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I want to print the fibonacci series using two threads. Like 1st number should be printed by 1st thread and then 2nd number by 2nd thread and so on

I want fibonacci series to be printed by threads and the 1st number of the series should be printed by 1st thread then 2nd number by 2nd thread then 3rd by 1st thread and 4th by 2nd and so on.
I tried this code by using arrays like printing the array elements using thread but I am not able to switch between the threads.
class Fibonacci{
void printFibonacci() {
int fibArray[] = new int[10];
int a = 0;
int b = 1;
fibArray[0] = a;
fibArray[1] = b;
int c;
for(int i=2;i<10;i++) {
c = a+b;
fibArray[i] = c;
a = b;
b = c;
}
for(int i=0;i<10;i++) {
if(Integer.parseInt(Thread.currentThread().getName())%2==0 && (i%2==0))
{
System.out.println("Thread " +Thread.currentThread().getName()+" "+fibArray[i]);
try{
wait();
}catch(Exception e) {}
}
else if(Integer.parseInt(Thread.currentThread().getName())%2!=0 && (i%2!=0))
{
System.out.println("Thread " +Thread.currentThread().getName()+" "+fibArray[i]);
}
}
}
}
public class FibonacciUsingThread {
public static void main(String[] args) throws Exception {
Fibonacci f = new Fibonacci();
Thread t1 = new Thread(()->
{
f.printFibonacci();
});
Thread t2 = new Thread(()->
{
f.printFibonacci();
});
t1.setName("0");
t2.setName("1");
t1.start();
t1.join();
t2.start();
}
}

The following line in your code is causing t1 to finish before t2 can start.
t1.join();
Apart from this, you need to synchronize on the method, printFibonacci.
You can do it as follows:
class Fibonacci {
synchronized void printFibonacci() throws InterruptedException {
int fibArray[] = new int[10];
int a = 0;
int b = 1;
fibArray[0] = a;
fibArray[1] = b;
int c;
for (int i = 2; i < 10; i++) {
c = a + b;
fibArray[i] = c;
a = b;
b = c;
}
for (int i = 0; i < 10; i++) {
String currentThreadName = Thread.currentThread().getName();
if (currentThreadName.equals("1")) {
if (i % 2 == 0) {
System.out.println("Thread " + Thread.currentThread().getName() + " " + fibArray[i]);
notify();
} else {
wait();
}
} else if (currentThreadName.equals("0")) {
if (i % 2 == 1) {
System.out.println("Thread " + Thread.currentThread().getName() + " " + fibArray[i]);
notify();
} else {
wait();
}
}
}
}
}
public class Main {
public static void main(String[] args) {
Fibonacci f = new Fibonacci();
Thread t1 = new Thread(() -> {
try {
f.printFibonacci();
} catch (InterruptedException e) {
e.printStackTrace();
}
});
Thread t2 = new Thread(() -> {
try {
f.printFibonacci();
} catch (InterruptedException e) {
e.printStackTrace();
}
});
t1.setName("0");
t2.setName("1");
t1.start();
t2.start();
}
}
Output:
Thread 1 0
Thread 0 1
Thread 1 1
Thread 0 2
Thread 1 3
Thread 0 5
Thread 1 8
Thread 0 13
Thread 1 21
Thread 0 34

Apart from all being said and already answered, I would just like to add one alternative approach to Fibonacci sequence implemetation, without arrays and in-advance dimensioning:
public class Fibonacci {
private int index = -1;
private int previous = 0;
private int last = 1;
synchronized public int getNext() {
index++;
if( index == 0 ) return previous;
if( index == 1 ) return last;
int next = last + previous;
if( next < 0 ) throw new ArithmeticException( "integer overflow" );
previous = last;
last = next;
return next;
}
}
Limited only by overflow of numeric data type, in this case integer.

