Wile E. Coyote was doing his best to catch the Road Runner. For years he spent countless hours and most of his family fortune in futile attempts to chase that extremely fast (and extremely lucky) bird. Wile did not only fail to catch the Road Runner, but also found himself in various situations where he himself would fall into his own traps. He didn’t mind the pain as much as he minded the humiliation.
One day Wile realized that in order to outsmart the Road Runner he must be smarter than the Road Runner. And so, Wile decided to go to San Jose State University, disguised as a foreign exchange student from Death Valley, CA, and get his degree in Computer Science, assuming that all the math, physics and programming would make him smarter than the Road Runner, who seems to know nothing but how to run fast and say “beep beep.”
In his first semester Wile took CS 46A. During the fourth lab Wile was asked by the lab manual to write a program that prompts the user for a double, finds its absolute value and raises it to the power of 5 (the double, not the user). Wile spent some time planning his algorithm and eventually wrote the following program:
/** This class reads a double number from the user and finds the value of |number|^5 (absolute value to the power of 5). @author Wile E. Coyote @version 1.0 */ import javax.swing.JOptionPane; public class Calculate { public static void main(String[] args) { //getting a number from the user String input = JOptionPane.showInputDialog( "Enter a number or press cancel to exit: "); while(input != null) { double number = Double.parseDouble(input); double answer = number; // Finding number’s absolute value: if (answer < 0) answer = -answer; double positiveNumber = answer; // finding answer ^5 for (int i = 0; i < 5 ;i++) answer = positiveNumber * answer; // printing result System.out.println("|" + number + "|^5 = " + answer); //getting a number from the user String input = JOptionPane.showInputDialog( "Enter a number or press cancel to exit: "); } System.exit(0); } }
Take a look at Wile’s code and find the portion of code where the absolute value is found and where the number is raised to the 5th power. Note that Wile implemented his program using an if statement and a for loop. Also, he used an extra variable called positiveNumber.
Wile’s roommate, Duffy Duck, who also took CS 46A with Wile, wrote a similar program:
/** This class reads a double number from the user and finds the value of |number|^5 (absolute value to the power of 5). To do that the program uses the abs and the pow methods from the Math class. @author Duffy Duck @version 1.0 */ import javax.swing.JOptionPane; public class CalculateBetter { public static void main(String[] args) { String input = JOptionPane.showInputDialog( "Enter a number or press cancel to exit: "); while(input != null) { double number = Double.parseDouble(input); double answer = Math.abs(number); answer = Math.pow(answer, 5.0); // printing result System.out.println("|" + number + "|^5 = " + answer); //getting a number from the user input = JOptionPane.showInputDialog( "Enter a number or press cancel to exit: "); } System.exit(0); } }
How did Duffy Duck find the absolute value and the 5th power of the number? Duffy used the abs and the pow methods from the Math class, thus not needing to use an extra variable. When Wile saw Duffy’s program he wondered how did Duffy know about these methods and how to use them. In fact, he wondered, how can anyone know how to use all the available Java methods?
The answer is simple. No one is expected, nor should they even try, to master the entire Java library. There are so many classes and methods available that it would be practically impossible to learn them all. However, Java programmers have a tool they can utilize to find classes and methods they can use in their code, just like Duffy used the two methods from the Math class.
Open your favorite web browser and go to:
http://java.sun.com/j2se/1.4.2/docs/api/index.html
This is the API specification for the Java 2 Platform. In other words, this is a library containing all the available packages, classes and methods that are part of the official and standard Java language. The main frame lists all the various packages and the smaller frame on the bottom left lists all the classes. Find the Math class and click on it. Now in the main frame, find the specification for the abs method. Notice that there are several abs methods.
Problem 1: (1 point) How do these several abs methods differ from one another? List two differences in a file called abs.txt.
Take a look at the summary of one of the abs method. Here we can see the method’s signature - the name of the method, the parameter we send to it (if any) and the return value (if any). There is also a short description of the method. Find the method used by Duffy Duck and click on it. Now we get a more elaborate description of the method, what it does, special cases it handles etc.
This API documentation is very handy and allows us to find classes and methods to use in our own programs. It also provides us with just enough information to know how to use these methods and objects of these classes, and all without us worrying about the actual implementation and code. We don’t really know how these methods are implemented, and we don’t really care. All we know is that we send a number to the abs method and the return value is the number’s absolute value. Cool.
