Java Interview Questions and Answers covered in this post:
What is the difference between Local variable and Instance
variable?
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Local Variable |
Instance Variable |
|
1. |
Local variable declares inside the method, block and
constructor. |
Instance variable declared outside the method, block and
constructor but inside the class. |
|
2. |
We have to manually initialize the local variable before use. |
When instance variable gets created default value assigned to that
variable. |
|
3. |
Scope of local variable up to method execution. |
Instance variable scope is wider than the local variable. |
Can you declare constructor inside interface?
- Constructor cannot be declared inside interface.
- Constructor is mainly used to initialize the value. We cannot instantiate the interface so we cannot declare the constructor inside an interface.
Can we declare a class as Abstract without having a single abstract
method?
- You can declare an abstract class without a
single abstract method declared inside an abstract class.
- Make a note if you declared the already abstract method then it is mandatory to declare the class as abstract class.
What is the purpose of the final keyword where it can be
applicable?
- Final keyword can be applied to the following.
1.
Variable: Final variable value cannot change once
assigned.
2. Method: Final method cannot override.
3. Class: Final class cannot be sub-classed.
What are Boxing and Unboxing?
- The conversion of primitive data type into its corresponding wrapper class
is called boxing.
- The conversion of wrapper class type into its corresponding primitive type
is called unboxing.
- Please refer to the below example for boxing and unboxing.
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package simplifiedjava.crackedInterview; public class BoxingUnboxingDemo { public static void main(String[] args) {
int num
= 100;
Integer wrapperNum1 = new
Integer(num);
Integer wrapperNum2
= 200; System.out.println("Auto Boxing "+ wrapperNum1 + " and " + wrapperNum2);
Integer wrapperNum3 = new
Integer(300);
num = wrapperNum3;
System.out.println("Unboxing "+ num);
}
}
Output :
Auto Boxing 100 and 200
Unboxing 300 |
 
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How to create an immutable class or What are the steps to create
immutable objects?
- Please refer to the below steps to create an immutable class.
1. Create a final class so a subclass is not possible.
2. Make all instance variables final so no one can change the
value.
3. Do not provide any setter method just provide a getter
method.
- For your reference, I have created a new immutable
class.
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package simplifiedjava.crackedInterview; public final class ImmutableClassDemo { private final String empCode="E001";
public
String getEmpCode() {
return empCode;
}
} |
Which JVM Parameter is used to control stack size of thread?
- Parameter: -Xssn represents Stack Size;
- Every thread that is spawned during the execution of the program passed to java has n as its C stack size.
What is the use of ExecutorService interface?
- The Java ExecutorService is the interface that allows us to execute tasks
on threads asynchronously.
- The Java ExecutorService interface is present in the java.util.Concurrent
package.
- Executors execute() method takes a runnable object and performs the task
asynchronously.
- After making the call of execute() method we call the shutdown() method.
- Please refer to the below example.
|
package simplifiedjava.crackedInterview;
import
java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; public class ExecutorServiceDemo {
public static void
main(String[] args) {
ExecutorService executorService
= Executors.newFixedThreadPool(10); executorService.execute(new Runnable() {
@Override
public void
run() {
System.out.println("ExecutorService");
}
});
executorService.shutdown();
}
}
Ouput: ExecutorService |
-
Executors submit() method takes a runnable object and returns a Future
object. Later on, this object can be used to check the status of Runnable
whether it has completed execution or not.
- Please refer to the below example.
|
package simplifiedjava.crackedInterview;
import
java.util.concurrent.ExecutorService;
import
java.util.concurrent.Executors; import java.util.concurrent.Future; public class ExecutorServiceDemo {
public static void
main(String[] args) {
ExecutorService executorService
= Executors.newSingleThreadExecutor(); Future future = executorService.submit(new Runnable() {
@Override
public void
run() {
//System.out.println("ExecutorService");
}
});
System.out.println(future.isDone());
}
}
Output: false |
- Executors invokeAny() method takes a collection of callable objects. This
method returns the future object of the callable objects which are executed
first successfully.
