This article discusses the application of "Design by Contract" in object-oriented software development. It emphasizes the importance of reliability in software systems, defined as the absence of bugs. The article presents methodological guidelines to improve software reliability, focusing on the theory of design by contract, which underlies the Eiffel programming language. The key idea is to define clear contracts between software components, specifying preconditions and postconditions for each routine. These contracts help ensure that software components behave as expected, reducing the likelihood of bugs. The article explains how to use assertions to enforce these contracts. Preconditions specify the conditions that must be met for a routine to function correctly, while postconditions specify the properties that the routine guarantees to achieve. Class invariants, which apply to all instances of a class, further ensure consistency across all routines. The article also discusses the role of inheritance and dynamic binding in object-oriented programming. Redeclaration allows routines to be redefined in descendant classes, but this must be done carefully to maintain compatibility with the original specification. The theory of design by contract provides a framework for ensuring that redeclared routines honor the original contract. The article also addresses the issue of exception handling. Exceptions are used to handle abnormal cases, and the design by contract approach provides a systematic way to define and handle these exceptions. The contract theory helps ensure that exceptions are handled properly, preventing software from entering an inconsistent state. The article concludes by emphasizing the importance of using assertions and contracts to improve software reliability. By clearly defining the responsibilities of each software component, developers can create more robust and maintainable systems. The use of design by contract is a powerful tool for ensuring that software behaves as intended, reducing the likelihood of bugs and improving overall software quality.This article discusses the application of "Design by Contract" in object-oriented software development. It emphasizes the importance of reliability in software systems, defined as the absence of bugs. The article presents methodological guidelines to improve software reliability, focusing on the theory of design by contract, which underlies the Eiffel programming language. The key idea is to define clear contracts between software components, specifying preconditions and postconditions for each routine. These contracts help ensure that software components behave as expected, reducing the likelihood of bugs. The article explains how to use assertions to enforce these contracts. Preconditions specify the conditions that must be met for a routine to function correctly, while postconditions specify the properties that the routine guarantees to achieve. Class invariants, which apply to all instances of a class, further ensure consistency across all routines. The article also discusses the role of inheritance and dynamic binding in object-oriented programming. Redeclaration allows routines to be redefined in descendant classes, but this must be done carefully to maintain compatibility with the original specification. The theory of design by contract provides a framework for ensuring that redeclared routines honor the original contract. The article also addresses the issue of exception handling. Exceptions are used to handle abnormal cases, and the design by contract approach provides a systematic way to define and handle these exceptions. The contract theory helps ensure that exceptions are handled properly, preventing software from entering an inconsistent state. The article concludes by emphasizing the importance of using assertions and contracts to improve software reliability. By clearly defining the responsibilities of each software component, developers can create more robust and maintainable systems. The use of design by contract is a powerful tool for ensuring that software behaves as intended, reducing the likelihood of bugs and improving overall software quality.