Problem 2: (2 points) Write a Java program to calculate the number of seconds that have passed since the stroke of midnight in London on New Years Eve, 1970. Your program should not be long, just a few lines of code. (Hint - don’t try and calculate the result yourself, browse through the Java API and find some help there…). Save your file as Seconds.java.
Notice that all the documentation is in an HTML format. Now that you know some HTML you could build such pages for the various classes you design in your homework assignments. Of course, this could use up a lot of time and energy. Luckily, we have a tool called javadoc, which is provided as part of the Java Development Kit. And so, if we follow the rules and comment our code properly, with one line in the DOS command prompt we can generate similar API pages based on our code. This could be a very useful thing for two reasons: First, that would enable other people to familiarize themselves with your work, thus enabling them to use it in their own programs. Second, building such pages is crucial for your getting a good grade in this lab.
In order to generate javadoc pages we first need to worry about proper commenting of our code. The javadoc tool would look for specific standard structures in our comments and for special javadoc tags, all beginning with the ‘@’ sign. In fact, we already encountered these tags in lab 2 (@author, @version, @return etc), only then you didn’t really know what they meant and pretty much copied them from the examples. Let’s look at another class written by Wile E. Coyote as an example and officially introduce you to the various tags:
/** This class describes Train objects. A train has a name and speed in miles per hour (speed must be between 0 and the constant MAX_SPEED). Train objects will be used to run over the Road Runner. @author Wile E. Coyote @version 1.0 */ public class Train { /** Constructs a new Train object with speed 0. @param aName - this Train's name. */ public Train(String aName) { name = aName; speed = 0; } /** Increases this Train's speed. @param moreMPH - by how much to increase this Train's speed. @throws IllegalArgumentException if newSpeed is bigger than MAX_SPEED. */ public void accelerate(int moreMPH) { checkMaxSpeed(speed + moreMPH); speed += moreMPH; } /** Decreases this Train’s speed. @param lessMPH - by how much to decrease this Train's speed. @throws IllegalArgumentException if newSpeed is negative. */ public void decelerate(int lessMPH) { checkMinSpeed(speed - lessMPH); speed -= lessMPH; } /** Returns this Train's name. @return this Train's name. */ public String getName() { return name; } /** Returns this Train's current speed. @return this Train's current speed. */ public int getSpeed() { return speed; } /** Returns true if this Train is moving, false if it is idle. @return true if this Train is moving, false if it is idle. */ public boolean isMoving() { return (speed != 0); } /** Stops this Train by changing its speed to 0. */ public void stop() { speed = 0; } /** Returns a String representation of this Train. @return a String representation of this Train. */ public String toString() { return "[Train " + name + " is now going at " + speed + " MPH]"; } /** Checks if the value of newSpeed is bigger than MAX_SPEED, thus illegal. @param newSpeed - the speed to test. @throws IllegalArgumentException if newSpeed is bigger than MAX_SPEED. */ private void checkMaxSpeed(int newSpeed) { if (newSpeed > MAX_SPEED) { throw new IllegalArgumentException( "can't go faster than " + MAX_SPEED + "."); } } /** Checks if the value of newSpeed is negative, thus illegal. @param newSpeed - the speed to test. @throws IllegalArgumentException if newSpeed is negative. */ private void checkMinSpeed(int newSpeed) { if (newSpeed <= 0) { throw new IllegalArgumentException( "can't have negative speed."); } } // instance fields private String name; private int speed; private final int MAX_SPEED = 500; }
As in lab 2, we see how our code begins with a brief description of the class, what it does and what is the function of an object of this class. This description can be as long and as elaborate as you think is necessary. Remember, this is a general description, not a detailed one and should give an overall picture of the class, so only include information you think is important.
After the class description we find two tags:
Following the class declaration we see that the constructor and all methods are preceded by a short description of what the method does. Note that all comment blocks start with a “/**” which contains two stars. This is important to generate the javadoc later on. Again, the information you supply here should be part of a general description of what each method does.
