General Responsibility Assignment Software Patterns (or Principles), abbreviated GRASP, is a set of 'nine fundamental principles in object design and responsibility assignment'[1]:6 first published by Craig Larman in his 1997[citation needed] book Applying UML and Patterns.
- Cohesion and coupling 1. Coupling (computer programming) In software engineering, coupling is the degree of interdependence between software modules; a measure of how closely connected two routines or modules are; the strength of the relationships between modules. Coupling is usually contrasted with cohesion.
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The different patterns and principles used in GRASP are controller, creator, indirection, information expert, low coupling, high cohesion, polymorphism, protected variations, and pure fabrication.[2] All these patterns solve some software problem common to many software development projects. These techniques have not been invented to create new ways of working, but to better document and standardize old, tried-and-tested programming principles in object-oriented design.
COUPLING and COHESION COUPLING An indication of the strength of interconnections between program units. Highly coupled have program units dependent on each other. Loosely coupled are made up of units that are independent or almost independent. Modules are independent if they can function completely without the presence of the other.
Larman states that 'the critical design tool for software development is a mind well educated in design principles. It is not UML or any other technology.'[3]:272 Thus, the GRASP principles are really a mental toolset, a learning aid to help in the design of object-oriented software.
Patterns[edit]
In object-oriented design, a pattern is a named description of a problem and solution that can be applied in new contexts; ideally, a pattern advises us on how to apply its solution in varying circumstances and considers the forces and trade-offs. Many patterns, given a specific category of problem, guide the assignment of responsibilities to objects.
Information expert[edit]
Problem: What is a basic principle by which to assign responsibilities to objects?
Solution: Assign responsibility to the class that has the information needed to fulfill it.
Information expert (also expert or the expert principle) is a principle used to determine where to delegate responsibilities such as methods, computed fields, and so on.
Using the principle of information expert, a general approach to assigning responsibilities is to look at a given responsibility, determine the information needed to fulfill it, and then determine where that information is stored.
This will lead to placing the responsibility on the class with the most information required to fulfill it.[3]:17:11
Related Pattern or Principle: Low Coupling, High Cohesion
Creator[edit]
The creation of objects is one of the most common activities in an object-oriented system. Which class is responsible for creating objects is a fundamental property of the relationship between objects of particular classes.
Problem: Who creates object A?
Solution: In general, Assign class B the responsibility to create object A if one, or preferably more, of the following apply:
- Instances of
Bcontain or compositely aggregate instances ofA - Instances of
Brecord instances ofA - Instances of
Bclosely use instances ofA - Instances of
Bhave the initializing information for instances ofAand pass it on creation.[3]:16:16.7
Related Pattern or Principle: Low Coupling, Factory pattern
Controller[edit]
The controller pattern assigns the responsibility of dealing with system events to a non-UI class that represents the overall system or a use case scenario. A controller object is a non-user interface object responsible for receiving or handling a system event.
Problem: Who should be responsible for handling an input system event?
Solution: A use case controller should be used to deal with all system events of a use case, and may be used for more than one use case. For instance, for the use cases Create User and Delete User, one can have a single class called UserController, instead of two separate use case controllers.
The controller is defined as the first object beyond the UI layer that receives and coordinates ('controls') a system operation. The controller should delegate the work that needs to be done to other objects; it coordinates or controls the activity. It should not do much work itself. The GRASP Controller can be thought of as being a part of the application/service layer[4] (assuming that the application has made an explicit distinction between the application/service layer and the domain layer) in an object-oriented system with common layers in an information system logical architecture.
Related Pattern or Principle: Command, Facade, Layers, Pure Fabrication
Indirection[edit]
The indirection pattern supports low coupling and reuses potential between two elements by assigning the responsibility of mediation between them to an intermediate object. An example of this is the introduction of a controller component for mediation between data (model) and its representation (view) in the model-view control pattern. This ensures that coupling between them remains low.
Problem: Where to assign responsibility, to avoid direct coupling between two (or more) things? How to de-couple objects so that low coupling is supported and reuse potential remains higher?
Solution: Assign the responsibility to an intermediate object to mediate between other components or services so that they are not directly coupled.
The intermediary creates an indirection between the other components.
Low coupling[edit]
Coupling is a measure of how strongly one element is connected to, has knowledge of, or relies on other elements. Low coupling is an evaluative pattern that dictates how to assign responsibilities for the following benefits:
- lower dependency between the classes,
- change in one class having a lower impact on other classes,
- higher reuse potential.
High cohesion[edit]
High cohesion is an evaluative pattern that attempts to keep objects appropriately focused, manageable and understandable. High cohesion is generally used in support of low coupling. High cohesion means that the responsibilities of a given element are strongly related and highly focused. Breaking programs into classes and subsystems is an example of activities that increase the cohesive properties of a system. Alternatively, low cohesion is a situation in which a given element has too many unrelated responsibilities. Elements with low cohesion often suffer from being hard to comprehend, reuse, maintain and change.[3]:314–315</ref>
Polymorphism[edit]
According to the polymorphism principle, responsibility for defining the variation of behaviors based on type is assigned to the type for which this variation happens. This is achieved using polymorphic operations. The user of the type should use polymorphic operations instead of explicit branching based on type.
Problem: How to handle alternatives based on type? How to create pluggable software components?
Solution: When related alternatives or behaviors vary by type (class), assign responsibility for the behavior—using polymorphic operations—to the types for which the behavior varies. (Polymorphism has several related meanings. In this context, it means 'giving the same name to services in different objects'.)
