Develop Reference

How to print number 1 to 31 with three threads in JAVA?

I am trying to print number 1 to 31 In order with Three Threads.
When the number became 31, I want to output "(current Thread) defeated."
Also the number can be output up to 3.
So I want to output like
Thread One print: 1 2
Thread Two print: 3 4 5
Thread Three print: 6 7
Thread One print: 8
......
Thread Three print: 31
Thread Three defeat
and program ends.
public class MyRunnable implements Runnable {
Stack sta = new Stack();
public void run() {
while(sta.getStack()<=31){
int stack = 1;
stack++;
sta.setStack(stack);
System.out.println(Thread.currentThread().getName() + "print: "+ stack);
}
}
}
////////////
public class Stack {
private int stack = 1;
public synchronized void setStack (int stack) {
for(int cnt = 1; cnt<=31; cnt ++) {
try {
Thread.sleep(100);
} catch (InterruptedException e) {
}
System.out.println(Thread.currentThread().getName() + "defeated.");
}
}
public int getStack() {
return this.stack;
}
}
////////////////////
public class ThreadTest {
public static void main(String[] args) {
Runnable threadRun = new MyRunnable();
Thread myThread1 = new Thread(threadRun);
Thread myThread2 = new Thread(threadRun);
Thread myThread3 = new Thread(threadRun);
myThread1.setName("Thread One");
myThread2.setName("Thread Two");
myThread3.setName("Thread Three");
myThread1.start();
myThread2.start();
myThread3.start();
}
}
I made 3 classes.
But it prints only "Thread One output defeat"
I don't know what is problem.
It won't print Thread in order.
I'm not good at using this platform so sorry for looking weird.

I tried a bit and came up with this:
public class MyRunnable implements Runnable {
Stack sta = new Stack();
#Override
public void run() {
while (sta.getStack() <= 31) {
int stack = sta.getStack() + 1;
sta.setStack(stack);
if (stack <= 31) {
System.out.println(Thread.currentThread().getName() + " print: " + stack);
}
}
}
}
public class Stack {
private int stack = 0;
public synchronized void setStack(int stack) {
this.stack = stack;
try {
Thread.sleep(100);
} catch (InterruptedException e) {
}
if (stack > 31) {
System.out.println(Thread.currentThread().getName() + " defeated.");
}
}
public synchronized int getStack() {
return this.stack;
}
}
public class ThreadTest {
public static void main(String[] args) {
Runnable threadRun = new MyRunnable();
Thread myThread1 = new Thread(threadRun);
Thread myThread2 = new Thread(threadRun);
Thread myThread3 = new Thread(threadRun);
myThread1.setName("Thread One");
myThread2.setName("Thread Two");
myThread3.setName("Thread Three");
myThread1.start();
myThread2.start();
myThread3.start();
}
}

How to write below multithreaded program best way

I am new to multithreading, and get to know about the functionality of wait, notify and notifyAll. I want three threads to execute one after another and print alphabets from A to Z.
I have tried below code and it seems working also, but I doubt if this is the best possible way to tackle the problem. Is there any other way, I can make it more simple and better ? It seems some portion of my code is repeating.
package demo.threading;
class Flags {
boolean flagA = true;
boolean flagB = false;
boolean flagC = false;
}
class Container {
Flags flags = new Flags();
int charVal = (int) 'A';
void producer1() {
try {
while (charVal <= (int) 'Z') {
synchronized (this) {
if (!flags.flagA)
wait();
else {
System.out.println(Thread.currentThread().getName() + " Produced : " + (char) charVal);
flags.flagA = false;
flags.flagB = true;
charVal++;
notifyAll();
Thread.sleep(1000);
}
}
}
} catch (InterruptedException ex) {
ex.printStackTrace();
}
}
void producer2() {
try {
while (charVal <= (int) 'Z') {
synchronized (this) {
if (!flags.flagB)
wait();
else {
System.out.println(Thread.currentThread().getName() + " Produced : " + (char) charVal);
flags.flagB = false;
flags.flagC = true;
charVal++;
notifyAll();
Thread.sleep(1000);
}
}
}
} catch (InterruptedException ex) {
ex.printStackTrace();
}
}
void producer3() {
try {
while (charVal <= (int) 'Z') {
synchronized (this) {
if (!flags.flagC)
wait();
else {
System.out.println(Thread.currentThread().getName() + " Produced : " + (char) charVal);
flags.flagC = false;
flags.flagA = true;
charVal++;
notifyAll();
Thread.sleep(1000);
}
}
}
} catch (InterruptedException ex) {
ex.printStackTrace();
}
}
}
public class Main {
public static void main(String[] args) {
Container container = new Container();
Thread t1 = new Thread(() -> container.producer1(), "Thread 1");
Thread t2 = new Thread(() -> container.producer2(), "Thread 2");
Thread t3 = new Thread(() -> container.producer3(), "Thread 3");
t1.start();
t2.start();
t3.start();
}
}
Output should be :
Thread 1 Produced : A
Thread 2 Produced : B
Thread 3 Produced : C
Thread 1 Produced : D
Thread 2 Produced : E
Thread 3 Produced : F

