Activity: Implement Component

Follow the Programming Guidelines when implementing the classes.

The primary basis for implementation is the classes, with public operations, attributes and associations. It is important be aware that not all public operations, attributes and associations are defined during design.

The secondary basis for implementation is the use-case realizations, which shows how the classes and objects interact to perform the use case.

It is recommended that you implement the classes incrementally; compile, link and run some regression tests a couple of times a day.

Before you implement a class from scratch, consider adapting existing implementation classes, typically by subclassing or by instantiation.

Implement Operations

To implement operations, do the following:

• Choose an algorithm
• Choose data structures appropriate to the algorithms
• Define new classes and operations as necessary
• Code the operation

Choose an Algorithm

Many operations are simple enough to be implemented straight away from the operation and its specification.

Nontrivial algorithms are primarily needed for two reasons: to implement complex operations for which a specification is given, and to optimize operations for which a simple but inefficient algorithm serves as definition.

Choose Data Structures Appropriate To the Algorithms

Choosing algorithms involves choosing the data structure they work on. Many implementation data structures are container classes, such as, arrays, lists, queues, stacks, sets, bags, and variations of these. Most object-oriented languages, and programming environments provide class libraries with these kinds of reusable components.

Define New Classes and Operations as Necessary

New classes may be found to hold intermediate results for example, and new low-level operations may be added on the class to decompose a complex operation. These operations are often private to the class, that is, not visible outside the class itself.

Code the Operation

Write the code for the operation, starting with its interface statement, e.g. the member function declaration in C++, subprogram specification in Ada, or method in Visual Basic. Follow the Programming Guidelines.

Implement States

There are two basic approaches to implementing an object's states.

• Direct implementation of a state-machine mechanism

This is the most obvious way of implementing a state machine, with a variable that holds the state.

• Using concurrent processes

An object can be implemented as a task or, light-weight process in the programming language or operating system. This is the most general approach because it preserves the inherent concurrency of real objects. Events are implemented using inter-task calls using the facilities in the programming language or operating system. Use this solution with care because it might require the introduction of many tasks.

If the object's behavior is complex, in terms of states, then it has often been described with a state diagram in design. This state diagram will serve as an important input when you implement the class. For more information, refer to Guidelines: State Diagram.

Use Delegation to Reuse Implementation

If a class or parts of a class can be implemented reusing an existing class, use delegation rather than inheritance.

Delegation means that the class is implemented with the help of other classes. The class references an object of the other class by using a variable. When an operation is called, the operation calls an operation in the referenced object (of the reused class), for actual execution. Thus, you can say that it delegates responsibility to the other class.

Implement Associations

A one-way association is implemented as a pointer - an attribute which contains an object reference. If the multiplicity is one, then it is implemented as a simple pointer. If the multiplicity is many, then it is a set of pointers. If the many end is ordered, then a list can be used instead of a set.

A two-way association is implemented as attributes in both directions, using techniques for one-way associations.

A qualified association is implemented as a lookup table (for example, a Smalltalk Dictionary class) in the qualifying object. The selector values in the lookup table are the qualifiers, and the target values are the objects of the other class.

If the qualifier values must be accessed in order, then the qualifiers can be arranged into a sorted array or a tree. In this case, access time will be proportional to log N where N is the number of qualifier values.

If the qualifiers are drawn from a compact finite set, then the qualifier values can be mapped into an integer range and the association can be efficiently implemented as an array. This approach is more attractive if the association is mostly full rather than being sparsely populated and is ideal for fully populated finite sets.

Most object-oriented languages and programming environments provide class libraries with reusable components to implement different kinds of associations.

Implement Attributes

Implement attributes either as a variable of a built-in primitive, a reusable component class, or define a new class. Defining a new class is often more flexible, but introduces unnecessary indirection. For example, an employee's Social Security number can either be implemented as an attribute of type String or as a new class.

Alternative implementations of an attribute.

It may also be the case that groups of attributes are combined into new classes, as the following example shows. Both implementations are correct.

The attributes in Line are implemented as associations to a Point class.

Provide Feedback to Design

If a design error is discovered in any of the steps, rework feedback has to be provided to the design. If the required change is small, and the same individual is designing and implementing the class, the there is no need for a formal change request. The individual can do the change in the design.

If the required change affects several classes, for example a change in a public operation, then a formal change request should be submitted to a CCB (change control board). See Activity: Fix a Defect.

Evaluate the Code

Before you start unit testing, there are some checks you can do. Testing is more expensive, so try to do some of the following:

• Always compile the code. Set the compiler's warning level to the most detailed level.
• Mentally check the operations. Read through the code, trying to follow all the paths, and identify all exception conditions. Do this as soon as anything new is implemented.
• Use tools to check the code for errors. For example, a static code rule checker.