Java Reference
In-Depth Information
Our
CowboyHat
class also has five methods, so you can do five different things with a
CowboyHat
object. One of these is a special method called a
constructor
, which creates a
CowboyHat
object - this
is the method with the name,
CowboyHat
, that is the same as the class name. The items between the
parentheses that follow the name of the constructor specify data that is to be passed to the method when
it is executed - that is, when a
CowboyHat
object is created.
In practice you might need to define a few other methods for the class to be useful;
you might want to compare
CowboyHat
objects for example, to see if one was larger
than another. However, at the moment you just need to get an idea of how the code
looks. The details are of no importance here, as we will return to all this in Chapter 5.
Java Program Statements
As you saw in the
CowboyHat
class example, the code for each method in the class appears between
braces, and it consists of
program statements
. A semicolon terminates each program statement. A
statement in Java can spread over several lines if necessary, since the end of each statement is
determined by the semicolon, not by the end of a line. Here is a Java program statement:
hatOn = false;
If you wanted to, you could also write this as:
hatOn =
false;
You can generally include spaces and tabs, and spread your statements over multiple lines to enhance
readability if it is a particularly long statement, but sensible constraints apply. You can't put a space in the
middle of a name for instance. If you write
hat
On
, for example, the compiler will read this as two words.
Encapsulation
At this point we can introduce another bit of jargon you can use to impress or bore your friends -
encapsulation
. Encapsulation refers to the hiding of items of data and methods within an object. This is
achieved by specifying them as
private
in the definition of the class. In the
CowboyHat
class, the
instance variables,
owner
,
type
,
size
, and
hatOn
were encapsulated. They were only accessible
through the methods defined for the class. Therefore the only way to alter the values they contain is to
call a method that does that. Being able to encapsulate members of a class in this way is important for
the security and integrity of class objects. You may have a class with data members that can only take
on particular values. By hiding the data members and forcing the use of a method to set or change the
values, you can ensure that only legal values are set.
We mentioned earlier another major advantage of encapsulation - the ability to hide the
implementation of a class. By only allowing limited access to the members of a class, you have the
freedom to change the internals of the class without necessitating changes to programs that use the class.
As long as the external characteristics of the methods that can be called from outside the class remain
unchanged, the internal code can be changed in any way that you, the programmer, want.
