Develop Reference

printing alternative output from 2 threads using semaphores

I am learning about the use of semaphores and multi threading in general but am kind of stuck. I have two threads printing G and H respectively and my objective is to alternate the outputs of each thread so that the output string is like this;
G
H
G
H
G
H
Each of the two classes has a layout similar to the one below
public class ClassA extends Thread implements Runnable{
Semaphore semaphore = null;
public ClassA(Semaphore semaphore){
this.semaphore = semaphore;
}
public void run() {
while(true)
{
try{
semaphore.acquire();
for(int i=0; i<1000; i++){
System.out.println("F");
}
Thread.currentThread();
Thread.sleep(100);
}catch(Exception e)
{
System.out.println(e.toString());
}
semaphore.release();
}
}
}
below is my main class
public static void main(String[] args) throws InterruptedException {
Semaphore semaphore = new Semaphore(1);
ClassA clasA = new ClassA(semaphore);
Thread t1 = new Thread(clasA);
ClassB clasB = new ClassB(semaphore);
Thread t2 = new Thread(clasB);
t1.start();
t2.join();
t2.start();
The output I am getting is way too different from my expected result. can anyone help me please? did I misuse the semaphore? any help?

Semaphores can't help you solve such a task.
As far as I know, JVM doesn't promise any order in thread execution. It means that if you run several threads, one thread can execute several times in a row and have more processor time than any other. So, if you want your threads to execute in a particular order you can, for the simplest example, make a static boolean variable which will play a role of a switcher for your threads. Using wait() and notify() methods will be a better way, and Interface Condition will be the best way I suppose.
import java.io.IOException;
public class Solution {
public static boolean order;
public static void main(String[] args) throws IOException, InterruptedException {
Thread t1 = new ThreadPrint("G", true);
Thread t2 = new ThreadPrint("O", false);
t1.start();
t2.start();
t2.join();
System.out.println("Finish");
}
}
class ThreadPrint extends Thread {
private String line;
private boolean order;
public ThreadPrint(String line, boolean order) {
this.line = line;
this.order = order;
}
#Override
public void run() {
int z = 0;
while (true) {
try {
for (int i = 0; i < 10; i++) {
if (order == Solution.order) {
System.out.print(line + " ");
Solution.order = !order;
}
}
sleep(100);
} catch (Exception e) {
System.out.println(e.toString());
}
}
}
}
BTW there can be another problem cause System.out is usually an Operation System buffer and your OS can output your messages in an order on its own.
P.S. You shouldn't inherit Thread and implement Runnable at the same time
public class ClassA extends Thread implements Runnable{
because Thread class already implements Runnable. You can choose only one way which will be better for your purposes.
You should start a thread then join to it not vice versa.
t1.start();
t2.join();
t2.start();

As others have pointed out, locks themselves do not enforce any order and on top of that, you cannot be certain when a thread starts (calling Thread.start() will start the thread at some point in the future, but this might take a while).
You can, however, use locks (like a Semaphore) to enforce an order. In this case, you can use two Semaphores to switch threads on and off (alternate). The two threads (or Runnables) do need to be aware of each other in advance - a more dynamic approach where threads can "join in" on the party would be more complex.
Below a runnable example class with repeatable results (always a good thing to have when testing multi-threading). I will leave it up to you to figure out why and how it works.
import java.util.concurrent.*;
public class AlternateSem implements Runnable {
static final CountDownLatch DONE_LATCH = new CountDownLatch(2);
static final int TIMEOUT_MS = 1000;
static final int MAX_LOOPS = 10;
public static void main(String[] args) {
ExecutorService executor = Executors.newCachedThreadPool();
try {
AlternateSem as1 = new AlternateSem(false);
AlternateSem as2 = new AlternateSem(true);
as1.setAlternate(as2);
as2.setAlternate(as1);
executor.execute(as1);
executor.execute(as2);
if (DONE_LATCH.await(TIMEOUT_MS, TimeUnit.MILLISECONDS)) {
System.out.println();
System.out.println("Done");
} else {
System.out.println("Timeout");
}
} catch (Exception e) {
e.printStackTrace();
} finally {
executor.shutdownNow();
}
}
final Semaphore sem = new Semaphore(0);
final boolean odd;
AlternateSem other;
public AlternateSem(boolean odd) {
this.odd = odd;
}
void setAlternate(AlternateSem other) { this.other = other; }
void release() { sem.release(); }
void acquire() throws Exception { sem.acquire(); }
#Override
public void run() {
if (odd) {
other.release();
}
int i = 0;
try {
while (i < MAX_LOOPS) {
i++;
other.acquire();
System.out.print(odd ? "G " : "H ");
release();
}
} catch (Exception e) {
e.printStackTrace();
}
DONE_LATCH.countDown();
}
}

