Contracts and interfaces

A way to manage interactions between objects is to establish some specific rules so that an object declares what it needs and another object declares what it implements, that is a contract. As in a business context, a contract is an agreement between two or more parties for the doing of something specified. For example, in the interaction between two software components, such as objects, we can establish how an object can ask another object to perform some actions in order to achieve a specific goal. With reference to our example of a software house that wants to hire developers, the contract between it and the candidates establishes that the developer must be able to write code.

So, while a contract is a general agreement between two software components in order to achieve a specific goal, an interface is the formal terms of the contract, the details of the agreement that the components must be compliant with. We can say that an interface is the concrete part of...