CSCI 201 -- References

Variables

Let's start by taking a look at variables in Java.

Storage of Variables

Because variables are used so frequently, they need to be stored 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 block of memory. The system handles all the details of managing its size so you don't have to worry about it.

Variable Passing

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, 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 in different classes do not share memory, the only way methods can share information is via parameters and return values. A calling method can send information to the called method by giving values to that method's parameters. A called method can return information to the calling method using a return value. The following Stack N' Heap demonstrates the use of return values.

Code
{
int myVar = 5;
myVar = dosomething2 ( myVar );
System.out.println( myVar );
}

public int dosomething2 ( int temp ) {
return ( temp + 15 );
}
Stack
Heap
Temporary

Storage of Objects

Variables can also refer to objects. The stack space for the variable only needs to contain the reference to the object. The objects themselves are stored on the heap, another area of memory that is much larger than the stack.. The system cannot find an object once it is stored on the heap unless it knows its address; object addresses are stored in reference variables.

Objects that cannot be accessed through a reference variable are useless. However, they may still take up may space in the heap. To reclaim this space, a procedure called garbage collection is periodicly run by the Java system. This process scans the stack and heap to find objects that can be removed. The system handles this automatically; you needn't worry about it.

Passing Variables

As mentioned above, a calling 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) begins 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");. The values passed to the called method become the initial values of the called method's parameters.

Passing Primitive Data Types

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

Passing Reference Variables

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; however, this does not change any variables used by the calling method. Now you can 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:

Code
{
intHolder myVar = new intHolder ( 5 );
dosomething4 ( myVar );
System.out.println( myVar.getInt() );
}

public void dosomething4 ( intHolder temp ) {
temp.setInt ( 20 );
}
Stack
Heap
Temporary

Downloading the project framework

Download References.zip, a ZIP file containing a NetBeans project named References and unZIP this project into your csci/201 directory. Try to make your Projects panel look something like the following picture before continuing.
Projects panel

Reference Variables

The file DeuxTons.java defines a simple class that represents an object with two colors; a "top" color and 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. The colors will differ from this example, because new colors are randomly chosen on each run. The eight pictures will probably fill your screen. If you terminate any picture, all will disappear.
Before modification

Your Assignment

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 method 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 are allowed to modify to complete this lab is References.java.

Original two objects

The first two pictures are Click's of ColorPair1 and ColorPair2 immediately after their creation. We have two local variables, ColorPair1 and ColorPair2, within your main method that point to different DeuxTons objects. Each of the two DeuxTons objects point have reference variables that point to different Java Color objects.

Objects before modification Picture before modification

Swap the objects

Your first task is to swap the two objects referenced by ColorPair1 and ColorPair2. After that swap, the order in which the DeuxTons objects are displayed will be referenced. You can also check the hashCode displayed after I'm to make sure you have correctly switched the two.

Objects after reference swap Piecture after reference swap

Swap colors between the objects

Now you are going to swap the top colors of each DeuxTons object. Note that you do not change the object references of ColorPair1 and ColorPair2. You change that state of their objects!

Objects after top color swap Picture after top color swap

Swap colors within the objects

Your final modification is to swap the bottom and top colors of each DeuxTons object.

Objects after top/bottom color swap Pictures after top/bottom color swap

The final result

A correctly working solution should resemble the following set of eight pictures.
After modification

Show your lab instructor your complete code with all modifications. The ultimate display should resemble the picure shown after. Pay careful attention to the object ID's displayed in the middle of each picture.

The instructor will probably look at your code to verify that DeuxTons.java has not been modified.