As "#Live and Let Live" pointed out, correctness-wise the main issues with your code is the missing synchronized clause and calling join of the first thread before starting the second thread.
IMO you could clean the code a bit by first separating a bite the concerns, namely, the class Fibonacci would only responsible for calculation the Fibonacci of a given array:
class Fibonacci{
void getFibonacci(int[] fibArray) {
int a = 0;
int b = 1;
fibArray[0] = a;
fibArray[1] = b;
int c;
for(int i=2;i<fibArray.length;i++) {
c = a+b;
fibArray[i] = c;
a = b;
b = c;
}
}
}
In this way, you keep your Fibonacci class concise without any thread-related code. Moreover, the getFibonacci is now more abstract; you can calculate the fib of more than just 10 elements like you had before.
Then on the class FibonacciUsingThread:
public class FibonacciUsingThread {
public static void main(String[] args) throws Exception {
int [] array_fib = new int[10];
Fibonacci f = new Fibonacci();
f.getFibonacci(array_fib);
Thread t1 = new Thread(()->
{
for(int i = 0; i < array_fib.length; i+=2)
System.out.println("Thread 1:" + array_fib[i]);
});
Thread t2 = new Thread(()->
{
for(int i = 1; i < array_fib.length; i+=2)
System.out.println("Thread 2:" + array_fib[i]);
});
t1.start();
t2.start();
t1.join();
t2.join();
}
}
First, you calculate the Fibonaccis using the main thread, there is no point in having all the threads calculate the same thing. Afterward, you specified that Thread 1 and Thread 2 will print the even and odd positions, respectively.
Unless this is just an exercise to play around with threads and synchronization there is not much sense in using threads to do this kind of work. In your code, the part worth parallelizing is the calculation of the Fibonacci numbers themselves, not the printing part.
The code previously shown will not print the Fibonacci numbers in order, for that you need to ensure that the threads wait for one another after iterating through each element of the array. Hence, you need to adapt the code that will be executed by the threads, namely:
Thread t1 = new Thread(()->
{
synchronized (array_fib){
for(int i = 0; i < array_fib.length; i++)
if(i % 2 == 0) {
System.out.println("Thread 1:" + array_fib[i]);
try {
array_fib.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
else
array_fib.notify();
}
});
Thread t2 = new Thread(()->
{
synchronized (array_fib){
for(int i = 0; i < array_fib.length; i++)
if(i % 2 != 0) {
System.out.println("Thread 2:" + array_fib[i]);
try {
array_fib.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
else
array_fib.notify();
}
});
We can remove the code redundancy by extracting a method with the work that will be assigned to the Threads. For instance:
private static void printFib(String threadName, int[] array_fib, Predicate<Integer> predicate) {
for (int i = 0; i < array_fib.length; i++)
if (predicate.test(i)) {
System.out.println(threadName + " : " + array_fib[i]);
try {
array_fib.wait();
} catch (InterruptedException e) {
// do something about it
}
} else
array_fib.notify();
}
and the main code:
public static void main(String[] args) throws Exception{
int [] array_fib = new int[10];
Fibonacci f = new Fibonacci();
f.getFibonacci(array_fib);
Thread t1 = new Thread(()-> {
synchronized (array_fib){
printFib("Thread 1:", array_fib, i1 -> i1 % 2 == 0);
}
});
Thread t2 = new Thread(()-> {
synchronized (array_fib){
printFib("Thread 2:", array_fib, i1 -> i1 % 2 != 0);
}
});
t1.start();
t2.start();
t1.join();
t2.join();
}