Immediately after the method description we have several tags:
These comments are the important ones for generating the javadoc pages. Regular comments starting with “//”, such as the one just before our instance fields in the Train class, will not be included in the javadoc pages, though that does not mean you should not include them in your code. Another important thing to note is the use of the phrase this Train in the comments. Remember that this code will be used to create various Train objects, each with a different name and speed. Thus, when we write this Train we mean the implicit parameter - the parameter that precedes the method call. By the way, if a method is static we use the class name instead of an implicit parameter, just like Duffy Duck called the pow method by stating
Math.pow(answer, 5.0);
One more thing, as you can see, writing a Train class in order to run over the Road Runner did not really work well for our friend Wile E. Coyote:
Maybe he should come up with a better idea…
Ok, while Wile is trying to escape from the train we will go on and generate some javadoc pages. Copy the Train class into TextPad and create the file Train.java. Save the file in your working directory (we will assume you are saving your file at J:\Lab4>). Now open the DOS command prompt and go to that directory. When there, issue the following command:
j:\Lab4> javadoc -version -author Train.java
This is the javadoc command. Note that we added the words -version and -author to our command line. This enables us to tell the javadoc tool that we are using the @version and the @author tags, since the javadoc default does not include these tag.
After you typed the command look in your working directory and you should find several HTML files. Open the file index.html. You should now be looking at a page that is similar in format to the API pages we saw at the beginning of the lab, only this one describes our Train class. Here are screen shots of the HTML file showing the class summary, method summary and method details:
Look over the class documentation. Can you see the class description we wrote? How about the various method descriptions? Find the place where the @version and the @author tags are used in the HTML file. Now look at the details of the accelerate method. Can you see where the @param and @throws tags are used?
Problem 3: (1 point) Take a closer look at the content of your javadoc page. Does it give details of all the elements in the Train class? Obviously not, otherwise, why would we ask this question? Which elements are not included (hint: there are 5 missing elements)? What do these 5 element have in common? Write you answer is a file called whatsMissing.txt and save it.
Look at the five missing elements. They are all declared as private. javadoc is used to give us information of how to use a class or method. Private methods and private instance fields can only be used by the class itself, not by other programs that use that class. And so, as programmers who merely use objects and methods of the class, we don’t need to know anything about private element of the class. Hence, javadoc will not include any information about private elements of a class. However, that does not mean you shouldn’t include comments for private elements in your code. After all, in 5 years when someone else would need to maintain and update code you wrote your comments will be quite useful.
Here’s another interesting piece of information - you can imbed HTML tags in your javadoc pages. First, if you have a long description you can add tags to format your text, like the <P>, <BR> etc. You can also add links. For example, when you write your name in the @version tag, you can add an <A> tag with your e-mail address at the HREF attribute (using the mailto: value).
javadoc is a very useful tool. Though you’re not likely to use it as often to generate javadoc pages for your own code, you are very likely to use the online API documentation to find other Java classes and methods and use them in your own code (it is possible to download the entire API to your hard drive, but it is extremely big and available for free online anyway, so why bother?). However, when you write your code always keep in mind that at some point you may want to generate javadoc pages for it, so follow the rules and comment your code accordingly. (plus, the Computer Science department requires you to follow these rules).
As for out friend Wile E. Coyote - he decided to go back to some less sophisticated and more straight forward ways to catch the Road Runner…
Problem 4: (3 points) In a last attempt to outsmart the Road Runner, Wile E. Coyote builds an elevator that can take him from his lookout point forty-two floors up on the top of the cliff, down to the valley where the Road Runner roams. Look at the following method signatures and write the proper comments and code for the Elevator class. The method up moves the Elevator up a certain amount of floors. The method move moves the Elevator to a specific floor. The method checkFloor returns true if the requested floor is between 1 and 42 (inclusive), and false otherwise. Feel free to use the Train class for inspiration. Save your well-commented code in the file Elevator.java.
class Elevator: Constructor: public Elevator() Methods: public void up(int amount) public void down(int amount) public void move(int newFloor) public int getFloor() private boolean checkFloor(int testFloor) Instance Fields: private int floor; private final int MAX_FLOOR = 42;
Problem 5: (3 points) Generate the proper javadoc pages for the Elevator class. Don't forget to include the @author tag.
Submit the following files to the instructor. (Follow your instructor's guidelines to submit the files.)
Do not submit any .class files.