As pointed out before, if you want to do this "one after another", you actually don't need multiple threads. However, you can achieve this by using a Semaphore:
int numberOfThreads = 3;
Semaphore semaphore = new Semaphore(1);
for (int i = 1; i <= numberOfThreads; i++) {
new Thread(() -> {
try {
semaphore.acquire();
for (char c : "ABCDEFGHIJKLMNOPQRSTUVWXYZ".toCharArray()) {
System.out.println(Thread.currentThread().getName()
+ " produced: " + c + ".");
}
} catch (InterruptedException e) {
// NOP
} finally {
semaphore.release();
}
}, "Thread " + i).start();
}
I recommend exploring java.util.concurrent which is available since Java 5. It's a great help to keep your concurrent code concise and simple compared with Java's low-level concurrency primitives such as wait and notify. If you're really interested in that topic, Brian Goetz's "Java Concurrency in Practice" is a must-read.
EDIT:
public class ConcurrentAlphabet {
private volatile Thread current;
public static void main(String[] args) {
new ConcurrentAlphabet().print(3,
"ABCDEFGHIJKLMNOPQRSTUVWXYZ".toCharArray());
}
public void print(int numberOfThreads, char[] alphabet) {
Thread[] threads = new Thread[numberOfThreads];
for (int i = 1; i <= numberOfThreads; i++) {
int offset = i - 1;
threads[offset] = new Thread(() -> {
Thread me = Thread.currentThread();
Thread next = threads[(offset + 1) % numberOfThreads];
for (int index = offset; index < alphabet.length; index += numberOfThreads) {
synchronized (this) {
while (me != current) {
try {
wait();
} catch (InterruptedException e) { /* NOP */ }
}
System.out.println(me.getName() + " produced: " + alphabet[index] + ".");
current = next;
notifyAll();
}
}
}, "Thread " + i);
}
current = threads[0];
for (Thread t : threads) {
t.start();
}
}
}

package demo.thread;
public class ABCPuzzle {
private static class RunnableImpl implements Runnable {
private String nextThread;
private ExecServ execServ;
public RunnableImpl(ExecServ execServ, String nextThread) {
this.execServ = execServ;
this.nextThread = nextThread;
}
#Override
public void run() {
String threadName = Thread.currentThread().getName();
synchronized (execServ) {
try {
while (true) {
if (execServ.key > 'Z')
break;
if (threadName.equals(execServ.current)) {
System.out.println(threadName + " consuming " + execServ.key);
Thread.sleep(1000);
execServ.key++;
execServ.current = nextThread;
execServ.notifyAll();
} else
execServ.wait();
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
private static class ExecServ {
private String current, next;
private char key = 'A';
}
public static void main(String[] args) {
ExecServ execServ = new ExecServ();
execServ.current = "t1";
Thread t1 = new Thread(new RunnableImpl(execServ, "t2"), "t1");
Thread t2 = new Thread(new RunnableImpl(execServ, "t3"), "t2");
Thread t3 = new Thread(new RunnableImpl(execServ, "t4"), "t3");
Thread t4 = new Thread(new RunnableImpl(execServ, "t1"), "t4");
t1.start();
t2.start();
t3.start();
t4.start();
}
}
Output :
t1 consuming A
t2 consuming B
t3 consuming C
t4 consuming D
t1 consuming E
t2 consuming F
t3 consuming G
t4 consuming H
t1 consuming I
t2 consuming J