Protected variations[edit]
The protected variations pattern protects elements from the variations on other elements (objects, systems, subsystems) by wrapping the focus of instability with an interface and using polymorphism to create various implementations of this interface.
Problem: How to design objects, subsystems, and systems so that the variations or instability in these elements does not have an undesirable impact on other elements?
Solution: Identify points of predicted variation or instability; assign responsibilities to create a stable interface around them.
Pure fabrication[edit]
A pure fabrication is a class that does not represent a concept in the problem domain, specially made up to achieve low coupling, high cohesion, and the reuse potential thereof derived (when a solution presented by the information expert pattern does not). This kind of class is called a 'service' in domain-driven design.
Related Patterns and Principles• Low Coupling.• High Cohesion.
See also[edit]
References[edit]
- ^Craig Larman (2001). Applying UML and Patterns: An Introduction to Object-Oriented Analysis and Design and the Unified Process(PDF) (2nd ed.). Prentice Hall. ISBN0-13-092569-1.
- ^Muhammad Umair (2018-02-26). 'SOLID, GRASP, and Other Basic Principles of Object-Oriented Design'. DZone.
- ^ abcdCraig Larman (2004). Applying UML and Patterns: An Introduction to Object-Oriented Analysis and Design and Iterative Development (3rd ed.). Pearson. ISBN978-0131489066.
- ^'Application Layer like business facade?'. Yahoo! Groups (domaindrivendesign). Retrieved 2010-07-15.
Cohesion: Cohesion can be defined as the degree of the closeness of the relationship between its components. In general, it measures the relationship strength between the pieces of functionality within a given module in the software programming. It is an ordinal type of measurement, which is described as low cohesion or high cohesion.
In a good module, the various parts having high cohesion is preferable due to its association with many desirable traits of the software such as reliability, re-usability, robustness and understand-ability.
Coupling And Cohesion Example
On the other hand, a low cohesion is associated with the undesirable traits, including difficulty in maintaining, reusing and understanding. If the functionalities embedded in a class have much in common, then the cohesion will be increased in a system.
High cohesion leads to the increased module re-usability because the developers of the application will easily find the component they look for in the cohesive set of operations offered by the module.
The system maintainability will be increased due to logical changes in the domain effecting fewer modules.The module complexity also reduces, when there is a high cohesion in the programming.
i. Coincidental cohesion: the parts of a component are not related but simply bundled into a single component. Harder to understand and not reusable.
ii. Logical association: similar functions such as input, error handling, etc. put together. Functions fall in same logical class. May pass a flag to determine which ones executed. Interface difficult to understand. Code for more than one function may be intertwined, leading to severe maintenance problems. Difficult to reuse
iii. Temporal cohesion: all of statements activated at a single time, such as start up or shut down, are brought together. Initialization, clean up. Functions weakly related to one another, but more strongly related to functions in other modules so may need to change lots of modules when do maintenance.
iv. Procedural cohesion: a single control sequence, e.g., a loop or sequence of decision statements. Often cuts across functional lines. May contain only part of a complete function or parts of several functions. Functions still weakly connected, and again unlikely to be reusable in another product.
v. Communicational cohesion: operate on same input data or produce same output data. May be performing more than one function. Generally acceptable if alternate structures with higher cohesion cannot be easily identified. Still problems with reusability.
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vi. Sequential cohesion: output from one part serves as input for another part. May contain several functions or parts of different functions.
vii. Informational cohesion: performs a number of functions, each with its own entry point, with independent code for each function, all performed on same data structure. Different than logical cohesion because functions not intertwined.
viii. Functional cohesion: each part necessary for execution of a single function. e.g., compute square root or sort the array. Usually reusable in other contexts. Maintenance easier.
ix. Type cohesion: modules that support a data abstraction. Not strictly a linear scale. Functional much stronger than rest while first two much weaker than others. Often many levels may be applicable when considering two elements of a module. Cohesion of module considered as highest level of cohesion that is applicable to all elements in the module.
Coupling: In software engineering, the coupling can be defined as the measurement to which the components of the software depend upon each other. Normally, the coupling is contrasted with the cohesion. If the system has a low coupling, it is a sign of a well-structured computer system and a great design.
A low coupling combined with the high cohesion, it supports the mission of high readability and maintainability. The coupling term generally occurs together with the cohesion very frequently. The coupling is an indication of the strength of the interconnection between all the components in a system. The highly coupled systems have interconnections, in which the program units depend upon each other, whereas in the loosely coupled systems made up of components that are independent of each other and have no dependence on each other.
1. Content Coupling: Content Coupling is the highest type of coupling which occurs when one of the module relies on the other module’s internal working. It means a change in the second module will lead to the changes in the dependent module.
2. Common Coupling: It is the second highest type of coupling also known as Global Coupling. It occurs when the same global data are shared by the two modules. In this, the modules will undergo changes if there are changes in the shared resource.
3. External Coupling: This type of coupling occurs when an external imposed data format and communication protocol are shared by two modules. External Coupling is generally related to the communication to external devices.
4. Control Coupling: In this type of coupling, one module controls the flow of another and passes information from one to another.
Cohesion And Coupling Difference
5. Message Coupling: This type of coupling can be achieved by the state decentralization. It is the loosest type of coupling, in which the component communication is performed through message passing.
6. Data Coupling: The modules are connected by the data coupling, if only data can be passed between them.
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7. Stamp Coupling: In this type of coupling, the data structure is used to transfer information from one component to another.