Java中的线程同步/生产者-消费者。 重复打印数字1-10，然后再打印10-1

Question

I'm very new to java and found an exercise to work on basic thread synchronization. The problem is to print out 12345678910 10987654321 repeatedly until the program stops. Ten different threads should be used.

This is my code so far: I am first working on trying to get just the first number (number one to work), but it keeps giving me an exception

public static void main(String[] args){
   threadOne one = new threadOne();
   one.start();
    }
}

class updateNumber{
    private int i;
    synchronized void increase(int s){
        this.i=s;
        System.out.println(i);
    }
 } 

class threadOne extends Thread {
    private updateNumber grab;
     public void run() {
        try{
         grab.increase(1);
        }
        catch(Exception e){
         System.out.println("error in thread one");
        }
    }
}

I may be going about this the completely wrong way, but I've read a lot of documentation and am just thoroughly confused.

Answer 1

It looks like you didnt create a new instance of update

class threadOne extends Thread {
    private updateNumber grab;
     public void run() {
        try{
         grab.increase(1); // null pointer reference...<<<<<<<
        }
        catch(Exception e){
         System.out.println("error in thread one");
        }
    }
}

// you need to alocate memory to updateNumber like this

//private updateNumber grab = new updateNumber();

    class threadOne extends Thread {
        private updateNumber grab = new updateNumber(); 
         public void run() {
            try{
             grab.increase(1); 
            }
            catch(Exception e){
             System.out.println("error in thread one");
            }
        }
    }