Using Objects

Once you've created an object,
you probably want to use it for something.
You may need to use the value of one of its fields, change one of its fields,
or call one of its methods to perform an action.

Referencing an Object's Fields

Object fields are accessed by their name. You must use a name that is unambiguous.

You may use a simple name for a field within its own class. For example, we can add a statement
within the Rectangle class that prints the width and height:

System.out.println("Width and height are: " + width + ", " + height);

In this case, width and height are simple names.

Code that is outside the object's class must use an object reference or expression,
followed by the dot (.) operator, followed by a simple field name,
as in:

objectReference.fieldName

For example, the code in
the CreateObjectDemo class is outside the code for the
Rectangle class. So to refer to the origin,
width, and height fields within the Rectangle
object named rectOne, the CreateObjectDemo
class must use the names rectOne.origin,
rectOne.width, and rectOne.height, respectively.
The program uses two of these names to display the width
and the height of rectOne:

System.out.println("Width of rectOne: " + rectOne.width);

System.out.println("Height of rectOne: " + rectOne.height);

Attempting to use the simple names width and height
from the code in the CreateObjectDemo class doesn't make
sense — those fields exist only within an object —
and results in a compiler error.



Later, the program uses similar code to display information about
rectTwo. Objects of the same type have their own copy of
the same instance fields. Thus, each Rectangle object
has fields named origin, width, and height.
When you access an instance field through an object reference,
you reference that particular object's field. The two objects
rectOne and rectTwo in the CreateObjectDemo
program have different origin, width,
and height fields.





To access a field, you can use a named reference to an object,
as in the previous examples, or you can use any expression that returns
an object reference. Recall that the new operator returns
a reference to an object. So you could use the value returned from new
to access a new object's fields:

int height = new Rectangle().height;

This statement creates a new Rectangle object and immediately gets
its height. In essence, the statement calculates the default height of a
Rectangle. Note that after this statement has been executed,
the program no longer has a reference to the created Rectangle,
because the program never stored the reference anywhere.
The object is unreferenced, and its resources are free to be recycled by
the Java Virtual Machine.

Calling an Object's Methods

You also use an object reference to invoke an object's method. You append the method's simple name to the object reference,
with an intervening dot operator (.). Also, you provide, within enclosing parentheses,
any arguments to the method. If the method does not require any
arguments, use empty parentheses.

objectReference.methodName(argumentList);

   or

objectReference.methodName();

The Rectangle class has two methods: getArea() to compute the
rectangle's area and move() to change the rectangle's origin.
Here's the CreateObjectDemo code that invokes these two methods:

System.out.println("Area of rectOne: " + rectOne.getArea());

...

rectTwo.move(40, 72);

The first statement invokes rectOne's getArea() method and displays
the results. The second line moves rectTwo because the
move() method assigns new values to the object's origin.x
and origin.y.



As with instance fields, objectReference must be a reference to an
object. You can use a variable name, but you also can use any expression
that returns an object reference. The new operator returns an
object reference, so you can use the value returned from new to invoke a
new object's methods:

new Rectangle(100, 50).getArea()

The expression new Rectangle(100, 50) returns an object
reference that refers to a Rectangle object.
As shown, you can use the dot notation to invoke the new
Rectangle's getArea() method to compute the area
of the new rectangle.



Some methods, such as getArea(), return a value.
For methods that return a value, you can use the method invocation
in expressions. You can assign the return value to a variable,
use it to make decisions, or control a loop. This code assigns
the value returned by getArea() to the variable areaOfRectangle:

int areaOfRectangle = new Rectangle(100, 50).getArea();

Remember, invoking a method on a particular object is the same as sending a
message to that object. In this case, the object that getArea()
is invoked on is the rectangle returned by the constructor.

The Garbage Collector

Some object-oriented languages require that you keep track of all the objects
you create and that you explicitly destroy them when they are no longer
needed. Managing memory explicitly is tedious and error-prone. The Java
platform allows you to create as many objects as you want (limited, of
course, by what your system can handle), and you don't have to worry about
destroying them. The Java runtime environment deletes objects when it
determines that they are no longer being used. This process is called garbage collection.



An object is eligible for garbage collection when there are no more references
to that object. References that are held in a variable are usually
dropped when the variable goes out of scope. Or, you can explicitly drop an
object reference by setting the variable to the special value null.
Remember that a program can have multiple references to the same
object; all references to an object must be dropped before the object is
eligible for garbage collection.





The Java runtime environment has a garbage collector that periodically frees
the memory used by objects that are no longer referenced. The garbage collector
does its job automatically when it determines that the time is right.