Week1 -Threading

Question 1

Multi threading

Answer 1

two or more part that can run concurrently. They can run at the same time, while making optimal use of available resources, such as the CPU.

Question 2

Thread Life Cycle

Answer 2

The life cycle of a variety of stages: Born, Started, Run, Die.

Question 3

Thread States

Answer 3

The thread states are

• New
• Runnable
• Block
• Wait
• Timed_Waiting
• Terminate

Question 4

New

Answer 4

Is the born thread state. It stays in this state until the program starts the thread.

Question 5

Runnable

Answer 5

The thread has been started and is executing a task.

Question 6

Block

Answer 6

when a thread try’s to acquire an object lock, but said lock is being used by another thread. It will remain in this state until the other thread has finished an releases the lock

Question 7

Wait

Answer 7

a thread goes to wait state once its calls the wait() method on an object. It will need to wait indefinitely for another thread to perform a particular task.

Question 8

Timed_Waiting

Answer 8

thread is waiting for another thread to perform a particular action for a specific amount of time. The thread transitions back to Runnable after interval time has expired or when the event its waiting for occurs.

Question 9

Terminate

Answer 9

thread completes execution. terminated.

Question 10

Thread Priorities

Answer 10

helps the operation system to determine the order in which threads are scheduled. Thread priority does not guarantee the order in which the threads will execute. and are very platform dependent.

Question 11

Thread Priority range

Answer 11

MIN_PRIORITY - CONSTANT 1
MAX_PRIORITY CONSTANT 10.
By default every thread is given a priority of NORM_PRIORITY CONSTANT 5.

Question 12

Joining Threads

Answer 12

Allow one thread to wait for another thread to finish its execution. If t is a thread object whose threads are currently executing. Then t.join() will make sure that the t is terminated, before next instructions are executed by the program. If there are multiple threads calling join() method, it mean overloading on the joins allows the programmer to specify a waiting period.

Question 13

3 overload functions

Answer 13

join()
join(long millis)
join( long millis, int nanos)

Question 14

join()

Answer 14

waits for the thread to die. It will put the current thread on wait until the thread in which it was called is dead. If interrupted will throw an interrupted exception.

syntax: public final void join(){
do something
}

Question 15

join(long millis)

Answer 15

waits at the most millis millisecond for the thread to die.

syntax: public final synchronized void join (long millis){
do something
}

Question 16

join( long millis, int nanos)

Answer 16

waits for the millis millisecond plus nanos nanosecond for the thread to die.

syntax: public final void join(long millis, int nanos){
do something
}

Question 17

How many thread are creating when you start a java program?

Answer 17

Three thread are creating

• Main
• Garbage Collector
• Thread Scheduler

Question 18

Garbage Collector

Answer 18

The garbage collector intermittently removed de-referenced objects from memory. We can not force garbage collection but can request for it to run.
System.gc() - gently request garbage to run.
We can however intervene with garbage collection, in order to specify behavior at the time of garbage collection. java.lang.object.finalize().
A form of Deamon Thread - we don’t interact or control these. They are low priority background processes.

Question 19

final

Answer 19

is a reserved keyword in java. It can’t be used a identifier. It can be used with methods, variables (objects) and classes.
final class- can’t be extend
final method - cant be overriden
final variable - the value of the variable can not be changed once its initialized .

Question 20

finally

Answer 20

is used within the try/catch block. It guarantees that a section of the code will be executed even if an exception is thrown.

Question 21

finalized()

Answer 21

is the method of object class. This method id called just before an object is garbage collected. finalized() method overrides the dispose system resources, performs clean up activities and minimize memory leaks.

Question 22

Thread Scheduler

Answer 22

The Thread scheduler in java is the part of the JVM that decides which thread should run.
a form of Deamon thread- we don’t interact or control these. They are low priority background processes.

Question 23

How to create a thread.

Answer 23

• extend thread

- implement Runnable Interface

Question 24

extend thread

Answer 24

within the main args you start by calling the start() method , which is inhearited by the thread. It runs the method. extendThread.start(). A thread is already Runnable.

syntax: 
public class MyThread extends Thread{
public void  run(){
do something 
} }

Question 25

implement Runnable Interface

Answer 25

you start a Runnable  thread by creating a thread, pass the Runnable to it call start(). 
A Runnable  can subclass  something other that Thread using Runnable. Since each class can only extend  one other class. 
On the other hand anything that extend Thread is already considered a Thread . so its convenient to use sometimes

syntax:
public  class MyRunnable  implements Runnable {
public  void run(){
do something 
}}

Question 26

Thread Safe

Answer 26

one thread at a time can access if there’s an array list of synchronized elements.

Question 27

Can thread be garbage collected

Answer 27

Yes, an as long as its active it won’t get garbage collected.

Question 28

Synchronized

Answer 28

Keyword ensures only on thread can access a resource at a given time, such as methods variables objects and code blocks.
By synchronizing you can consistency but lose speed.
By allowing multiple threads to read (but not write) and synchronize writing creates a normal reading method and a synchronized writing method. Also, when a collection is thread safe its members are protected from being accessed by more than thread at a time.

Question 29

Thread Methods

Answer 29

start() - calls run
setPriority() - values from 1-10 changes the priority
getPriority - values from 1-10 gets priority
isAlive() - checks whether the thread is running
wait() - specify the amount to time to wait. Can be notified.
sleep() - will pause of a specified about of time . Can not be notified
notify() - waskes up waiting thread
notifyAll() - notifies all waiting threads
join() - waits for another thread to stop executing.Catches up with another thread.

Question 30

Deadlock

Answer 30

When multiple threads are forever blocking each other from trying to access the same resource.

Question 31

Starvation

Answer 31

when a thread is not granted CPU time because other threads grabbed it all. This is caused when high priority thread swallow up all the CPU from low priority thread.

Question 32

Saturation

Answer 32

Basically bottle necking. you are producing faster that they being consumed.

Question 33

Producers and Consumers

Answer 33

Its the producers job to generate data, put into the buffer and start again. While at the same time the consumer is consuming the data (removing it) from the buffer one piece at a time.

Question 34

Blocking Queue

Answer 34

a queue that is thread safe to put elements into and take elements from.

Question 35

What is a Thread?

Answer 35

A process or a single line of logic

Question 36

What is a subprocess

Answer 36

A child process of the main parents logic

Question 37

Do Threads really run concurrently in Java?

Answer 37

No its handled by the thread scheduler