As an alternative, you can use a fair Semaphore to alternate between threads, and an AtomicReference to keep the shared status. Here's an example:
import java.util.concurrent.Semaphore;
import java.util.concurrent.atomic.AtomicReference;
public class FibonacciConcurrent {
public static void main(String[] args) throws InterruptedException {
// needs to be fair to alternate between threads
Semaphore semaphore = new Semaphore(1, true);
// set previous to 1 so that 2nd fibonacci number is correctly calculated to be 0+1=1
Status initialStatus = new Status(1, 0, 1);
AtomicReference<Status> statusRef = new AtomicReference<>(initialStatus);
Fibonacci fibonacci = new Fibonacci(20, semaphore, statusRef);
Thread thread1 = new Thread(fibonacci);
Thread thread2 = new Thread(fibonacci);
thread1.start();
thread2.start();
thread1.join();
thread2.join();
}
private static final class Status {
private final long previous;
private final long current;
private final int currentIndex;
private Status(long previous, long current, int currentIndex) {
this.previous = previous;
this.current = current;
this.currentIndex = currentIndex;
}
}
private static final class Fibonacci implements Runnable {
private final int target;
private final Semaphore semaphore;
private final AtomicReference<Status> statusRef;
private Fibonacci(int target, Semaphore semaphore, AtomicReference<Status> statusRef) {
this.target = target;
this.semaphore = semaphore;
this.statusRef = statusRef;
}
#Override
public void run() {
try {
process();
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
throw new RuntimeException("Interrupted", e);
}
}
private void process() throws InterruptedException {
while (!Thread.currentThread().isInterrupted()) {
try {
semaphore.acquire();
Status status = statusRef.get();
String threadName = Thread.currentThread().getName();
if (status.currentIndex > target) return;
System.out.println(
threadName + ": fibonacci number #" + status.currentIndex + " - " + status.current);
long next = status.previous + status.current;
Status newStatus = new Status(status.current, next, status.currentIndex + 1);
statusRef.set(newStatus);
} finally {
semaphore.release();
}
}
}
}
}
Will print:
Thread-0: fibonacci number #1 - 0
Thread-1: fibonacci number #2 - 1
Thread-0: fibonacci number #3 - 1
Thread-1: fibonacci number #4 - 2
Thread-0: fibonacci number #5 - 3
Note that this solution does not only print on the threads - it does the actual calculation on the threads as well - e.g. when it's Thread A's turn, it uses the previous status that was calculated by Thread B to calculate the next fibonacci number.

how to give priority to the threads waiting in a semaphore?

I have used a semaphore to restrict the number of threads accessing a function. I want the thread to be awakened next should be chosen by some priority which i will be giving,not by default way that semaphore awaken's them ? How can we achieve this ?
Here is the implementation :
class MyMathUtil2 implements Runnable {
double a;
double b;
String name = "demo";
Thread t;
//static int currentCount = 0;
static int MAX_COUNT = 2;
private final Semaphore available = new Semaphore(MAX_COUNT, true);
MyMathUtil2(double v1, double v2) {
a = v1;
b = v2;
t = new Thread(this, name);
System.out.println("New thread: " + t);
t.start();
}
public void InternalPow(double a, double b) throws InterruptedException {
available.acquire();
try {
System.out.println("Power of " + a + " and " + b + " : " + Math.pow(a, b));
} finally {
available.release();
}
}
public void run() {
try {
InternalPow(a, b);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
class TestMyMathUtil2 {
public static void main(String args[]) {
new MyMathUtil2(10.2, 8);
new MyMathUtil2(11, 56);
new MyMathUtil2(10.2, 9);
new MyMathUtil2(2, 3);
new MyMathUtil2(4, 5);
}
}

