Objects
Objects are little black boxes of functionality that are fully independent of any code that references or uses them.
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A black box is an engineering term that works like this:
First we drop something in to a black box.
Then, we wait while our thing is "magically transformed" inside the black box.
Finally, we receive a new transformed thing back from the black box.
The beauty of a black box is that all we need to know is how to drop something into the black box and what to expect on the other side.
We do not need to understand the magic inside.
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Objects can be thought of as having two characteristics.
Objects have properties, and objects have methods.
- Properties describe the qualities of an object
- Methods specify the object's behavior.
The typical "introduction to objects" chapter will give you an example something like this:
| The Cat Object |
| Properties | Methods |
| Eye Color | Can Purr |
| Number of legs | Catches mice |
| Species | Rips up the couch |
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Although this is not a perfect analogy, sometimes when I am trying to figure out what an object should look like, I think of objects in terms of nouns, properties in terms of adjectives, and methods in terms of verbs.
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However, let's consider a more realistic object that you might have to deal with in your code.
Let's consider the HTML FORM "Select Box" object:
A Select Box object has several properties.
For one, Select Boxes have a SIZE that corresponds to the number of visible selections.
Select Boxes also can be set to allow either multiple or single selections.
Meanwhile, Select Boxes have methods.
For example, a select box knows how to create its popup list when a user clicks on the down arrow.
Also, if a user selects an item from the popup list, the select box knows how to select that item.
Further, the Select Box knows how to close the popup box when a user selects an item.
The table below presents a quick reference to our select box object.
| The Select Box Object |
| Properties | Methods |
| Size | Open Drop Down Box |
| Selection Policy | Select an Item |
| List of Items | Close Drop Down Box |
Achieving abstraction through objects is aided through four foundation characteristics of objects:
- Encapsulation
- Inheritance
- Overloading
- Polymorphism
Let's look at each of these concepts...
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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