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();
}
}
}

Producer Consumer using threads

I’m writing a program that implements the Producer Consumer problem in Java using multithreading concepts. Below are few details how I’m supposed to do it:
1) The main thread should create a buffer with capacity specified as a command line argument. The number of producer and consumer threads are also specified as command line arguments. I’m supposed to assign a unique number to each producer and consumer thread. How do I assign a unique number to producer and consumer threads?
2) The producer thread operates in an infinite loop. It produces a data item (a string) with the following format: <producer number>_<data item number>. For example the 1st data item from thread number 1 will be 1_1 and second data item from thread number 3 will be 3_2. How do create data items in such a format?
3) Then the Producer thread writes an entry into the producer log file (< producer number > “Generated” <data item>). Upon writing the log entry, it attempts to insert into the buffer. If insertion is successful, it creates an entry into the log file (<producer number> <data item> “Insertion successful”). How do I write such a code?
Below is the Java code I wrote.
import java.util.*;
import java.util.logging.*;
public class PC2
{
public static void main(String args[])
{
ArrayList<Integer> queue = new ArrayList<Integer>();
int size = Integer.parseInt(args[2]);
Thread[] prod = new Thread[Integer.parseInt(args[0])];
Thread[] cons = new Thread[Integer.parseInt(args[1])];
for(int i=0; i<prod.length; i++)
{
prod[i] = new Thread(new Producer(queue, size));
prod[i].start();
}
for(int i=0; i<cons.length; i++)
{
cons[i] = new Thread(new Consumer(queue, size));
cons[i].start();
}
}
}
class Producer extends Thread
{
private final ArrayList<Integer> queue;
private final int size;
public Producer(ArrayList<Integer> queue, int size)
{
this.queue = queue;
this.size = size;
}
public void run()
{
while(true){
for(int i=0; i<size; i++)
{
System.out.println("Produced: "+i+" by id " +Thread.currentThread().getId());
try
{
produce(i);
Thread.sleep(3000);
}
catch(Exception e)
{
Logger.getLogger(Producer.class.getName()).log(Level.SEVERE, null, e);
}
}}
}
public void produce(int i) throws InterruptedException
{
while(queue.size() == size)
{
synchronized(queue)
{
System.out.println("Queue is full "+Thread.currentThread().getName() +" is waiting, size: "+queue.size());
queue.wait();
}
}
synchronized(queue)
{
queue.add(i);
queue.notifyAll();
}
}
}
class Consumer extends Thread
{
private final ArrayList<Integer> queue;
private final int size;
public Consumer(ArrayList<Integer> queue, int size)
{
this.queue = queue;
this.size = size;
}
public void run()
{
while(true)
{
try
{ System.out.println("Consumed: "+consume());
Thread.sleep(1000);
}
catch(Exception e)
{
Logger.getLogger(Consumer.class.getName()).log(Level.SEVERE, null, e);
}
}
}
public int consume() throws InterruptedException
{
while(queue.isEmpty())
{
synchronized(queue)
{
System.out.println("Queue is empty "+Thread.currentThread().getName()+" is waiting, size: "+queue.size());
queue.wait();
}
}
synchronized (queue)
{
queue.notifyAll();
System.out.println("Consumed by id "+Thread.currentThread().getId());
return (Integer) queue.remove(0);
}
}
}
How can I carry out the above steps?