Well, a Semaphore does not support priority.
I suggest to use a ThreadPoolExecutor with 2 fixed threads and a PriorityBlockingQueue to solve this problem.
A ThreadPoolExecutor with 2 fixed threads can make sure that at any moment, there is at most 2 task running. The other tasks will be put in this PriorityBlockingQueue, and the thread pool will retrieve tasks from the queue based on a custom Comparator.
Here is an example. Every Runnable in this example is supposed to print a number. It submits the Runnables in reverse order: 1000, 999, ..., 1.
But the Runnable will be executed in nature order: 1, 2, ...., 1000 using a priority queue.
import java.util.Comparator;
import java.util.concurrent.PriorityBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
class ComparableRunnable implements Runnable {
public int index;
ComparableRunnable(int index) {
this.index = index;
}
public void run() {
System.out.println(Thread.currentThread().getName() + "-index : " + index);
try {
// sleep current thread, so the other thread can print
// this is not mandatory, without this, the result might not follow strict natural order
// for example, thread1 print 1,
// thread2 take 2 but did not print it immediatly,
// thread1 print 3,
// thread2 print 2
// the result will be 1, 3, 2,
Thread.sleep(10);
} catch (Exception e) {
}
}
public static void main(String[] args) {
int corePoolSize = 2; // fixed thread number
long ignore = 0L;
// comparator
Comparator<Runnable> comparator = new Comparator<Runnable>() {
#Override
public int compare(Runnable o1, Runnable o2) {
int index1 = ((ComparableRunnable)o1).index;
int index2 = ((ComparableRunnable)o2).index;
// you should implement this method based on your own order
return Integer.compare(index1, index2);
}
};
// use the comparator create a priority queue
PriorityBlockingQueue<Runnable> queue = new PriorityBlockingQueue<>(10, comparator);
ThreadPoolExecutor executor =
new ThreadPoolExecutor(corePoolSize, corePoolSize, ignore, TimeUnit.SECONDS, queue);
// Warm the thread pool up
// this is not mandatory, without this, it will print 1000, 999, 1, 2, ...., 998
// because the first two Runnbale will be executed once they are submitted
for (int i = 0; i < corePoolSize; i++) {
executor.execute(() -> {
try {
Thread.sleep(1000);
} catch (Exception e) {
}
});
}
// submit in 1000, 999, ..., 1 order
// print in 1, 2, 3, ..., 1000 order
for (int i = 1000; i > 0; i--) {
executor.execute(new ComparableRunnable(i));
}
}
}
Result:
pool-1-thread-1-index : 1
pool-1-thread-2-index : 2
pool-1-thread-1-index : 3
pool-1-thread-2-index : 4
pool-1-thread-2-index : 5
...
pool-1-thread-2-index : 996
pool-1-thread-2-index : 997
pool-1-thread-1-index : 998
pool-1-thread-2-index : 999
pool-1-thread-1-index : 1000

implementing synchronization to get the proper threads output properly

Currently when I run the program, the threads are running at random. For example the current output is :
Global.sharedBuffer[0] = 2
Global.sharedBuffer[1] = 1
Global.sharedBuffer[2] = 1
Global.sharedBuffer[3] = 1
Global.sharedBuffer[4] = 1
Global.sharedBuffer[5] = 1
Global.sharedBuffer[6] = 1
Global.sharedBuffer[7] = 1
Global.sharedBuffer[8] = 1
Global.sharedBuffer[9] = 1
Global.sharedBuffer[10] = 2
Global.sharedBuffer[11] = 4
Global.sharedBuffer[12] = 3
What I want is from sharedBuffer 0 to 9 all 1's , then from 10 - 19 all 2's and etc. I added a synchronization block thinking it would do this, however, it just stopped it from context switching every time a thread was called. How do you go about implementing this?
CODE:
import java.io.*;
import java.lang.*;
import java.util.*;
class MyThreadExample2 {
public static void main(String[] args) {
HelloThread ht1 = new HelloThread(1);
HelloThread ht2 = new HelloThread(2);
HelloThread ht3 = new HelloThread(3);
HelloThread ht4 = new HelloThread(4);
ht1.start();
ht2.start();
ht3.start();
ht4.start();
}
}
class Global {
public static int[] sharedBuffer = new int[1000];
public static int in = 0;
}
class HelloThread extends Thread {
int threadID;
HelloThread(int threadID) {
System.out.println("threadID: " + threadID);
this.threadID = threadID;
}
public synchronized void run() {
for (int i = 0; i < 100; i++) {
if((Global.in >= 0 || Global.in <= 99) && (this.threadID == 1))
Global.sharedBuffer[Global.in] = this.threadID;
else if((Global.in >= 100 || Global.in <= 199) && (this.threadID == 2))
Global.sharedBuffer[Global.in] = this.threadID;
else if((Global.in >= 200 || Global.in <= 299) && (this.threadID == 3))
Global.sharedBuffer[Global.in] = this.threadID;
else if((Global.in >= 300 || Global.in <= 399) && (this.threadID == 4))
Global.sharedBuffer[Global.in] = this.threadID;
System.out.println("Thread " + this.threadID + " has written "
+ this.threadID + " to Global.sharedBuffer[" + Global.in + "]\n");
Global.in++;
}
if (this.threadID == 4)
{
try {this.sleep(2000L);
}
catch (Throwable e) {e.printStackTrace();
}
System.out.println("The final buffer is **************\n");
for (int i = 0; i < 30; i++) {
System.out.println("Global.sharedBuffer[" + i + "] = " +
Global.sharedBuffer[i]);
} // for
} // if
} // run
} // end Thread

