Patterns, Implementation, Maintenance & Reuse

Design Patterns

• Solution to a commonly occurring design problem that can be applied over and over again in many different project context.
• A formal design pattern has a name, a description of the problem it solves, and a solution strategy
• Design patterns also incorporate best practices in their solutions, and result in very flexible, and reusable designs.
• Creational patterns
• address design issues that focus on how to make a system independent of how its objects are created which results in designs very robust in response to changes.
• Structural patterns
• describe ways to design objects that work with each other in very robust ways to realize new functionality
• Behavioral patterns
• deal with how to assign responsibilities to objects and how objects communicate with one another

Sample Design Patterns

• Singleton pattern
• used when we want to ensure that only a single instance from a class is instantiated
• class constructor is not made public.
• static variable “instance”, which in this case is a pointer to a Singleton object
• static method named Instance, that returns a pointer to the unique Singleton object
• Façade pattern
• can be used to provide a unified, more simplified interface that hides any complex relationships that might exist
• Observer pattern
• publish-subscribe pattern
• very useful pattern in situations where an object needs to notify other objects whenever its state changes

Implementation

• component reuse
• framework reuse
• level of reuse
• clear box: we can’t see the code
• translucent box: Not only can we see
• the code, but we can make changes to it
• black box: can see can’t change

Benefit of reuse

• Software reliability, as well as hardware reliability, is measured as “mean time to failure,” or MTTF.
• There are two main reasons why reused software has higher reliability than hand-crafted software.
• if I am building some software that I know will be reused by other people; I will take more care to do a good job.
• software has higher reliability is because of something we will cover in our module on assuring software quality
• less expensive
• capture expert knowledge
• standardization
• less develop time

Impediments to Reuse

• creating components
• paying
• not exact fit
• legal
• trust

Software maintenance

• why we need to do maintenance
• The better products actually received bigger maintenance budgets than the poorer ones
• Corrective maintenance is fixing bugs
• Adaptive maintenance is when we make changes to keep up with new technology such as new hardware or operating systems.
• perfective maintenance
• cost maintenance
• 60% to 80% of the total life cycle cost of software is in maintenance
• Another thing to keep in mind is that we are continually building more software that will need to be maintained.
• The only thing we can do is to reduce the unit cost of maintenance.