Multi threaded java program to print even and odd numbers alternatively

I was asked to write a two-threaded Java program in an interview. In this program one thread should print even numbers and the other thread should print odd numbers alternatively.
Sample output:
Thread1: 1
Thread2: 2
Thread1: 3
Thread2: 4
... and so on
I wrote the following program. One class Task which contains two methods to print even and odd numbers respectively. From main method, I created two threads to call these two methods. The interviewer asked me to improve it further, but I could not think of any improvement. Is there any better way to write the same program?
class Task
{
boolean flag;
public Task(boolean flag)
{
this.flag = flag;
}
public void printEven()
{
for( int i = 2; i <= 10; i+=2 )
{
synchronized (this)
{
try
{
while( !flag )
wait();
System.out.println(i);
flag = false;
notify();
}
catch (InterruptedException ex)
{
ex.printStackTrace();
}
}
}
}
public void printOdd()
{
for( int i = 1; i < 10; i+=2 )
{
synchronized (this)
{
try
{
while(flag )
wait();
System.out.println(i);
flag = true;
notify();
}
catch(InterruptedException ex)
{
ex.printStackTrace();
}
}
}
}
}
public class App {
public static void main(String [] args)
{
Task t = new Task(false);
Thread t1 = new Thread( new Runnable() {
public void run()
{
t.printOdd();
}
});
Thread t2 = new Thread( new Runnable() {
public void run()
{
t.printEven();
}
});
t1.start();
t2.start();
}
}

I think this should work properly and pretty simple.
package com.simple;
import java.util.concurrent.Semaphore;
/**
* #author Evgeny Zhuravlev
*/
public class ConcurrentPing
{
public static void main(String[] args) throws InterruptedException
{
Semaphore semaphore1 = new Semaphore(0, true);
Semaphore semaphore2 = new Semaphore(0, true);
new Thread(new Task("1", 1, semaphore1, semaphore2)).start();
new Thread(new Task("2", 2, semaphore2, semaphore1)).start();
semaphore1.release();
}
private static class Task implements Runnable
{
private String name;
private long value;
private Semaphore semaphore1;
private Semaphore semaphore2;
public Task(String name, long value, Semaphore semaphore1, Semaphore semaphore2)
{
this.name = name;
this.value = value;
this.semaphore1 = semaphore1;
this.semaphore2 = semaphore2;
}
#Override
public void run()
{
while (true)
{
try
{
semaphore1.acquire();
System.out.println(name + ": " + value);
value += 2;
semaphore2.release();
}
catch (InterruptedException e)
{
throw new RuntimeException(e);
}
}
}
}
}

Well, there are many alternatives. I would probably use a SynchronousQueue instead (I don't like low-level wait/notify and try to use higher-level concurrency primitives instead). Also printOdd and printEven could be merged into single method and no additional flags are necessary:
public class App {
static class OddEven implements Runnable {
private final SynchronousQueue<Integer> queue = new SynchronousQueue<>();
public void start() throws InterruptedException {
Thread oddThread = new Thread(this);
Thread evenThread = new Thread(this);
oddThread.start();
queue.put(1);
evenThread.start();
}
#Override
public void run() {
try {
while (true) {
int i = queue.take();
System.out.println(i + " (" + Thread.currentThread() + ")");
if (i == 10)
break;
queue.put(++i);
if (i == 10)
break;
} catch (InterruptedException e) {
throw new RuntimeException(e);
}
}
}
public static void main(String[] args) throws InterruptedException {
new OddEven().start();
}
}

