ObjectStream, the value of the variable can't be written to the stream instead when the class is retrieved from the ObjectStreamthe value of the variable becomes null.
Q30. What is Properties class in Java. Which class does it extends?
The Properties class represents a persistent set of properties. The Properties can be saved to a stream or loaded from a stream. Each key and its corresponding value in the property list is a string
Q31. Explain the concept of shallow copy vs deep copy in Java
In case of shallow copy, the cloned object also refers to the same object to which the original object refers as only the object references gets copied and not the referred objects themselves.
In case deep copy, a clone of the class and all all objects referred by that class is made.
Q32. How can you make a shallow copy of an object in Java
Use clone() method inherited by Object class
Q33. How would you make a copy of an entire Java object (deep copy) with its state?
Have this class implement Cloneable interface and call its method clone().
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