Strange problem with simple multithreading program in Java

Question

I am just starting play with multithreading programming. I would like to my program show alternately character '-' and '+' but it doesn't. My task is to use synchronized keyword. As far I have:

class FunnyStringGenerator{

    private char c;

    public FunnyStringGenerator(){
        c = '-';
    }

    public synchronized char next(){

        if(c == '-'){
            c = '+';
        }
        else{
            c = '-';
        }

        return c;
    }
}

class ThreadToGenerateStr implements Runnable{

    FunnyStringGenerator gen;

    public ThreadToGenerateStr(FunnyStringGenerator fsg){
        gen = fsg;
    }

    @Override
    public void run() {
        for(int i = 0; i < 10; i++){

            System.out.print(gen.next());
        }
    }


}

public class Main{


    public static void main(String[] args) throws IOException {

        FunnyStringGenerator FSG = new FunnyStringGenerator();

        ExecutorService exec = Executors.newCachedThreadPool();

        for(int i = 0; i < 20; i++){
            exec.execute(new ThreadToGenerateStr(FSG));
        }

    }

}

EDIT: I also testing Thread.sleep in run method instead for loop.

Answer 1

Your synchronized block in FunnyStringGenerator.next() is working fine. It will return '+' and '-' alternately.

However you have a race condition in ThreadToGenerateStr.run():

System.out.print(gen.next());

This is equivalent to:

char c = gen.next(); // Synchronized
System.out.print(c); // Not synchronized

The problem occurs when:

• Thread 1 calls gen.next(), getting a result of '-'
• Thread 2 calls gen.next(), getting a result of '+'
• Thread 2 calls System.out.print(), writing '+'
• Thread 1 calls System.out.print(), writing '-'

The result is that the '+' and '-' are written in the opposite order.

There are various possible workarounds, e.g.:

• Call both gen.next() and System.out.print() in a single synchronized block (as in dogbane's answer)
• Make gen.next() write the character to the stream instead of returning it
• Make gen.next() append the character to a shared BlockingQueue and have a dedicated I/O thread taking characters from this queue and printing them.