class SafeRaceBlock extends Thread {
    private int count;
    private SafeAccumulateBlock acc;

    public SafeRaceBlock( int count, SafeAccumulateBlock acc ) {
	this.count = count;
	this.acc = acc;
    }
    public void run() {
	for( int i = 0; i < count ; i++ ) {
	    acc.inc();
	}
    }
    public static void main( String[] args )  {
        long t = System.currentTimeMillis();
	SafeAccumulateBlock acc = new SafeAccumulateBlock();
	Thread t1 = new SafeRaceBlock( 9000, acc );
	Thread t2 = new SafeRaceBlock( 8000, acc );
	t1.start();
	t2.start();
	try {
	    t1.join();
	    t2.join();
	}
	catch( InterruptedException ex ) {
	}
	System.out.println( acc.getCount() + " == " + 17000 );
        t = System.currentTimeMillis() - t;
	System.out.println( "elapsed = " + t );
    }
}

class SafeAccumulateBlock {
    private int sum;

    public SafeAccumulateBlock() {
	sum = 0;
    }
    // the only difference is synchronized
    public void inc() {
	synchronized (this ) {
            int s = sum;
            for( int i = 0 ; i < 10000 ; i++ ) 
                ; // do nothing
            sum = s + 1;
        }
    }
    public int getCount() {
	return sum;
    }
}