Design by Contract, or programming by Contract, is not a new concept in programming, many languages already having such implementations.

契约式设计(或“契约式编程”)并不是编程领域的新概念，许多语言中已经有类似的实现了。

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This Contract is made in English and Russian languages in two originals, one of each versions shall be for the Seller and one for the Buyer.

本合同以英语和俄语的形式分别出具两份正本，每一个版本由卖方和买方各保存一份。

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The concept of Design by Contract allows you to define some constraints and invariants of your class. Statically typed languages have a primitive version of this.

契约设计这样的概念允许我们在类中定义一些约束和限制。

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Their options range from using online app-building tools, to taking crash courses in computer programming and coding languages, hiring a costly professional on contract, or recruiting a full-time developer.

WSJ: App Building, the Do-It-Yourself Way