最佳实践

public final V (V value) {  
   throw new UnsupportedOperationException();  
}

// 空指针校验
if (key == null) 
   throw new NullPointerException();

if (corePoolSize < 0 ||
    maximumPoolSize <= 0 ||
    maximumPoolSize < corePoolSize ||
    keepAliveTime < 0)
    throw new IllegalArgumentException();