Lab: Experimenting with Static Members

Instructions: Answer the following questions one at a time. After answering each question, check your answer (by clicking on the check-mark icon if it is available) before proceeding to the next question.

Getting Ready: Before going any further, you should:

Depending on your development environment, create either a directory or a project for this lab.
Setup your development environment.
Download the following files:

AccessCard.java
Driver.java

to an appropriate directory/folder. (In most browsers/OSs, the easiest way to do this is by right-clicking/control-clicking on each of the links above.)

1. Understanding Static Members: This part of the lab will help you understand the difference between static and non-static members.

Read and understand the AccessCard class and the main class named Driver.

Compile and execute Driver. What was output?


After constructing bernstein...
Prof. Bernstein is number 1
  Classrooms true
  Dorms      false
  Offices    true


After constructing fred...
Fred Flintstone is number 2
  Classrooms false
  Dorms      true
  Offices    false


After constructing rose...
Pres. Rose is number 3
  Classrooms true
  Dorms      true
  Offices    true


Before exiting...
Prof. Bernstein is number 1
  Classrooms true
  Dorms      false
  Offices    true


Fred Flintstone is number 2
  Classrooms false
  Dorms      true
  Offices    false


Pres. Rose is number 3
  Classrooms true
  Dorms      true
  Offices    true

Make all of the boolean attributes in the AccessCard class static. What change did you make?

Changed:

    private boolean     classroomAccess, dormAccess, officeAccess;

to:

    private static boolean     classroomAccess, dormAccess, officeAccess;

Compile AccessCard and execute Driver. What was output?


After constructing bernstein...
Prof. Bernstein is number 1
  Classrooms true
  Dorms      false
  Offices    true


After constructing fred...
Fred Flintstone is number 2
  Classrooms false
  Dorms      true
  Offices    false


After constructing rose...
Pres. Rose is number 3
  Classrooms true
  Dorms      true
  Offices    true


Before exiting...
Prof. Bernstein is number 1
  Classrooms true
  Dorms      true
  Offices    true


Fred Flintstone is number 2
  Classrooms true
  Dorms      true
  Offices    true


Pres. Rose is number 3
  Classrooms true
  Dorms      true
  Offices    true

Why do all of the AccessCard objects have the same access rights "before exiting"?
.

Static attributes belong to the class. So, all objects in the AccessCard class have the same classroomAccess, dormAccess, and officeAccess attributes.
Why do all of the AccessCard objects have attributes that allow access to classrooms, dorms, and offices? In other words, why are the access attributes of all of the objects true ?
.

The last call to the setAccessRights() method was passed true, true, true.

Fix this defect. In other words, make all of the boolean attributes in the AccessCard class non-static. What change did you make?

Changed:

    private static boolean     classroomAccess, dormAccess, officeAccess;

back to:

    private boolean     classroomAccess, dormAccess, officeAccess;

2. Understanding Static and Non-Static Contexts: This part of the lab will help you understand the concepts "static context" and "non-static context".

Modify the Driver class, replacing:

       rose.setAccessRights(true, true, true);

with:

       AccessCard.setAccessRights(true, true, true);

Compile Driver

What error was generated?


Driver.java:19: non-static method setAccessRights(boolean,boolean,boolean) cannot be referenced from a static context
       AccessCard.setAccessRights(true, true, true);
                 ^
1 error

Why was this message generated?
.

This error was generated because the new line in the Driver class is treating the setAccessRights() method as if it is static when it isn't.
What does it mean when someone says you are behaving like one of 10,000 monkeys at 10,000 keyboards?
.

If you put 10,000 monkeys at 10,000 keyboards, they will, eventually, write a Shakespearean play. Programmers are behaving like this when, instead of thinking about what they are doing, they make random changes in the hope that they will, eventually, stumble upon the correct answer.
Temporarily, behave like one of 10,000 monkeys at 10,000 keyboard. Specifically, in the AccessCard class, make the setAccessRights() method static in an attempt to correct this error.

Compile the AccessCard class. What error was generated?


