use of wait-notify, whats wrong in my approach

Question

I am trying to implement a case of shared variable like semaphores. In that I have 3 thread which should be run sequentially. below is my program. Could u tell me where I am wrong?

class Test2{    
    private volatile static int sharedvarNEXT = 1;
    public synchronized static int getSharedvarNEXT(){
        return sharedvarNEXT;   
    }
    public synchronized static void setSharedvarNEXT(int val){
        sharedvarNEXT = val;    
    }
    public static void main(String[] s){
        new classACaps().start();
        new classAsmall().start();
        new class1().start();
    }
}

class classACaps extends Thread{
    public void run(){
        try{
            for(char c='A';c<='Z';c++){
                if(Test2.getSharedvarNEXT() == 1){
                    System.out.println(c);
                    Test2.setSharedvarNEXT(2);
                    notify();
                } else {
                    wait();
                }
            }
        } catch(Exception e){
            System.out.println("caught in A");      
        }
    }
}

class classAsmall extends Thread{  
    public void run(){
        try{
            for(char c='a';c<='z';c++){
                if(Test2.getSharedvarNEXT() == 2){
                    System.out.println(c);      
                    Test2.setSharedvarNEXT(3);
                    notify();
                } else {
                    wait();
                }
            }
        } catch(Exception e){
            System.out.println("caught in a");      
        }
    }
}

class class1 extends Thread{
    public void run(){
        try{
            for(int c=1;c<=26;c++){
                if(Test2.getSharedvarNEXT() == 3){
                    System.out.println(c);
                    Test2.setSharedvarNEXT(1);
                    notify();
                }else {
                    wait();
                }
            }
        } catch(Exception e){
            System.out.println("caught in 1");      
        }
    }
}

expected outcome is like : A, a, 1, B, b, 2......

Answer 1

In that I have 3 thread which should be run sequentially.

Well you're not running sequentially - you're running the three concurrently, which is the nature of what happens when you start three separate threads.

Those threads aren't synchronizing against each other at all. It doesn't really sound like you should have three threads at all, given your requirements.

Additionally:

• You don't have any synchronized blocks; both wait() and notify() can only be called by a thread which owns the monitor on which they're called
• Using wait() and notify() on instances of Thread is strongly discouraged, as the internals of Thread use it themselves. I suggest you create a separate object just to synchronize/wait/notify on.
• Your exception handling completely ignores the exception, which has been hiding the first problem from you. Never ignore the exception - and try to avoid catching bare Exception at all, in general.

Answer 2

Calling notify() and wait() inside the run() method is basically calling

this.notify();

None of your Threads is synchronized on each this Thread instance.

You cannot notify() if you don't own the monitor, ie. synchronized on it. You're swallowing all the exceptions you get.