Core Java -101

Question 1

Can we do array[i] and i could be long type also?

Answer 1

No i must be only int

Question 2

What is a Runtime class?

Answer 2

Every Java Application has one instance of this , this will allow instance to interface with env it is running

Question 3

How to obtain the Runtime?

Answer 3

getRuntime

Question 4

What is addShutdownHook(Thread hook)

Answer 4

Registers a new virtual-machine shutdown hook.

Question 5

What is exec methods?

Answer 5

So this has many overloads & use to give the commands on the given environment

Question 6

exit(int status)

Answer 6

Terminates the currently running Java virtual machine by initiating its shutdown sequence.

Question 7

freeMemory()

Answer 7

Returns the amount of free memory in the Java Virtual Machine.

Question 8

gc()

Answer 8

Runs the garbage collector.

Question 9

halt(int status)

Answer 9

Forcibly terminates the currently running Java virtual machine.

Question 10

loadLibrary(String libname)

Answer 10

Loads the dynamic library with the specified library name.

Question 11

removeShutdownHook(Thread hook)

Answer 11

De-registers a previously-registered virtual-machine shutdown hook.

Question 12

totalMemory()

Answer 12

totalMemory()

Returns the total amount of memory in the Java virtual machine.

Question 13

What the exec method returns?

Answer 13

Process

Question 14

What is a java.lang.Process class?

Answer 14

The ProcessBuilder.start() and Runtime.exec methods create a native process and return an instance of a subclass of Process that can be used to control the process and obtain information about it. The class Process provides methods for performing input from the process, performing output to the process, waiting for the process to complete, checking the exit status of the process, and destroying (killing) the process.

Question 15

Does the methods of PRocess works in all env?

Answer 15

Not necessarily all, may fail is some basic env

Question 16

How can we obtain the I/O of Process?

Answer 16

Process does not have terminal or console, all standard I/O are redirected to the Parent process, and can be obtained with the methods like getInputStream, getErrorStream, getOutputStream,

Question 17

How the deadlock may happen while getting getInputStream like methods?

Answer 17

As some env support limited buffer size thus reading and writng to same buffer may cause deadlock

Question 18

Does the subProcess (process instance) do async?

Answer 18

Yes

Question 19

destroy()

Answer 19

Kills the subprocess.

Question 20

exitValue()

Answer 20

Returns the exit value for the subprocess.

Question 21

waitFor()

Answer 21

Causes the current thread to wait, if necessary, until the process represented by this Process object has terminated.

Question 22

Where do thread executes?

Answer 22

Within a process

Question 23

How can we execute a thread in a thread class?

Answer 23

Thread thread = new Thread(new Runnable())
thread.start
The start method will call the run method of runnable

Question 24

Can we use lambda expression to write the THread with runnable IF?

Answer 24

Yes since runnable is an functional IF

Thread thread = new Thread( () -> {
   /// do the task
});

Question 25

Why Implementing is better than extending?

Answer 25

This way we cannot alter the beahvious of the actual class

Add more

Question 26

Which package is for concurrency?

Answer 26

java.util.concurrency

Question 27

What does Executor does?

Answer 27

It is exactly to replace the Thread and likewise it provide only one method execute
runnable r
Executor e
e.execute(r)

Question 28

What does ExecutorService does?

Answer 28

It is an IF and extends the Executor and provides method like submit to accepts runnable and callable to return value

Question 29

What does ScheduledExecutorService does?

Answer 29

It is an IF and extends the ExecutorService and provides method to execute tasks at trepeated intervals

Question 30

What does executor makes use of?

Answer 30

They use threadpool and Worker threads

Question 31

Are there fixed thread pools, & what is the concept of it?

Answer 31

If a thread is terminated then a new thread created

Question 32

Does Executor class has static methods to create ExecutorSerivice

Answer 32

Yes, static ExecutorService newSingleTHreadExecutor()

Question 33

In static ExecutorService newSingleTHreadExecutor(), how many threads are created?

Answer 33

One

Question 34

How can we create a multiple thread pool with the Executor?

Answer 34

static ExecutorService newScheduledThreadPool(int)