AccessCard.java:58: error: non-static variable classroomAccess cannot be referen
ced from a static context
       classroomAccess = classroom;
       ^
AccessCard.java:59: error: non-static variable dormAccess cannot be referenced f
rom a static context
       dormAccess      = dorm;
       ^
AccessCard.java:60: error: non-static variable officeAccess cannot be referenced
 from a static context
       officeAccess    = office;
       ^
3 errors

Why were these errors generated?
.

These errors were generated because the static method setAccessRights() is using the non-static attributes classroomAccess, dormAccess, and officeAccess. (This can't be allowed because a static method can be used without an object having been created. Hence, it can be used in situations in which the attributes classroomAccess, dormAccess, and officeAccess don't exist.)
Continue to temporarily behave like one of 10,000 monkeys at 10,000 keyboards, and make things even worse. Specifically, make all of the boolean attributes in the AccessCard class static. What change did you make?
.
Changed:

private boolean classroomAccess, dormAccess, officeAccess;

to:

private static boolean classroomAccess, dormAccess, officeAccess;
Compile the AccessCard class. It, should compile since the static setAccessRights() method now uses only static attributes.
Compile the Driver class. It, too, should compile since the setAccessRights() method is in fact static.
Should you be happy that both classes compile?
.

Of course not, I already know that the attributes should not be static because every AccessCard object should have its own attributes.
Execute the Driver class. You should get the same wrong results you got earlier.

Fix the AccessCard class by making the boolean attributes non-static and by making the setAccessRights() method non-static. What change did you make?

Changed:

    private static boolean     classroomAccess, dormAccess, officeAccess;

    public static void setAccessRights(boolean classroom, boolean dorm, boolean office)

back to:

    private boolean     classroomAccess, dormAccess, officeAccess;

    public void setAccessRights(boolean classroom, boolean dorm, boolean office)

Compile AccessCard. It should compile.

Modify the Driver class, changing:

       AccessCard.setAccessRights(true, true, true);

back to:

       rose.setAccessRights(true, true, true);

Compile Driver. It should compile.
Execute Driver. It should execute properly.

3. Using Static Attributes: One problem with the current design of the AccessCard class is that its users (in this case the Driver) must keep track of the next account number to use. We can fix this with a static attribute.

Add the following attribute to the AccessCard class:

    private static int  nextAccountNumber;

In the AccessCard class's constructor, change:

       this.number = number;

to:

       this.number = nextAccountNumber;

Compile AccessCard and Execute Driver. What output was generated?


After constructing bernstein...
Prof. Bernstein is number 0
  Classrooms true
  Dorms      false
  Offices    true


After constructing fred...
Fred Flintstone is number 0
  Classrooms false
  Dorms      true
  Offices    false


After constructing rose...
Pres. Rose is number 0
  Classrooms true
  Dorms      true
  Offices    true


Before exiting...
Prof. Bernstein is number 0
  Classrooms true
  Dorms      false
  Offices    true


Fred Flintstone is number 0
  Classrooms false
  Dorms      true
  Offices    false


Pres. Rose is number 0
  Classrooms true
  Dorms      true
  Offices    true

This output is obviously incorrect, since each AccessCard object now has the same number. Why is it incorrect?
.
We forgot to increase nextAccountNumber. To fix this problem, add the following line to the bottom of the constructor: ++nextAccountNumber;

Compile AccessCard and execute Driver What output was generated?


After constructing bernstein...
Prof. Bernstein is number 0
  Classrooms true
  Dorms      false
  Offices    true


After constructing fred...
Fred Flintstone is number 1
  Classrooms false
  Dorms      true
  Offices    false


After constructing rose...
Pres. Rose is number 2
  Classrooms true
  Dorms      true
  Offices    true


Before exiting...
Prof. Bernstein is number 0
  Classrooms true
  Dorms      false
  Offices    true


Fred Flintstone is number 1
  Classrooms false
  Dorms      true
  Offices    false


Pres. Rose is number 2
  Classrooms true
  Dorms      true
  Offices    true

Remove the formal parameter number from the constructor in AccessCard since it is no longer needed.
Remove the actual parameter corresponding to number in each of the uses of the constructor in Driver.
Compile AccessCard and Driver.
The card numbers now start at 0 and I'd prefer that they start at 10. So, we obviously need to initialize nextAccountNumber. As a first attempt, add the following line to the top of the constructor in AccessCard:
```
       nextAccountNumber = 10;       
    
```

Compile AccessCard and execute Driver. What output was generated?