I’m supposed to assign a unique number to each producer and consumer
thread. How do I assign a unique number to producer and consumer
threads?
Add an instance (non-static) variable to the Producer/Consumer classes. When you initialize the new Producer/Consumer Objects, pass in the unique number. You can keep track of what number you're on with an int counter in your main class.
2) The producer thread operates in an infinite loop. It produces a
data item (a string) with the following format: < producer number >_<
data item number > . For example the 1st data item from thread number
1 will be 1_1 and second data item from thread number 3 will be 3_2.
How do create data items in such a format?
Use synchronized methods and/or atomic variables. Look into Java Concurrency.
3) Then the Producer thread writes an entry into the producer log file
(< producer number > “Generated” < data item >). Upon writing the log
entry, it attempts to insert into the buffer. If insertion is
successful, it creates an entry into the log file (< producer number >
< data item > “Insertion successful”). How do I write such a code?
My answer is the same as the previous question: read about Java concurrency. Spend an hour reading about synchronization, locks, and atomic variables and I guarantee you will easily write your program.

For producer consumer problem best solution is BlockingQueue. I was testing a few things so designed same kind of program now modified it as per your need.
See if it helps.
import java.util.concurrent.*;
public class ThreadingExample {
public static void main(String args[]){
BlockingQueue<Message> blockingQueue = new ArrayBlockingQueue<Message>(100);
ExecutorService exec = Executors.newCachedThreadPool();
exec.execute(new Producer(blockingQueue));
exec.execute(new Consumer(blockingQueue));
}
}
class Message{
private static int count=0;
int messageId;
Message(){
this.messageId=count++;
System.out.print("message Id"+messageId+" Created ");
}
}
class Producer implements Runnable{
private BlockingQueue<Message> blockingQueue;
Producer(BlockingQueue<Message> blockingQueue){
this.blockingQueue=blockingQueue;
}
#Override
public void run(){
while(!Thread.interrupted()){
System.out.print("Producer Started");
try {
blockingQueue.put(new Message());
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("Producer Done");
}
}
}
class Consumer implements Runnable{
private BlockingQueue<Message> blockingQueue;
Consumer(BlockingQueue<Message> blockingQueue){
this.blockingQueue=blockingQueue;
}
#Override
public void run(){
while(!Thread.interrupted()){
System.out.print("Concumer Started");
try{
Message message = blockingQueue.take();
System.out.print("message Id"+message.messageId+" Consumed ");
}
catch(InterruptedException e){
e.printStackTrace();
}
System.out.println("Concumer Done");
}
}
}

I tried the following which might work for you, except for the buffer condition on 3, which you can add the part of the code by yourself.
Hope this helps.
public class Message {
private String msg;
public Message(String msg) {
super();
this.msg = msg;
}
public String getMsg(){
return msg;
}
}
import java.util.concurrent.BlockingQueue;
public class Producer implements Runnable {
private BlockingQueue<Message> queue;
private boolean run = true;
public Producer(BlockingQueue<Message> queue) {
super();
this.queue = queue;
}
public void setRun(boolean val) {
this.run = val;
}
#Override
public void run() {
int i = 0;
while (run) {
Message msg = new Message(Thread.currentThread().getName() + "_"+ i);
try {
Thread.sleep(i * 100);
queue.put(msg);
System.out.println("Producer: "+Thread.currentThread().getName()+" produced and added to the queue: "+msg.getMsg());
} catch (InterruptedException e) {
e.printStackTrace();
}
i++;
if(i==10){
setRun(false);
System.out.println(Thread.currentThread().getName()+" stopped");
}
}
}
}
import java.util.concurrent.BlockingQueue;
public class Consumer implements Runnable{
private BlockingQueue<Message> queue;
private boolean run = true;
public Consumer(BlockingQueue<Message> queue) {
super();
this.queue = queue;
}
public void setRun(boolean val){
this.run = val;
}
#Override
public void run() {
while(run){
try {
Thread.sleep(100);
Message msg = queue.take();
System.out.println("Consumer: "+Thread.currentThread().getName()+" generated/consumed "+msg.getMsg());
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
}
import java.util.Scanner;
import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.BlockingQueue;
public class ProducerConsumerMain {
public static void main(String[] args) {
System.out
.println("please enter the number of producer:consumer:size of the queue in order");
Scanner scan = new Scanner(System.in);
Thread[] prodThreads = new Thread[scan.nextInt()];
Thread[] consThreads = new Thread[scan.nextInt()];
BlockingQueue<Message> queue = new ArrayBlockingQueue<Message>(scan.nextInt());
for (int i = 0; i < prodThreads.length; i++) {
prodThreads[i] = new Thread(new Producer(queue), "" + i);
prodThreads[i].start();
}
for (int i = 0; i < consThreads.length; i++) {
consThreads[i] = new Thread(new Consumer(queue), "" + i);
consThreads[i].start();
}
}
}

