Concurrency

Question 1

Sequential

Answer 1

Use the von Neumann architecture. After one task is finished, the program can start the next one. It stops if one of them fails.

Question 2

Parallelism

Answer 2

Tasks run on separate threads.

Question 3

Concurrency

Answer 3

Multiple tasks can be processed at (not exactly) the same time by switching very fast between processes.

Question 4

Process

Answer 4

an executing program

Question 5

Thread

Answer 5

one independent task within a process

Question 6

Scheduling

Answer 6

-The distribution of thread execution in time slots
- Threads are scheduled separately

Question 7

Context switch for processes

Answer 7

• used by the operating systems to constantly switch between processes
• saves the current process info and loads the new process info

Question 8

How are processes and threads connected?

Answer 8

A process consists of mini-processes called threads. By default a process only consists of a single ”main” thread, but new threads can be spawned to handle different tasks.

Question 9

How are threads in java created?

Answer 9

• by extending the Thread class
• by implementing the Runnable interface

Question 10

What are the states that a thread can have?

Answer 10

• new
• runnable
• blocked
• waiting
• time-waiting
• terminated

Question 11

Thread state: new

Answer 11

A thread that has been created, but not in execution.

Question 12

Thread state: runnable

Answer 12

Memory for the thread has been allocated and the thread is either running or ready for running

Question 13

Thread state: BLOCKED, WAITING, TIMED WAITING

Answer 13

These states all reflect that the thread is currently waiting for something. In the case of BLOCKED,
the thread could be waiting to enter a synchronized method. A thread can enter the WAITING
state if wait() , join() (both without timeout specified) or park() is called. A thread will enter the
TIMED WAITING state if sleep() is called or wait() and join() have a timeout specified.

Question 14

Thread state: TERMINATED

Answer 14

A thread that has exited is in this state.

Question 15

What is the life span of a thread?

Answer 15

new -> runnable <=> blocked -> runnable -> dead

Question 16

Why does a process have multiple threads?

Answer 16

Switching between processes is expensive because it needs to do a context switch. It’s cheaper to switch between threads because they share execution information.

Question 17

What is the difference netween run() and start() methods ?

Answer 17

Calling run() will only execute the code
within the run method without actually starting the thread. By calling start() the code in the run() method will be executed.

Question 18

Why choose implementing Runnable instead of extending the Thread class?

Answer 18

• code is more loosely coupled when using an interface vs extending a class
• extending Thread limits us from extending the class from another class
• pass the same Runnable object to different threads
• we are (most likely) not overriding anything from the Thread class

Question 19

What is ExecutorService?

Answer 19

Another popular way for creating and managing threads; it works with a thread pool; the ExecutorService object is created using
the Factory pattern

Question 20

What is a thread pool?

Answer 20

A collection of threads where each thread is waiting for a task

Question 21

What are the kinds of tasks that we can submit to the executor service?

Answer 21

Runnable and callable

Question 22

Why use executors instead of manually creating threads?

Answer 22

Executors have more control over the execution context. For a few threads it is not necessary to use executors, but for an application with a lot more threads it makes sense to separate thread management from the rest.

Question 23

What does thread safety refer to?

Answer 23

It refers to multiple threads trying to access (read or write) the same resource at the same time. The sections that can cause issues are called critical sections.

Question 24

How to ensure thread safety?

Answer 24

The ‘synchronized’ keyword(for a method or block of code) - using a lock, it ensures that no other thread can call the method while it is already in use by a thread.

Question 25

What are atomic operations?

Answer 25

Operations where everything happens at once (in contrast to, for instance, ‘++’ which consists of three operations under the hood) (in java: reading and writing of reference and primitive variables, except for long and double)

Question 26

How do you shut down an Executor? (It is important to do so)

Answer 26

You can do that by calling shutdown() or shutdownNow(). The first one makes the executor to stop accepting new tasks and shut down when all tasks are complete, whereas the last one interrupts the currently running tasks.