After constructing bernstein...
Prof. Bernstein is number 10
  Classrooms true
  Dorms      false
  Offices    true


After constructing fred...
Fred Flintstone is number 10
  Classrooms false
  Dorms      true
  Offices    false


After constructing rose...
Pres. Rose is number 10
  Classrooms true
  Dorms      true
  Offices    true


Before exiting...
Prof. Bernstein is number 10
  Classrooms true
  Dorms      false
  Offices    true


Fred Flintstone is number 10
  Classrooms false
  Dorms      true
  Offices    false


Pres. Rose is number 10
  Classrooms true
  Dorms      true
  Offices    true

Why does every AccessCard object now have the number 10?
.

Because nextAccountNumber, which is static, is being initialized every time the constructor is called.

To fix this problem, first delete the following line in the constructor in AccessCard:

       nextAccountNumber = 10;

Now, change:

    private static int  nextAccountNumber;

to:

    private static int  nextAccountNumber = 10;

Compile AccessCard and execute Driver. What output was generated?


After constructing bernstein...
Prof. Bernstein is number 10
  Classrooms true
  Dorms      false
  Offices    true


After constructing fred...
Fred Flintstone is number 11
  Classrooms false
  Dorms      true
  Offices    false


After constructing rose...
Pres. Rose is number 12
  Classrooms true
  Dorms      true
  Offices    true


Before exiting...
Prof. Bernstein is number 10
  Classrooms true
  Dorms      false
  Offices    true


Fred Flintstone is number 11
  Classrooms false
  Dorms      true
  Offices    false


Pres. Rose is number 12
  Classrooms true
  Dorms      true
  Offices    true

This output is now correct.
To make sure you really understand what's going on, make nextAccountNumber non-static.

Compile AccessCard and execute Driver. What output was generated?


After constructing bernstein...
Prof. Bernstein is number 10
  Classrooms true
  Dorms      false
  Offices    true


After constructing fred...
Fred Flintstone is number 10
  Classrooms false
  Dorms      true
  Offices    false


After constructing rose...
Pres. Rose is number 10
  Classrooms true
  Dorms      true
  Offices    true


Before exiting...
Prof. Bernstein is number 10
  Classrooms true
  Dorms      false
  Offices    true


Fred Flintstone is number 10
  Classrooms false
  Dorms      true
  Offices    false


Pres. Rose is number 10
  Classrooms true
  Dorms      true
  Offices    true

Why does every AccessCard object again have a number of 10?
.

Now it's because each AccessCard object has its own nextAccountNumber attribute. Make sure you understand why this is a problem!
Fix nextAccountNumber by making it static.
Compile AccessCard.

4. Using Static Methods: This part of the lab will help you understand one use of static methods.

Add the following method to the AccessCard class:

    public static void setFirstAccountNumber(int n)
    {
       nextAccountNumber = n;       
    }

Compile AccessCard.

Add the following line to the top of the main() method in Driver:

       AccessCard.setFirstAccountNumber(5000);

Compile and execute Driver.

What output was generated?


After constructing bernstein...
Prof. Bernstein is number 5000
  Classrooms true
  Dorms      false
  Offices    true


After constructing fred...
Fred Flintstone is number 5001
  Classrooms false
  Dorms      true
  Offices    false


After constructing rose...
Pres. Rose is number 5002
  Classrooms true
  Dorms      true
  Offices    true


Before exiting...
Prof. Bernstein is number 5000
  Classrooms true
  Dorms      false
  Offices    true


Fred Flintstone is number 5001
  Classrooms false
  Dorms      true
  Offices    false


Pres. Rose is number 5002
  Classrooms true
  Dorms      true
  Offices    true