This trivial example checks whether an entry already exists with the current key before writing new data, in order to avoid an exception that would occur if data with this key already existed.

这个简单示例可检查在编写新数据之前当前键中是否已经有一个项，以避免因存在具有此键的数据而发生的异常。

youdao

Unlock the door with this key.

用这把钥匙开门。

youdao

Homeland Security Secretary Janet Napolitano, a key figure behind this week's aid package, will visit Mexico early next month along with U.S.

VOA: standard.2009.03.25

The U.S.official faced large hurdles this week, with all sides far apart on the key issue of Jewish settlements in the West Bank.

VOA: standard.2009.09.13

"The key to long term success in this area is our outreach with foreign governments."

VOA: standard.2010.07.23

The article continues with this key passage.

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Mr. HILBORN: The chief warden comes with this giant key, you know it sort of takes two hands.

NPR: Conservationists Split on How to Curb Poaching

She said having a cycling network would make cycling a "desirable choice, " with this a key time to get more people on a bike before the "Olympic buzz dwindles".

BBC: Cyclist