在尝试使用HashMap
时,我注意到了一些奇怪的事情。
使用4个线程,每个线程都试图插入(key,value),其中key从0到9999,value是一个常量字符串。当所有线程完成后,map.size()
返回值大于10,000。这是怎么发生的?这是否意味着map包含重复的键?
我遍历了map.entrySet()
并发现某些键的计数确实超过1。如果我在map中为这样的键执行get()
,会返回什么值?
以下是我尝试的代码
final HashMap<String, String> vals = new HashMap<>(16_383);
Runnable task = new Runnable() {
@Override
public void run() {
for (int i = 0; i < 10000; i++) {
vals.put(""+i, Thread.currentThread().getName());
}
}
};
Thread thread = new Thread(task, "a");
Thread thread1 = new Thread(task, "b");
Thread thread2 = new Thread(task, "c");
Thread thread3 = new Thread(task, "d");
thread.start();
thread1.start();
thread2.start();
thread3.start();
thread.join();
thread1.join();
thread2.join();
thread3.join();
System.out.println(Thread.currentThread().getName() + "vals "+ vals.size());
System.out.println(Thread.currentThread().getName() + "vals "+ vals.entrySet().size());
System.out.println(Thread.currentThread().getName() + "vals "+ vals.keySet().size());