0.1 Jbs1010-objects and encapsulation  (Page 5/7)

Sending a message

A person who speaks in OOP-speak might say that pressing one of the frequency-selector buttons on the front of the radio sends a message to the radio object, asking it to perform an action (tune to a particular station) . That person might also say that storing a new frequency that corresponds to a particular button entails sending a message to the radio object asking it to change its state.

Calling a method

Java-speak is a little more specific than general OOP-speak. In Java-speak, we might say that pressing one of the selector buttons on the front of the radio calls a method on the radio object. The behavior of the method is to cause the object to perform an action.

As a practical matter, the physical manifestation of sending a message to an object in Java is to cause that object to execute one of its methods.

Similarly, we might say that storing a new frequency that corresponds to a particular button calls a setter method on the radio object.

(In an earlier paragraph, I said that I could follow a specific procedure to set the frequency value associated with a button to correspond to one of the radio stations in Dallas. Note the use of the words set and setter in this jargon.)

Behavior

In addition to state, objects are often also said to have behavior . The overall behavior of an object is determined by the combined behaviors of its individual methods.

For example, one of the behaviors exhibited by our radio object is the ability to play the radio station at a particular frequency. When a frequency is selected by pressing a selector button, the radio knows how to translate the radio waves at that frequency into audio waves compatible with our range of hearing, and to send those audio waves out through the speakers.

Thus, the radio object behaves in a specific way in response to a message asking it to tune to a particular frequency.

Where do objects come from?

In order to mass-produce car radios, someone must first create a set of plans, (drawings, or blueprints) for the radio. Once the plans are available, the manufacturing people can produce millions of nearly identical radios.

A class definition is a set of plans

The same is true for software objects. In order to create a software object in Java, it is necessary for someone to first create a plan.

In Java, we refer to that plan as a class .

The class is defined by a Java programmer. Once the class definition is available, that programmer, (or other programmers) , can use it to produce millions of nearly identical objects.

(While millions may sound like a lot of objects, I'm confident that since Java was released into the programming world around 1997, Java programmers around the world have created millions of objects using the standard Java class named Button .)