Lecture 9

Question 1

Three types of Map?

Answer 1

HashMap (no ordering), LinkedHashMap (linked pointers, traversing happens in specified order), TreeMap (sorted using compareTo of interface comparable, keys must implement comparable).

Question 2

How to traverse an unordered set?

Answer 2

Use an iterator. Interface Collection provides a method iterator. Methods: hasNext(), next().

Question 3

What happens after a traversal usin an iterator?

Answer 3

The used iterator is useless. Cannot be reset.

Question 4

What’s a computer program:

Answer 4

Represents a set of instructions.

Question 5

What is a process?

Answer 5

The sequence of instructions executed by CPU. An instance of the program. Each process consists of threads (sub processes).

Question 6

What are threads Charachteristics?

Answer 6

Execute indecently of each other m. Multithreading is two or more threads in single process. All threads share same resources in process (ie memory). Depend on parent process. Also called lightweight processes.

Question 7

What is concurrency?

Answer 7

Two or more processes/threads running simultaneously. Either at same time (parallel computing) or seemingly (switching back and forth). Concurrency achieved by multiple CPUs or CPU Cores, or frequent switching by scheduler.

Question 8

Concurrency can happen on system level or…

Answer 8

Within a single process. System=multitasking, concurrency of processes, Single=multithreading, concurrency of threads.

Question 9

Why are multithreading and concurrency useful?

Answer 9

To cooperate with other programs (a server with multiple clients), avoid users waiting for response, make programs faster.

Question 10

Java allows programs to create multiple threads which execute either…

Answer 10

Concurrently (multiple CPU/Cores) or sequentially(Scheduler using time slicing).

Question 11

4 types of threads relationships?

Answer 11

Unrelated threads: don’t interact. Run in parallel.
Related but unsynchronized: work on same data without ness for sync.
Mutually exclusive: work on shared resources, must avoid race conditions. Communicating mutually exclusive: must notify each other of their state change.

Question 12

Toe create a Thread need a Thread class and a…

Answer 12

Task class which implements interface Runnable. This task class must implement run(). A thread performs a task, ie the run().

Question 13

Create a thread:

Answer 13

Create object of class thread. Pass to the constructor of thread object which implements runnable.
Thread myT = new Thread(myTask);

Question 14

How to start a thread?

Answer 14

Invoke method start(); of the thread object (not run() of the task object!).

myT.start();

Question 15

List six states a thread can be in.

Answer 15

New - created but not yet running. 
Runnable - start() invoked.
Running - actually running.
Blocked - sleeps or waits for event
Interrupted - by another thread.
Dead - has ended.

Question 16

What are race conditions?

Answer 16

When multiple threads access a shared resource concurrently in an unprotected way. Without proper synchronization operations interleave each other.

Question 17

What is thread safe?

Answer 17

Code is thread safe if it accesses share resources which doesn’t cause race conditions.

Question 18

What is an atomic operation?

Answer 18

Cannot be interrupted by other threads. And yet it’s not guaranteed, depends on JVM version.

Question 19

How best to avoid race conditions?

Answer 19

Use keyword synchronized.

synchronized(this) {
//Critical section
}

Question 20

What is the critical section?

Answer 20

Where the concurrent access of a shared resource takes place.

Question 21

How is synchronized typically used?

Answer 21

For entire methods:
Public synchronized void someMethod() {
// critical section
}

Question 22

How does synchronized work?

Answer 22

Every Java object comes with a lock. A thread tries to acquire the lock when using a method. Once acquired other threads block until the first thread releases the lock, ie leaves the synchronized method.

Question 23

What is the Kava approach to multithreading synchronization called?

Answer 23

The monitor approach. It provides for mutual exclusion (using synchronization keyword) and thread cooperation using condition variables.

Question 24

Another name for Maps?

Answer 24

Associate arrays, or dictionaries.