As mentioned, access modifiers may be use to separate representative parts of a program from non-representative parts. The term information hiding is often used for a similar purpose. Information hiding is a software design principle that is used to limit accessibility of attributes in a program component.
The term component is a general term used for such program elements – in this book a component corresponds to an object-descriptor. The term client is often use for components that use a given component. We use the terms component and client below, but a component is as mentioned an object-descriptor, that may define a module, class or singular object.
The primary goal of information hiding is to prevent extensive modification to clients whenever the implementation details of a component are changed. This is done by hiding implementation details that are not relevant for the use of a given component.
The term encapsulation is often used in connection to information hiding. One may think of information hiding is a design principle and encapsulation as a technical term. A component like a class is said to encapsulate attributes and hiding implementation details. A component hides information by encapsulating the information into a component which presents an interface
The term interface of a component is another important term in software design. The interface of a component is the set of attributes that are accessible by clients of the component. The attributes of a given component defined as %public thus constitutes the interface of the component. Clients of the interface of a component will access the component purely through its interface. This way, if the implementation changes, the clients do not have to change.
The term Application Programming Interface (API) is another common term. It refers to the interface of a component, but is most often used for software modules and/programs used by other modules or programs, but technically it is an interface.
In many schools of software design, it is a common design principle that the interface of a component should be restricted to a set of methods. Data-items are primarily considered implementation details and access to data-items should therefore be limited. In object-oriented design as advocated in this book, an interface may also include local classes and/or intrinsic objects.
Information hiding, encapsulation and interface are technical terms of software design. They go hand inland with programming as modeling. One way of practicing information hiding s to make sure that only representative parts of a module are visible to clients and thus provides the interface. Non-representative parts correspond to implementation. Modules, classes and object-descriptors are language mechanisms for supporting encapsulation supplemented by access modifiers.
Information hiding serves as a criterion that can be used to decompose a program into modules. The principle is also useful for reducing coupling within a system.
A common use of information hiding is to hide the physical storage layout for data so that if it is changed, the change is restricted to a small subset of the total program. For example, if a three-dimensional point (x, y, z) is represented in a program with three floating-point scalar variables and later, the representation is changed to a single array variable of size three, a module designed with information hiding in mind would protect the remainder of the program from such a change.
