langref.org - java - Parallel

Create a multithreaded "Hello World"

Create a program which outputs the string "Hello World" to the console, multiple times, using separate threads or processes.

Example:

-Output-

Thread one says Hello World!
Thread two says Hello World!
Thread four says Hello World!
Thread three says Hello World!

-Notice that the threads can print in any order.

java

for (int i = 0; i < 4; i++) {
final int nr = i ;
new Thread(new Runnable() {
public void run() {
System.out.println("Thread " + new String[] { "one", "two", "three", "four" }[nr] + " says Hello World!");
}
}).start();
}

Create read/write lock on a shared resource.

Create multiple threads or processes who are either readers or writers. There should be more readers then writers.

(From Wikipedia):

Multiple readers can read the data in parallel but an exclusive lock is needed while writing the data. When a writer is writing the data, readers will be blocked until the writer is finished writing.

Example:

-Output-

Thread one says that the value is 8.
Thread three says that the value is 8.
Thread two is taking the lock.
Thread four tried to read the value, but could not.
Thread five tried to write to the value, but could not.
Thread two is changing the value to 9.
Thread two is releasing the lock.
Thread four says that the value is 9.
...

--Notice that when a needed resource is locked, a thread can set a timer and try again in the future, or wait to be notified that the resource is no longer locked.

java

public class ParallelPermutations {

Lock lock = new ReentrantLock();

Integer value = 8;

public static void main(String[] args) {
new ParallelPermutations(Arrays.asList(args));
}

public ParallelPermutations(List<String> words) {
for (int i = 0; i < 20; i++) {
final Integer cnt = i ;
if ( i % 3 == 0) {
new Thread(new Runnable() {
public void run() {
if ( ! lock.tryLock() ) {
System.out.println("Thread " + cnt + " tried to write the value, but could not.") ;
lock.lock();
}
value = (int) (Math.random() * 10);
System.out.println("Thread " + cnt + " is changing the value to " + value ) ;
lock.unlock();
System.out.println("Thread " + cnt + " is releasing the lock.") ;
}
}).start();
} else {
new Thread(new Runnable() {
public void run() {
if ( ! lock.tryLock() ) {
System.out.println("Thread " + cnt + " tried to read the value, but could not.") ;
lock.lock() ;
}
System.out.println("Thread " + cnt + " says that the value is " + value + ".") ;
lock.unlock();
}
}).start();
}
}
}
}

Separate user interaction and computation.

Allow your program to accept user interaction while conducting a long running computation.

Example:

Hello user! Please input a string to permute: (input thread)
abcdef
Passing on abcdef... (input thread)
Please input another string to permute: (input thread)
lol
Passing on lol... (input thread)
Done Work On abcdef! (worker thread)
["abcdef", "abcefd", ... ] (worker thread)
Please input another string to permute: (input thread)
EXIT
Quitting, I

'll let my worker thread know... (input thread)

We'

re quitting! Alright! (worker thread)

--Notice, that this could be accomplished on the command line or within a GUI. The point is that computation and user interaction should take place on separate threads of control.

java

public class BackgroundComputation {

final protected Queue<Thread> threads = new ConcurrentLinkedQueue<Thread>() ;

public BackgroundComputation() {
BufferedReader r = new BufferedReader(new InputStreamReader(System.in));
try {
while (true) {
System.out.print("Enter string to permutate: ");
final String word = r.readLine();
if ("EXIT".equals(word) ) {
System.out.println("I'll let my worker thread know... (input thread)") ;
while (! threads.isEmpty())
threads.poll().stop(new ThreadDeath()) ;
break ;
}
Thread t = new Thread(new Runnable() {
public void run() {
try {
Set<String> permutationSet = new HashSet<String>();
for (int i = 0; i < word.length(); i++)
for (int j = i + 1; j <= word.length(); j++)
permutations("", word.substring(i, j), permutationSet);
System.out.println();
System.out.println("Received results: " + permutationSet);
System.out.print("Enter string to permutate: ");
} catch (ThreadDeath e) {
System.out.println("We're quitting! Alright!");
}
}

private void permutations(String prefix, String word, Set<String> permutations) {
int N = word.length();
if (N == 0)
permutations.add(prefix);
else
for (int i = 0; i < N; i++)
permutations(
prefix + word.charAt(i),
word.substring(0, i) + word.substring(i + 1, N),
permutations
);
}
});
t.start();
threads.add(t);
}
} catch (IOException ioe) {
System.out.println("IO error trying to read your name!");
System.exit(1);
}
}

public static void main(String[] args) {
new BackgroundComputation() ;
}
}