• Threads often need to share a common resource ie a file, with one thread reading from the file while another thread writes to the file
• this is an example of a producer/consumer relationship

Race conditions

• race conditions occur when multiple, asynchronously executing threads access the same object returning unexpected (wrong) results
• they can be avoided by synchronizing the methods which access the shared resource

/* Example of Synchronized Threads from
   "Java 2 Certification" by Jamie Jaworski p182
   
   Example shows how synchronized methods and object locks
   are used to coordinate access to a common object by
   multiple threads.
*/

class Thread3 extends Thread {
    static String[] msg = { "Java", "is", "hot,", "aromatic,",
                            "and", "invigorating." };
                            
    public static void main(String[] args) {
        Thread3 t1 = new Thread3("t1: ");
        Thread3 t2 = new Thread3("t2: ");
        
        t1.start();
        t2.start();
        
        boolean t1IsAlive = true;
        boolean t2IsAlive = true;
        
        do {
            if(t1IsAlive && !t1.isAlive()) {
                t1IsAlive = false;
                System.out.println("t1 is dead.");
            }
            
            if(t2IsAlive && !t2.isAlive()) {
                t2IsAlive = false;
                System.out.println("t2 is dead.");
            }
        } while(t1IsAlive || t2IsAlive);
    }                            
                                
    public Thread3(String id) {
        super(id);
    }
    
    void randomWait() {
        try {
            Thread.currentThread().sleep((long)(3000*Math.random()));
        } catch(InterruptedException e) {
            System.out.println("Interrupted!");
        }
    }    

    
    public synchronized void run() {
        SynchronizedOutput.displayList(getName(), msg);
    }
}   

class SynchronizedOutput {

    // if the 'synchronized' keyword is removed, the message
    // is displayed in interleaved fashion
    public static synchronized void displayList(String name,
                                                String list[] )
    {
        for(int i=0; i            Thread3 t = (Thread3) Thread.currentThread();
            t.randomWait();
            System.out.println(name + list[i]);
        }
    }                                                    
}