Multi-threading only works if you can formulate tasks which can be performed independently of others. You have to avoid shared variables and if you can’t avoid it, the access must be properly guarded, which usually implies limiting the concurrency of the thread execution.
For your task it is simple to formulate independent tasks as each thread shall write into a different region of the array:
public class ThreadingExample {
public static void main(String[] args) {
final int numThread=4, chunkSize=10;
int[] array=new int[numThread*chunkSize];
Thread[] thread=new Thread[numThread];
// create threads and define their jobs
for(int t=0, p=0; t<numThread; t++, p+=chunkSize) {
thread[t]=new Thread(new FillInJob(array, t, p, chunkSize));
}
// start the threads
for(Thread t: thread) t.start();
// now all running concurrently
// wait for completion
try {
for(Thread t: thread) t.join();
} catch(InterruptedException ex) {
throw new AssertionError(ex);
}
// use result
System.out.println(java.util.Arrays.toString(array));
}
}
class FillInJob implements Runnable {
private final int[] targetArray;
private final int myID, startIndex, endIndex;
FillInJob(int[] target, int id, int start, int size) {
targetArray=target;
myID=id;
startIndex=start;
endIndex=start+size;
}
public void run() {
for(int ix=startIndex; ix<endIndex; ix++)
targetArray[ix]=myID;
}
}

How to create a cyclic exchange of three threads?

How to create a cyclic exchange of three threads? That is: first thread must send data to second, second to third and third thread must send data to first.
I wrote some code, but threads exchange in random oder.
class DataClass {
int value;
String message;
DataClass(int v, String s) {
value = v;
message = s;
}
int getValue() {
return (value);
}
String getMassage() {
return (message);
}
}
class Loop implements Runnable {
int counter;
String name;
Exchanger<DataClass> exchanger;
Loop(int startValue, String id, Exchanger<DataClass> ex) {
counter = startValue;
name = id;
exchanger = ex;
System.out.println(name + ": created");
}
public void run() {
System.out.println(name + ": started");
DataClass data = new DataClass(counter, name);
for (int i = 0; i < 3; ++i) {
try {
DataClass newData = exchanger.exchange(data);
counter += newData.getValue();
System.out.println(name + ": from "
+ newData.getMassage() + ": data: "
+ newData.getValue() + ": state = " + counter);
} catch (InterruptedException e) {
System.err.println(e.toString());
}
}
System.out.println(name + ": ended");
}
}
public class ExchangerDemo {
public static void main(String args[]) {
System.out.println("Main process started");
Exchanger<DataClass> exchanger = new Exchanger<DataClass>();
Loop loop1 = new Loop(1, "First", exchanger);
Loop loop2 = new Loop(2, "Second", exchanger);
Loop loop3 = new Loop(3, "Third", exchanger);
new Thread(loop1).start();
new Thread(loop2).start();
new Thread(loop3).start();
System.out.println("Main process ended");
}
}

For your dependency you should make three classes, and have three distinct Exchange objects (one in each). So thread1 would be between 1 and 2 (output of 1 to 2), thread 2's would be between 2 and 3 and thread 3's exhanger would be between itself and 1. Remember the exchanger's would guard only until it had its input from its feeder, to till it passes to its receiver.
Also synchronized is not as bad as the books make out. use it. Watch http://www.youtube.com/watch?v=WTVooKLLVT8
Also for reference Best way of running two threads alternatively?
Also why do you need three threads? Can you use a thread pool and have each task to the 3 things ?

