Application Programming Interface (API)
So what do we know about an object and what do we need to do to use an object?
Well, the only thing the programmer needs to know is how to get information from the object when the information is needed or how to tell the object to perform some action when some action is required.
For example, consider the case in which a web browser wants to know which item has currently been selected in the Choice Box because the user has clicked the submit button and it needs to formulate the HTTP request.
In this case, the web browser software will simply ask the select box object which item is currently selected and the select box object will answer.
So how does the web browser software ask the select box object for the currently selected item?
Typically, objects provide an Application Programer's Interface (API) (also called an interface) that allows other objects (like web browser software or a larger application) to get information or to ask the object to perform an action.
An API is a set of publicly available methods that yield or effect certain pieces of information about the state of an object.
In the case above, the choice box object will have an API method that will return the selected item.
Generically, the interaction between the web browser software and the object might work like this:
|
Web Browser Software: |
Hey Choice Box object, the user just pushed the
submit button and I need to put together a URL encoded string for
the HTTP request. What item is currently selected? |
| Choice Box Object |
"Apples" is currently selected. |
| Web browser happily goes
off and formulates the HTTP request without ever needing to know
how the select box object performs its magic. |
Why should we go through the process of asking for the data instead of simply accessing the object's property itself?
Why do we have to go through an "accessor method"?
Well, "encapsulating" objects and providing API methods to the outside world allows us to hide the object's properties and methods that is necessary for black box abstraction.
In this way we assure that regardless of how we change the choice box object in the future, so long as the API stays the same, all objects and routines that utilize the choice box object will not break.
Nobody cares about the code inside an object, they only care about the object's API.
If we allowed browsers to access the options data structure in order to determine which option was selected, we would be incapable of redefining how options are stored even if we found a better way to do it.
This would be because in order to change the way the Choice object worked, we would also have to change how the browsers accessed the data.
If we support hundreds of browsers, this would be a nightmare.
Instead, if we force the browser to always use the accessor when getting data, then the choice can change how it internally holds choices without ever affecting the browser.
So long as it keeps the API stays the same, browsers are not affected by changes in the choice object.
This makes writing and modifying code much, much easier and goes a long way toward solving the spaghettification dilemma.
NEXT
Selena Sol contributes to the JavaBoutique's Introduction to Java. Selena curently works for Barclays Capital in London, one of the leading global investment banks in Europe and has worked as a software developer for the National Center for Human Genome research, Microline Software, Neuron Data, and Electric Eye in Singapore. Selena is perhaps best-known for creating the Public Domain Web Script Archive (Extropia) and writing several books on Web Programming (Perl, CGI, Java).
Email: selena@extropia.com
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