Please refer the below code. You can change the constant values based on the command line arguments. I have tested the code, its working as per your requirement.
import java.util.LinkedList;
import java.util.Queue;
public class ProducerConsumerProblem {
public static int CAPACITY = 10; // At a time maximum of 10 tasks can be
// produced.
public static int PRODUCERS = 2;
public static int CONSUMERS = 4;
public static void main(String args[]) {
Queue<String> mTasks = new LinkedList<String>();
for (int i = 1; i <= PRODUCERS; i++) {
Thread producer = new Thread(new Producer(mTasks));
producer.setName("Producer " + i);
producer.start();
}
for (int i = 1; i <= CONSUMERS; i++) {
Thread consumer = new Thread(new Consumer(mTasks));
consumer.setName("Consumer " + i);
consumer.start();
}
}
}
class Producer implements Runnable {
Queue<String> mSharedTasks;
int taskCount = 1;
public Producer(Queue<String> mSharedTasks) {
super();
this.mSharedTasks = mSharedTasks;
}
#Override
public void run() {
while (true) {
synchronized (mSharedTasks) {
try {
if (mSharedTasks.size() == ProducerConsumerProblem.CAPACITY) {
System.out.println("Producer Waiting!!");
mSharedTasks.wait();
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
while (mSharedTasks.size() != ProducerConsumerProblem.CAPACITY) {
try {
Thread.sleep(50);
} catch (InterruptedException e) {
}
String produceHere = Thread.currentThread().getName()
+ "_Item number_" + taskCount++;
synchronized (mSharedTasks) {
mSharedTasks.add(produceHere);
System.out.println(produceHere);
if (mSharedTasks.size() == 1) {
mSharedTasks.notifyAll(); // Informs consumer that there
// is something to consume.
}
}
}
}
}
}
class Consumer implements Runnable {
Queue<String> mSharedTasks;
public Consumer(Queue<String> mSharedTasks) {
super();
this.mSharedTasks = mSharedTasks;
}
#Override
public void run() {
while (true) {
synchronized (mSharedTasks) {
if (mSharedTasks.isEmpty()) { // Checks whether there is no task
// to consume.
try {
mSharedTasks.wait(); // Waits for producer to produce!
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
while (!mSharedTasks.isEmpty()) { // Consumes till task list is
// empty
try {
// Consumer consumes late hence producer has to wait...!
Thread.sleep(100);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
synchronized (mSharedTasks) {
System.out.println(Thread.currentThread().getName()
+ " consumed " + mSharedTasks.poll());
if (mSharedTasks.size() == ProducerConsumerProblem.CAPACITY - 1)
mSharedTasks.notifyAll();
}
}
}
}
}

public class ProducerConsumerTest {
public static void main(String[] args) {
CubbyHole c = new CubbyHole();
Producer p1 = new Producer(c, 1);
Consumer c1 = new Consumer(c, 1);
p1.start();
c1.start();
}
}
class CubbyHole {
private int contents;
private boolean available = false;
public synchronized int get() {
while (available == false) {
try {
wait();
} catch (InterruptedException e) {
}
}
available = false;
notifyAll();
return contents;
}
public synchronized void put(int value) {
while (available == true) {
try {
wait();
} catch (InterruptedException e) {
}
}
contents = value;
available = true;
notifyAll();
}
}
class Consumer extends Thread {
private CubbyHole cubbyhole;
private int number;
public Consumer(CubbyHole c, int number) {
cubbyhole = c;
this.number = number;
}
public void run() {
int value = 0;
for (int i = 0; i < 10; i++) {
value = cubbyhole.get();
System.out.println("Consumer #"
+ this.number
+ " got: " + value);
}
}
}
class Producer extends Thread {
private CubbyHole cubbyhole;
private int number;
public Producer(CubbyHole c, int number) {
cubbyhole = c;
this.number = number;
}
public void run() {
for (int i = 0; i < 10; i++) {
cubbyhole.put(i);
System.out.println("Producer #" + this.number
+ " put: " + i);
try {
sleep((int) (Math.random() * 100));
} catch (InterruptedException e) {
}
}
}
}