Is there any better way to write the same program?
Well, the thing is, the only good way to write the program is to use a single thread. If you want a program to do X, Y, and Z in that order, then write a procedure that does X, then Y, then Z. There is no better way than that.
Here's what I would have written after discussing the appropriateness of threads with the interviewer.
import java.util.concurrent.SynchronousQueue;
import java.util.function.Consumer;
public class EvenOdd {
public static void main(String[] args) {
SynchronousQueue<Object> q1 = new SynchronousQueue<>();
SynchronousQueue<Object> q2 = new SynchronousQueue<>();
Consumer<Integer> consumer = (Integer count) -> System.out.println(count);
new Thread(new Counter(q1, q2, 2, 1, consumer)).start();
new Thread(new Counter(q2, q1, 2, 2, consumer)).start();
try {
q1.put(new Object());
} catch (InterruptedException ex) {
throw new RuntimeException(ex);
}
}
private static class Counter implements Runnable {
final SynchronousQueue<Object> qin;
final SynchronousQueue<Object> qout;
final int increment;
final Consumer<Integer> consumer;
int count;
Counter(SynchronousQueue<Object> qin, SynchronousQueue<Object> qout,
int increment, int initial_count,
Consumer<Integer> consumer) {
this.qin = qin;
this.qout = qout;
this.increment = increment;
this.count = initial_count;
this.consumer = consumer;
}
public void run() {
try {
while (true) {
Object token = qin.take();
consumer.accept(count);
qout.put(token);
count += increment;
}
} catch (InterruptedException ex) {
throw new RuntimeException(ex);
}
}
}
}

How about a shorter version like this:
public class OddEven implements Runnable {
private static volatile int n = 1;
public static void main(String [] args) {
new Thread(new OddEven()).start();
new Thread(new OddEven()).start();
}
#Override
public void run() {
synchronized (this.getClass()) {
try {
while (n < 10) {
this.getClass().notify();
this.getClass().wait();
System.out.println(Thread.currentThread().getName() + ": " + (n++));
this.getClass().notify();
}
} catch (InterruptedException ex) {
ex.printStackTrace();
}
}
}
}
There is a bit of a trick to kick-start the threads properly - thus the need to an extra notify() to start the whole thing (instead of have both processes wait, or required the main Thread to call a notify) and also to handle the possibility that a thread starts, does it's work and calls notify before the second thread has started :)

My initial answer was non-functional. Edited:
package test;
public final class App {
private static volatile int counter = 1;
private static final Object lock = new Object();
public static void main(String... args) {
for (int t = 0; t < 2; ++t) {
final int oddOrEven = t;
new Thread(new Runnable() {
#Override public void run() {
while (counter < 100) {
synchronized (lock) {
if (counter % 2 == oddOrEven) {
System.out.println(counter++);
}
}
}
}
}).start();
}
}
}

Multithreading: How to get a thread to execute more frequently than the others?

I've recently been learning about semaphores to specify the ordering of threads, but I'm curious about how to control the frequency as well. Below is a program that prints *, a digit, and then a letter to the screen. Always in that order (e.g. *1A). How can I make it so certain threads print more than once before the others? (e.g. *32A)
import java.lang.Thread;
import java.util.concurrent.*;
public class ThreadSync {
private static boolean runFlag = true;
private static Semaphore canPrintSymbol = new Semaphore(1);
private static Semaphore canPrintDigit = new Semaphore(0);
private static Semaphore canPrintLetter = new Semaphore(0);
public static void main( String[] args ) {
Runnable[] tasks = new Runnable[17];
Thread[] threads = new Thread[17];
// Create 10-digit threads
for (int d = 0; d < 10; d++) {
tasks[d] = new PrintDigit((char)('0' + d));
threads[d] = new Thread(tasks[d]);
threads[d].start();
}
// Create 6-letter threads
for (int d = 0; d < 6; d++) {
tasks[d + 10] = new PrintLetter((char)('A' + d));
threads[d + 10] = new Thread(tasks[d + 10]);
threads[d + 10].start();
}
// Create a thread to print asterisk
tasks[16] = new PrintSymbol('*');
threads[16] = new Thread(tasks[16]);
threads[16].start();
// Let the threads run for a period of time
try { Thread.sleep(500); }
catch (InterruptedException ex) { ex.printStackTrace(); }
runFlag = false;
// Interrupt the threads
for (int i = 0; i < 17; i++) threads[i].interrupt();
}
public static class PrintSymbol implements Runnable {
private char c;
public PrintSymbol(char c) {
this.c = c;
}
public void run() {
while (runFlag) {
try {
canPrintSymbol.acquire();
}
catch (InterruptedException ex) {
ex.printStackTrace();
}
System.out.printf("%c\n", c);
canPrintDigit.release();
}
}
}
public static class PrintDigit implements Runnable {
private char c;
public PrintDigit(char c) { this.c=c; }
public void run() {
while (runFlag) {
try {
canPrintDigit.acquire();
}
catch (InterruptedException ex) {
ex.printStackTrace();
}
System.out.printf("%c\n", c);
canPrintLetter.release();
}
}
}
public static class PrintLetter implements Runnable {
private char c;
public PrintLetter(char c) {
this.c = c;
}
public void run() {
while (runFlag) {
try {
canPrintLetter.acquire();
}
catch (InterruptedException ex) {
ex.printStackTrace();
}
System.out.printf("%c\n", c);
canPrintSymbol.release();
}
}
}
}

