In this lab, you will study reference variables.
Because variables are used so frequently, they need to be in a place where they can be quickly accessed. The stack provides this place. Each local variable, whether it is a primitive data type or a reference variable, is stored on the stack. When you use a variable's name, the computer quickly locates that variable in memory and retrieves its value.
Physically, the stack is nothing more than a big block of memory. The system handles all the details of managing its size so you don't have to worry about it.
When you call a method, that method is given a portion of memory at the top of the stack to store its local variables and parameters. Because each method uses its own portion of the stack, we know that one method cannot modify the local variables of another methods.
When a method returns (or finishes running), the portion of memory on top of the stack used by that method is freed by the system. In other words, that method's variables no longer exist in memory. The following Stack N' Heap demonstrates this:
Code
{
int myVar = 5; dosomething ( ); System.out.println( myVar ); } public void dosomething ( ) { int myVar = 18; System.out.println( myVar ); } |
Stack
|
Heap
|
Temporary
|
Because methods do not share memory, the only way methods can share information is via return values and parameters. A method can return information to the method that called it using a return value. A method can send information to the method it is calling by giving values to that method's parameters. The following Stack N' Heap demonstrates the use of return values.
All objects are stored on the heap. The system cannot find an object once it is stored on the heap unless it knows its address; an object addresses are stored in reference variables. Although objects that are not referenced by a reference variable are useless, they still take up space in memory. To reclaim this space, a procedure called garbage collection is run. This process scans the heap to find objects that can be removed. The system handles this automatically; you needn't worry about it.
As mentioned above, a method can send information to a called method
by giving values to that method's parameters. A method's
parameters are declared in the parenthesis immediately following the
method's name. The syntax for declaring a parameter is very simple,
it is identical to declaring a variable. For instance:
the method header
public void dosomething (int param1, String param2)
begin a method named
dosomething
that has two parameters:
an integer named param1
, and a
String
reference variable named param2
.
In order to call this method,
you must provide an initial value for each of these parameters, such as
dosomething(5, "This is a test");
.
(Note
that a comma separates the two expressions supplied for the parameters.) The
values "passed" to the method when you call it become the initial
values of that method's parameters.
When you call a method you must specify the initial values of its parameters; these values can be changed as the method runs. Using the assignment operator, you can change the value stored in a parameter, but this will have no effect on the variable stored in the calling method:
Code
{
int myVar = 5; dosomething3 ( myVar ); } public void dosomething3 ( int temp ) { temp = temp + 15; } |
Stack
|
Heap
|
Temporary
|
Remember that reference variables store addresses. When a parameter is a reference variable, it must be given an object's address as its initial value. You can use the assignment operator to change the address stored in a reference parameter. You can also use the address stored in the reference parameter to change the instance variables of the object to which it refers. These changes will exist after the method stops running. The following Stack N' Heap illustrates this:
Download the jar file: References.jar to your csci/201 directory,
unjar the file (Help),
create a new NetBeans project called References
,
and mount the References
directory created when you unjarred the archive file (Help).
Your directory structure should look something like the picture below:
The file DeuxTons.java defines a simple class
that represents an object with two colors; one, a "top" color and,
the another, a "bottom" color.
The file References.java contains a main
method that instantiates two DeuxTons
objects.
The main
method makes eight calls
of the static Click
method of the class
TwoToneDisplay
, four times for each
DeuxTons
object.
The Click
method takes a "picture" of a
DeuxTons
object. It displays the picture
inside a small frame with labels similar to
3'th picture. That label indicates the
picture's number within the "roll".
The picture contains two circles. The top circle
has the "top" color of the DeuxTons
object.
The bottom circle has the "bottom" circle.
Between the two colored circles, you will see a line proclaiming
something like I'm 342589. That number is an
object ID, obtained by calling the hashCode
method.
This ID is guaranteed to be unique for every Java run-time object.
Go ahead and run Reference
's main
method. You should see the eight Click
's.
They will probably fill your screen.
If you click the X on any picture, all will disappear.
Within the main
method of References
there are comments that
describe modifications you are to make to
DeuxTons
objects or to references to
DeuxTons
objects during this lab.
After each modification, main
calls
the Click
twice to display the result of
your modification.
Your job is to add code to the main
method
so that it performs the modifications described in the comments in
References.java.
There are three
modifications that you that you must make.
Some of these modifications will require that you call
the getTop
, getBottom
,
setTop
, and setBottom
methods
of DeuxTons
.
You should look at the code within DeuxTons.java to
learn more about these methods; however,
the only file you should modify to complete this lab is
References.java.
Show your lab instructor your complete code with all modifications.
The ultimate display should resemble the following images.
Pay careful attention to the object ID's displayed in the middle
of each picture.