新手求助线程小问题，感谢大侠不吝赐教

小弟最近在自学java，在线程处遇到一卖票程序，有几处百思不得其解，望各位大侠不吝赐教，小弟谢过
class A implements Runnable
{
    public int tickets = 100;
    String str = new String("ironman");
    public void run()
    {
        
        while(true)
        {
                synchronized(str)
            {
                if(tickets>0)
                {
               
                    System.out.printf("%s线程正在卖出第%d张票\n",Thread.currentThread().getName(),tickets);
                    tickets--;
                }
               
                else
                {
                    break;
                }
            }
        }
    }
}
class TestTickets_2
{
    public static void main(String[] args)
    {
        A aa = new A();
        Thread str1 = new Thread(aa);
        str1.start();
        Thread str2 = new Thread(aa);
        str2.start();   
    }
}
问题1：为何只会有一个线程卖票，无法实现交替卖票？
问题2：若将主线程改为
class TestTickets_2
{
    public static void main(String[] args)
    {
        Thread str1 = new Thread(new A());
        str1.start();
        Thread str2 = new Thread(new A());
        str2.start();
        
    }
}
为何会出现每张票被重复卖出的现象？
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请大家帮帮忙，小弟不胜感激，谢谢大家
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搞定了，呵呵
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