Distributing each thread a Particular Range

I am using ThreadPoolExecutor in my multithreading program, I want each thread should have particular range of ID's if ThreadSize is set as 10 and Start = 1 and End = 1000 then each thread would have range of 100 id's(basically by dividing end range with thread size) that it can use without stepping on other threads.
Thread1 will use 1 to 100 (id's)
Thread2 will use 101 to 200 (id's)
Thread3 will use 201 to 300 (id's)
-----
-----
Thread10 will use 901 to 1000
I know the logic basically, the logic can be like this-
Each thread gets `N = (End - Start + 1) / ThreadSize` numbers.
Thread number `i` gets range `(Start + i*N) - (Start + i*N + N - 1)`.
As I am working with ThreadPoolExecutor for the first time, so I am not sure where should I use this logic in my code so that each Thread is Using a predefined ID's without stepping on other threads. Any suggestions will be appreciated.
public class CommandExecutor {
private List<Command> commands;
ExecutorService executorService;
private static int noOfThreads = 3;
// Singleton
private static CommandExecutor instance;
public static synchronized CommandExecutor getInstance() {
if (instance == null) {
instance = new CommandExecutor();
}
return instance;
}
private CommandExecutor() {
try {
executorService = Executors.newFixedThreadPool(noOfThreads);
} catch(Exception e) {
System.out.println(e);
}
}
// Get the next command to execute based on percentages
private synchronized Command getNextCommandToExecute() {
}
// Runs the next command
public synchronized void runNextCommand() {
// If there are any free threads in the thread pool
if (!(((ThreadPoolExecutor) executorService).getActiveCount() < noOfThreads))
return;
// Get command to execute
Command nextCommand = getNextCommandToExecute();
// Create a runnable wrapping that command
Task nextCommandExecutorRunnable = new Task(nextCommand);
executorService.submit(nextCommandExecutorRunnable); // Submit it for execution
}
// Implementation of runnable (the real unit level command executor)
private static final class Task implements Runnable {
private Command command;
public Task(Command command) {
this.command = command;
}
public void run() {
// Run the command
command.run();
}
}
// A wrapper class that invoked at every certain frequency, asks CommandExecutor to execute next command (if any free threads are available)
private static final class CoreTask implements Runnable {
public void run() {
CommandExecutor commandExecutor = CommandExecutor.getInstance();
commandExecutor.runNextCommand();
}
}
// Main Method
public static void main(String args[]) {
// Scheduling the execution of any command every 10 milli-seconds
Runnable coreTask = new CoreTask();
ScheduledFuture<?> scheduledFuture = Executors.newScheduledThreadPool(1).scheduleWithFixedDelay(coreTask, 0, 10, TimeUnit.MILLISECONDS);
}
}

Whether this is a good idea or not I will leave it for you to decide. But to give you a hand, I wrote a little program that does what you want... in my case I am just summing over the "ids".
Here is the code:
public class Driver {
private static final int N = 5;
public static void main(String args[]) throws InterruptedException, ExecutionException{
int startId = 1;
int endId = 1000;
int range = (1 + endId - startId) / N;
ExecutorService ex = Executors.newFixedThreadPool(N);
List<Future<Integer>> futures = new ArrayList<Future<Integer>>(N);
// submit all the N threads
for (int i = startId; i < endId; i += range) {
futures.add(ex.submit(new SumCallable(i, range+i-1)));
}
// get all the results
int result = 0;
for (int i = 0; i < futures.size(); i++) {
result += futures.get(i).get();
}
System.out.println("Result of summing over everything is : " + result);
}
private static class SumCallable implements Callable<Integer> {
private int from, to, count;
private static int countInstance = 1;
public SumCallable(int from, int to) {
this.from = from;
this.to = to;
this.count = countInstance;
System.out.println("Thread " + countInstance++ + " will use " + from + " to " + to);
}
// example implementation: sums over all integers between from and to, inclusive.
#Override
public Integer call() throws Exception {
int result = 0;
for (int i = from; i <= to; i++) {
result += i;
}
System.out.println("Thread " + count + " got result : " + result);
return result;
}
}
}
which produces the following output (notice that in true multi-thread fashion, you have print statements in random order, as the threads are executed in whatever order the system decides):
Thread 1 will use 1 to 200
Thread 2 will use 201 to 400
Thread 1 got result : 20100
Thread 3 will use 401 to 600
Thread 2 got result : 60100
Thread 4 will use 601 to 800
Thread 3 got result : 100100
Thread 5 will use 801 to 1000
Thread 4 got result : 140100
Thread 5 got result : 180100
Result of summing over everything is : 500500