Short answer is you can't. At least not to my knowledge. There are hints you can give to the OS like yielding your thread. This means it yields it processing to the next thread. Other then that all you can really do is influence the priority. But all these are just hints to the OS. The OS ultimately determines the order in which the threads are executed. This is one of the main things to keep in mind when working with multiple threads. It is generally not a good idea to have a dependency between separate threads which makes the order of execution important.

Cyclic Barrier in java

I have a list which needs to be populated by three parties(threads,lets say).I am using cyclic barrier to achieve this functionality. Everything works fine except that I am not able to use the resulted list without inducing a forced sleep. Below is the code :
public class Test{
List<Integer> item = new Vector<Integer>();
public void returnTheList(){
CyclicBarrier cb = new CyclicBarrier(3, new Runnable() {
#Override
public void run() {
System.out.println("All parties are arrived at barrier, lets play -- : " + CyclicBarrierTest.getTheList().size());
//Here I am able to access my resulted list
}
});
CyclicBarrierTest sw1 = new CyclicBarrierTest(cb, new ZetaCode(1500), s);
CyclicBarrierTest sw2 = new CyclicBarrierTest(cb, new ZetaCode(1500),s);
CyclicBarrierTest sw3 = new CyclicBarrierTest(cb, new ZetaCode(1500),s);
Thread th1 = new Thread(sw1, "ZetaCode1");
Thread th2 = new Thread(sw2, "ZetaCode2");
Thread th3 = new Thread(sw3, "ZetaCode3");
th1.start();
th2.start();
th3.start();
}
public static void main(String args[]){
System.out.println("asdfasd");
Test test = new Test();
//ActionClass ac = new ActionClass();
test.returnTheList();
System.out.println("Inside the main method...size of the final list : " +test.item.size() );
}
Below is my CyclicBrrierTest class :
public class CyclicBarrierTest implements Runnable{
private CyclicBarrier barrier;
private Object obj;
static volatile String s = "";
volatile List<Integer> finalIntList = new Vector<Integer>();
public CyclicBarrierTest(CyclicBarrier barrier, Object obj, String s){
this.barrier = barrier;
this.obj = obj;
}
#Override
public void run(){
try{
System.out.println(Thread.currentThread().getName() + " is waiting on barrier and s is now : " + finalIntList.size());
ZetaCode simple = (ZetaCode)obj;
finalIntList.addAll(simple.getTheItemList());
barrier.await();
System.out.println(Thread.currentThread().getName() + " has crossed the barrier");
}catch(InterruptedException ex){
System.out.println("Error.." + ex.getMessage());
}catch(Exception e){
System.out.println("Error.." + e.getMessage());
}
}
public List<Integer> getTheList(){
return finalIntList;
}
So if I run this code without giving any delay the print statement in my main method gives me the length of my list as zero,however after giving an appropriate sleep it gives me the expected output.I want to achieve the same without giving any delay.Any help would be appreciated.
Thanks in advance.

It seems you'd want to use a CountDownLatch, not a CyclicBarrier here. The CyclicBarrier is working exactly as intended - your main method just isn't waiting for it to be tripped by all 3 threads. When you give it a sleep statement, the other 3 threads just happen to finish before main wakes up again.
A CyclicBarrier is useful when you need N workers to all reach the same 'checkpoint' before proceeding, and the workers themselves are the only ones who care. However, you have an N + 1 user here, the main thread, who wants to know when they're all done, and CyclicBarrier doesn't support that use case.
Note, of course that you can also use both of them.

