Computer programming is the process of designing, writing, testing, debugging, and maintaining the source code of computer programs. This source code is written in one or more programming languages. The purpose of programming is to create a program that performs specific operations or exhibits a...
Object-oriented programming is a programming paradigm using "objects" – data structures consisting of data fields and methods together with their interactions – to design applications and computer programs. Programming techniques may include features such as data abstraction,...
In computer science, a predicate is called an invariant to a sequence of operations provided that: if the predicate is true before starting the sequence, then it is true at the end of the sequence.-Use:...
In computer science, an object is any entity that can be manipulated by the commands of a programming language, such as a value, variable, function, or data structure...
In object-oriented programming, a class is a construct that is used as a blueprint to create instances of itself – referred to as class instances, class objects, instance objects or simply objects. A class defines constituent members which enable these class instances to have state and behavior...
In object-oriented programming, a method is a subroutine associated with a class. Methods define the behavior to be exhibited by instances of the associated class at program run time...
of the class should preserve the invariant. The class invariant constrains the state stored in the object.
Class invariants are established during construction and constantly maintained between calls to public methods. Temporary breaking of class invariance between private method calls is possible, although not encouraged.
An object invariant, or representation invariant, is a programming construct consisting of a set of invariant properties that remain uncompromised regardless of the state of the object
Object (computer science)
In computer science, an object is any entity that can be manipulated by the commands of a programming language, such as a value, variable, function, or data structure...
. This ensures that the object will always meet predefined conditions, and that methods
Method (computer science)
In object-oriented programming, a method is a subroutine associated with a class. Methods define the behavior to be exhibited by instances of the associated class at program run time...
may, therefore, always reference the object without the risk of making inaccurate presumptions. Defining class invariants can help programmers and testers to catch more bugs during software testing
Software testing
Software testing is an investigation conducted to provide stakeholders with information about the quality of the product or service under test. Software testing can also provide an objective, independent view of the software to allow the business to appreciate and understand the risks of software...
.
Class invariants and inheritance
The useful effect of class invariants in object-oriented software is enhanced in the presence of inheritance. Class invariants are inherited, that is, "the invariants of all the parents of a class apply to the class itself."
Inheritance can allow descendant classes to alter implementation data of parent classes, so it would be possible for a descendant class to change the state of instances in a way that made them invalid from the viewpoint of the parent class. The concern for this type of misbehaving descendant is one reason object-oriented software designers give for favoring composition over inheritance (i.e., inheritance breaks encapsulation).
However, because class invariants are inherited, the class invariant for any particular class consists of any invariant assertions coded immediately on that class, logically "and-ed" with all the invariant clauses inherited from the class's parents. This means that even though descendant classes may have access to the implementation data of the their parents, the class invariant can prevent them from manipulating those data in any way that produces an invalid instance at runtime.
Assertions
Common programming languages like C++ and Java support assertions
Assertion (computing)
In computer programming, an assertion is a predicate placed in a program to indicate that the developer thinks that the predicate is always true at that place.For example, the following code contains two assertions:...
by default, which can be used to define class invariants. A common pattern to implement invariants in classes is for the constructor of the class to throw an exception if the invariant is not satisfied. Since methods preserve the invariants, they can assume the validity of the invariant and need not explicitly check for it.
The Java Modeling Language is a specification language for Java programs, using Hoare style pre- and postconditions and invariants, that follows the design by contract paradigm...
that provides a more robust way of defining class invariants.
Native support
The class invariant is an essential component of design by contract. So, programming languages that provide full native support for design by contract, such as Eiffel
Eiffel (programming language)
Eiffel is an ISO-standardized, object-oriented programming language designed by Bertrand Meyer and Eiffel Software. The design of the language is closely connected with the Eiffel programming method...
The D programming language is an object-oriented, imperative, multi-paradigm, system programming language created by Walter Bright of Digital Mars. It originated as a re-engineering of C++, but even though it is mainly influenced by that language, it is not a variant of C++...
, will also provide full support for class invariants.
Java
This is an example of a class invariant in the Java programming language with Java Modeling Language
Java Modeling Language
The Java Modeling Language is a specification language for Java programs, using Hoare style pre- and postconditions and invariants, that follows the design by contract paradigm...
.
The invariant must hold to be true after the constructor is finished and at the entry and exit of all public member
functions. Public member functions should define precondition
Precondition
In computer programming, a precondition is a condition or predicate that must always be true just prior to the execution of some section of code or before an operation in a formal specification....
In computer programming, a postcondition is a condition or predicate that must always be true just after the execution of some section of code or after an operation in a formal specification. Postconditions are sometimes tested using assertions within the code itself...
to help ensure the class invariant.
public class Date {
int /*@spec_public@*/ day;
int /*@spec_public@*/ hour;
D is the fourth letter in the basic modern Latin alphabet.- History :The Semitic letter Dâlet may have developed from the logogram for a fish or a door. There are various Egyptian hieroglyphs that might have inspired this. In Semitic, Ancient Greek, and Latin, the letter represented ; in the...
programming language has native support of class invariants, as well as other contract programming
Design by contract
Design by contract , also known as programming by contract and design-by-contract programming, is an approach to designing computer software...
techniques.
Here is an example from the official documentation.
class Date {
int day;
int hour;
invariant {
assert(1 <= day && day <= 31);
assert(0 <= hour && hour < 24);
}
}
