In addition, a wide variety of third-party class libraries are available on the Internet.

此外，在Internet上还提供了多种第三方类库。

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A third party class must be rewritten to extend only from the abstract class.

第三方的类必须被重写，只能从抽象类继承。

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A third party class must be rewritten to extend only from the abstract class.

第三方的类必须被重写，只能从抽象类继承。

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