- Please refer to the below example.
|
package simplifiedjava.crackedInterview;
import
java.util.HashSet;
import
java.util.Set;
import
java.util.concurrent.Callable;
import
java.util.concurrent.ExecutionException;
import
java.util.concurrent.ExecutorService;
import
java.util.concurrent.Executors; import java.util.concurrent.Future; public class ExecutorServiceDemo { public static void main(String[] args) throws InterruptedException, Exception {
// invoke method demo
ExecutorService executorService
= Executors.newSingleThreadExecutor();
Set<Callable<String>> callableSet = new
HashSet<Callable<String>>();
callableSet.add(new
Callable<String>() {
public
String call() throws
Exception{
return "Job 1";
} });
callableSet.add(new
Callable<String>() {
public
String call()throws
Exception{
return "Job 2";
} });
callableSet.add(new
Callable<String>() {
public
String call()throws
Exception{
return "Job 3";
} });
String executedJob = executorService.invokeAny(callableSet);
System.out.println(executedJob);
executorService.shutdown();
}
}
Output: Job 2 |
- The invokeAll() method takes in a Collection of Callable objects having
tasks and returns a list of Future objects containing the result of all the
tasks.
|
package simplifiedjava.crackedInterview;
import
java.util.HashSet;
import
java.util.List;
import
java.util.Set;
import
java.util.concurrent.Callable;
import
java.util.concurrent.ExecutionException;
import
java.util.concurrent.ExecutorService;
import
java.util.concurrent.Executors; import java.util.concurrent.Future; public class ExecutorServiceDemo { public static void main(String[] args) throws InterruptedException, Exception {
// invoke method demo
ExecutorService executorService
= Executors.newSingleThreadExecutor();
Set<Callable<String>> callableSet = new
HashSet<Callable<String>>();
callableSet.add(new
Callable<String>() {
public
String call() throws
Exception{
return "Job 1";
} });
callableSet.add(new
Callable<String>() {
public
String call()throws
Exception{
return "Job 2";
} });
callableSet.add(new
Callable<String>() {
public
String call()throws
Exception{
return "Job 3";
} }); List<Future<String>> executedJob = executorService.invokeAll(callableSet);
for(Future future: executedJob) {
System.out.println(future.get());
} executorService.shutdown();
}
}
Output:
Job 2
Job 3
Job 1
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What is the role of Future interface in Multithreading?
- A Future represents the result of an asynchronous computation.
- Methods are provided to check if the computation is complete, to wait for
its completion, and to retrieve the result of the computation.
- The result can only be retrieved using method get when the computation has
completed, blocking if necessary until it is ready.
- Cancellation is performed by the cancel method.
- Additional methods are provided to determine if the task was completed
normally or was cancelled.
- Once a computation has been completed, the computation cannot be cancelled.
- If you would like to use a Future for the sake of cancellability but not
provide a usable result, you can declare types of the form Future<?>
and return null as a result of the underlying task.
What is the fill ratio in collection framework and what is the default
fill ratio?
- Fill ratio is a measure that decides when to increase the size of the
collection.
- Default fill ratio is 75%. It means when your collection object gets full
by 75% then collection size will increase the space internally.
- Every collection object has its own way or different formula to increase
the size internally.
- Java interview questions and answers all MNC - Click here
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- Core java interview questions for 3 years experience - Click here
- Core java interview questions and answers for 3-5 years exp - Click here
- Core java interview questions and Answers for 5 - 7 Years exp - Click here
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- Java interview questions on exception handling - Click here
- Interview questions on multithreading in java for experienced - Click here
- Interview questions on serialization in java for experienced - Click here
- Interview questions on inner class in java for experienced - Click here
- Interview questions on Collections in java for experienced - Click here
Questions covered in upcoming post:
1. How would you implement Multiple Inheritance in java?
2. In your program you have static block, non-static block, constructor and main(). What will be the sequence of execution.
3. What is the purpose to make a constructor private.
4. What is instanceOf in java. Where it can be used.
5. What is the difference between equals() and ==.
6. What is Comparable and Comparator. What is the difference between Comparable and Comparator.
7. What is the difference between checked exception and unchecked exceptions.
8. Can you explain why NullPointerException occurred and how to handle it?
9. What is Thread and what is process. What is the difference between these two.
10.What are the advantages and disadvantages of Serialization.
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Thank you techies for visiting this blog. I hope you enjoyed this blog and got more technical knowledge. I have tried to cover all types of questions and provided examples tested on eclipse as much as I can. Guys, please don’t just mug up the questions and answers. Try to clear your concepts with examples. Try to write a code on eclipse once you read the concepts. It will help you to memorize the concepts for a very long time. Simultaneously you will be prepared for interview programs as well. It will help you to survive in the IT sector for a long time. It may be easy to crack an interview but it's really tough to survive in the IT industry with inadequate knowledge and skills. I have collected all the questions from actual interviews attended by my friends, colleagues, college mate and classmate. I have covered frequently asked questions as well as challenging questions. I have included many programs to understand the concept thoroughly. I will try to explain the concept with the help of a real-time program in eclipse. You can share more questions which are not covered in this blog or post. Always welcome your suggestions and queries. I will definitely try to resolve it. Please comment your queries and new set of questions under the comment sections. I will create a new post for those questions.
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