public class HashTest {
    static Map<String, Integer> map = new HashMap<String, Integer>();
    // static Map<String, Integer> map = new Hashtable<>();
    public static void main(String[] args) {
        for (int i = 0; i < 2; i++) {
            new Thread(() -> {
                int raddom = new
 
 Random().nextInt();
                String threadName = Thread.currentThread().getName();
                map.put(threadName, raddom);

                System.out.println(Thread.currentThread().getName() + "\t" + map.get(threadName));
            }).start();
        }
    }
}

HashMap

public class
 
 HashMap<K,V> extends AbstractMap<K,V>
    implements Map<K,V>, Cloneable, Serializable {

    public V put(K key, V value);

    public V get(Object key);
}

// 可以通過Collections將非線性安全轉為執行緒安全
Map<Object, Object> map = Collections.synchronizedMap(new HashMap<>());

Hashtable

public class Hashtable<K,V> extends Dictionary<K,V> implements Map<K,V>, Cloneable, java.io.Serializable {
    // 執行緒安全 同步方法synchronized
    public synchronized V put(K key, V value);

    // 執行緒安全 同步方法synchronized
    public synchronized V get(Object key);
}    

上面示例使用Hashtable就會每次都能獲取到資料