In this code we have 4 tasks . Task1, Task2, Task3 producing int values and Task4 will add all the int values . Task4 is waiting after calling await() for Task1, Task2, Task3 to produce values.When they produce values they call await() method and Task 4 will add their values and print the o/p and call reset() method so the barrier will reset. After reset this process will continue again
package practice;
import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;
public class CyclicbarrierExample {
public static void main(String[] args) {
CyclicBarrier c = new CyclicBarrier(4);
Task1 t1 = new Task1(c);
Task2 t2 = new Task2(c);
Task3 t3 = new Task3(c);
Task4 t4 = new Task4(c);
t1.start();
t2.start();
t3.start();
t4.start();
}
}
class Task1 extends Thread {
CyclicBarrier c;
static int t1 ;
public Task1(CyclicBarrier c) {
this.c = c;
}
#Override
public void run() {
while (true) {
t1 = t1 + 1;
try {
c.await();
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
} catch (BrokenBarrierException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
}
class Task2 extends Thread {
CyclicBarrier c;
static int t2;
public Task2(CyclicBarrier c) {
this.c = c;
}
#Override
public void run() {
while (true) {
t2 = t2 + 1;
try {
c.await();
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
} catch (BrokenBarrierException e) {
e.printStackTrace();
}
}
}
}
class Task3 extends Thread {
CyclicBarrier c;
static int t3;
public Task3(CyclicBarrier c) {
this.c = c;
}
#Override
public void run() {
while (true) {
t3 = t3 + 1;
try {
c.await();
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
} catch (BrokenBarrierException e) {
e.printStackTrace();
}
}
}
}
class Task4 extends Thread {
CyclicBarrier c;
static int t4;
static int count=0;
public Task4(CyclicBarrier c) {
this.c = c;
}
#Override
public void run() {
while (count<10) {
try {
c.await();
} catch (InterruptedException e) {
e.printStackTrace();
} catch (BrokenBarrierException e) {
e.printStackTrace();
}
t4 = Task1.t1 + Task2.t2 + Task3.t3;
System.out.println(t4);
try {
c.reset();
} catch (Exception e) {
System.out.println("yo");
}
count++;
}
}
}

Is my producer/consumer solution correct?

I'm trying to learn more about threads and thought that coming up with a solution to the producer/consumer problem would be a good start. One of the constraints I put on the solution was that the consumer does not know ahead of time how much the producer is producing. The code runs as expected and I've run it many many times, but that doesn't mean that it is free of flaws. Are there any problems with this solution?
package Multithreading.ProducerConsumer;
import java.util.LinkedList;
import java.util.concurrent.Semaphore;
public class ProducerConsumer
{
private class Producer implements Runnable
{
#Override
public void run()
{
for(int i = 0; i < 1000; i++)
{
try
{
canProduce.acquire();
mutex.acquire();
queue.add(i);
mutex.release();
canConsume.release();
}
catch (InterruptedException ex)
{
;
}
}
try
{
canConsume.acquire();
isTryingToFinish = true;
canConsume.release();
}
catch (InterruptedException ex)
{
;
}
}
}
private class Consumer implements Runnable
{
#Override
public void run()
{
while(!isDone)
{
try
{
canConsume.acquire();
mutex.acquire();
System.out.println(queue.pop());
if(isTryingToFinish && queue.isEmpty())
{
isDone = true;
}
mutex.release();
canProduce.release();
}
catch (InterruptedException ex)
{
;
}
}
}
}
Semaphore canProduce;
Semaphore canConsume;
Semaphore mutex;
boolean isTryingToFinish = false;
boolean isDone = false;
final static int bufferSize = 100;
LinkedList<Integer> queue;
public ProducerConsumer()
{
queue = new LinkedList<>();
canProduce = new Semaphore(bufferSize);
canConsume = new Semaphore(0);
mutex = new Semaphore(1);
}
public void Go() throws InterruptedException
{
Thread p = new Thread(new Producer());
Thread c = new Thread(new Consumer());
p.start();
c.start();
p.join();
c.join();
System.out.println("Job Complete!");
}
public static void main(String[] args) throws InterruptedException
{
ProducerConsumer p = new ProducerConsumer();
p.Go();
}
}

You could look at MSDN's 'Example 2: Synchronizing two threads: a producer and a consumer'. It's c# but that should not be a problem.