Java - does not loop in multithreaded program

I am still a java newbie and trying to play around learning threads. My question is that it does not loop 5 times. It runs one time and exits. I am using a.class to lock on the class object, such that both the threads are locking on the same object monitor.
class a implements Runnable {
Thread thr;
int count;
String time;
a(String s) {
thr = new Thread(this, s);
thr.start();
}
public void run() {
count++;
if (Thread.currentThread().getName().compareTo("one") == 0) {
synchronized (a.class) {
try {
for (int i = 0; i < 5; i++) {
System.out.println("Now running thread " + Thread.currentThread().getName() + " with count " + count);
time = "Tick";
System.out.println(time);
notify();
while (time == "Tock") {
wait();
}
}
} catch (Exception e) {
}
}
} else if (Thread.currentThread().getName().compareTo("two") == 0) {
synchronized (a.class) {
try {
for (int j = 0; j < 5; j++) {
System.out.println("Now running thread " + Thread.currentThread().getName() + " with count " + count);
time = "Tock";
System.out.println(time);
notify();
while (time == "Tick") {
wait();
}
}
} catch (Exception e) {
}
}
}
}
}
public class b {
public static void main(String args[]) {
a obj1 = new a("one");
a obj2 = new a("two");
}
}

Here you go, with the original code:
class a implements Runnable {
Thread thr;
int count;
static String time = "Tock";
a(String s) {
thr = new Thread(this, s);
thr.start();
}
public void run() {
count++;
if (Thread.currentThread().getName().compareTo("one") == 0) {
synchronized (a.class) {
try {
for (int i = 0; i < 5; i++) {
while (time.equals("Tock")) {
a.class.wait();
}
System.out.println("Now running thread "
+ Thread.currentThread().getName()
+ " with count " + count);
time = "Tock";
System.out.println(time);
a.class.notify();
}
} catch (Exception e) {
e.printStackTrace();
}
}
} else if (Thread.currentThread().getName().compareTo("two") == 0) {
synchronized (a.class) {
try {
for (int j = 0; j < 5; j++) {
while (time.equals("Tick")) {
a.class.wait();
}
System.out.println("Now running thread "
+ Thread.currentThread().getName()
+ " with count " + count);
time = "Tick";
System.out.println(time);
a.class.notify();
}
} catch (Exception e) {
e.printStackTrace();
}
}
}
}
}
public class Test {
public static void main(String args[]) {
a obj1 = new a("one");
a obj2 = new a("two");
}
}
The problem was that you were calling wait and notify on the implicit this object, when the lock was being held on the a.class object, hence you must call wait/notify on a.class. That was it.
I also did a small restructuring, since I assume you wanted them to print Tick and Tock in an alternating sequence, right?

The answer to why you only loop once is that you call notify() on an object that is not locked and thus an IllegalMonitorStateException is thrown and caught by the empty catch statement.
This is one way to do it. Not saying that it is the best. I tried to keep it close to your code:
public class TickTock {
static final int N = 4;
Object lock = new Object();
int token;
class Worker extends Thread {
int id;
Worker(int id) {
this.id = id;
}
#Override
public void run() {
try {
synchronized (lock) {
for (int i = 0; i < 5; i++) {
while (id != token%N) lock.wait();
System.out.println(id + " " + i);
token++;
lock.notifyAll();
}
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
void start() {
for (int i = 0; i < N; i++) {
new Worker(i).start();
}
}
public static void main(String[] args) {
new TickTock().start();
}
}

When comparing strings (and objects in general), you should use equals as opposed to == (which is generally reserved for primitives): while(time.equals("Tock")). == on strings will often times result in false when you want it to (and think it should) return true, and hence your loop will exit before expected.