How can design by contract improve object-oriented design?

1 What is DBC?

DBC is based on the idea of a contract between a supplier and a client. The supplier provides a service or a function, and the client uses it for a certain purpose. The contract defines the terms and conditions of the service, such as what the supplier expects from the client (preconditions), what the client can expect from the supplier (postconditions), and what remains unchanged by the service (invariants). The contract also defines the consequences of violating the terms, such as throwing an exception or terminating the program.

2 How to apply DBC?

DBC can be applied to any object-oriented language, either natively or through libraries or tools. The basic steps involve defining the contract for each class and method using annotations, comments, or assertions. It's important to use clear and precise language to describe the preconditions, postconditions, and invariants. To check the contract, you can use a tool or a library that can verify the contract and report any violations. Additionally, it's important to document the contract for each class and method using a standard format or a tool to generate the documentation from the contract annotations or comments. For example, in Java you can use the @Contract annotation from the JetBrains library to specify the contract for a method, the Cofoja library to enable contract checking at runtime using Java agents, and the Javadoc tool to generate the documentation from the @Contract annotation.

3 What are the benefits of DBC?

Design by Contract (DBC) can improve your object-oriented design by providing several benefits. It can make your code more readable and understandable by explicitly stating the assumptions and guarantees of each component, as well as help avoid ambiguity and confusion about the behavior and functionality. Additionally, DBC ensures that your code follows the principle of least surprise by enforcing the contract, and helps to detect and prevent contract violations early. Moreover, DBC makes your code more modular and decoupled by defining clear interfaces and boundaries between components, which increases the compatibility and interoperability of your code by establishing a common language and protocol for communication.

4 What are the challenges of DBC?

DBC is not a silver bullet for software design. It also has some challenges and limitations, such as overhead from contract checks and assertions, a trade-off between contract strength and flexibility, and the need for testing to complement DBC. Performance and memory can be impacted by adding contract checks, and code complexity and verbosity can increase with the addition of annotations or comments. A stronger contract can offer more reliability and correctness, but reduce adaptability and extensibility. On the other hand, a weaker contract can provide more flexibility but increase risk and uncertainty. Testing can verify functionality and quality, but not all scenarios or inputs; similarly, DBC can specify the contract but not guarantee its correctness or validity.