Lab 2 - The DrJava Development Environment

Purpose

This lab introduces the DrJava programming environment. You will use DrJava to develop and test programs for both lab and homework assignments.

Setup

• Create a subdirectory named nhLab in your home directory if you do not already have one. This can be done in a terminal window:

• Using a browser, download http://www.cs.uno.edu/~fred/nhText/Labs/lab2.zip into your nhLab directory and unzip.
• You should now have a subdirectory in your nhLab directory named lab2. This subdirectory will contain several files with an extension .java. These are text files containing program segments written in Java. They are called source files.

DrJava

DrJava is an easy to use software development environment for writing Java programs. It was created by the Computer Science Department at Rice University, and is currently under active development by the JavaPLT group at Rice. DrJava is designed primarily for students, and provides an interactive facility for developing Java code. It includes an editor and debugger, and incorporates the JUnit testing framework and javadoc documentation generating tool.

DrJava is discussed in Appendix II of the text. You can download DrJava for use on your home system from http://drjava.sourceforge.net/ or http://drjava.org.

Starting DrJava

• Open a terminal window and type drJava at the prompt:

Be sure to key upper and lower case as shown. After a few moments a window similar to that shown below will appear.

DrJava's main window is divided into three panels. The largest area to the right is the definitions pane. You write and edit class definitions in the definitions pane. The open document list displays a list of open documents. You can view and edit a particular document by selecting it from the list. When you click on a document in the list, it will be displayed in the definitions pane. The bottom panel contains several tabbed panes. Clicking the tab displays the pane. For the present, we will be concerned with the interactions pane and the compiler output pane. You can enter and evaluate java statements and expressions in the interactions pane. Compilation results are displayed in the compiler output pane.

Configuring DrJava

• Choose the Preferences option from the Edit pull-down menu. A Preferences window similar to that shown below will appear.

Click each of the Categories to get an idea of the options that can be set.

Since both DrJava and Java are being actively developed, the preference options available may not be exactly as described here. Furthermore, some of the options will have been set for you already. Your lab instructor will tell you what you need to do to properly configure DrJava. DrJava will remember your option settings. Options do not need to be set every time you run DrJava.

• Click the category Resource Locations. The location of the local libraries (nhUtilities) should be listed in the Extra Classpath box. It will be something like /apps/java. Your home directory should also be listed. If you do not see this, ask your lab instructor.
• Click the category Javadoc. The box labeled Custom Javadoc Parameters should contain the following:

If it does not, check with your lab instructor.

• Click the category Miscellaneous. Verify that the item Automatically Close Block Comments is checked. If it is not, click the check box and click Apply.
• Click the category Display Options. Note that there is a check box labeled Show All Line Numbers. If this box is checked, any document displayed in the definitions pane will be shown with numbered lines. Though some programmers like this option, we suggest that you leave this box unchecked for now.
• Dismiss the Preferences window by pressing OK.

Exercising the interactions pane

• In the DrJava interactions pane, key

Be sure that you are keying next to the last "> " prompt in the window. Key the line exactly as shown, including the semicolon at the end. Press the "Return" or "Enter" key at the end of each line.

This creates an int variable named i in the DrJava environment:

• Store the value 17 in the variable:

        > i = 17;    

The equals sign "=" doesn't mean "equals." It means "store the value given on the right into the variable named on the left." We can see the value stored in the variable by keying the name of the variable:

Note there is no semicolon at the end of the line. (We show DrJava's response in slanted font.)

• Change the value stored in the variable to 18:

        > i = 18;    

• Now do the following:

Keying the expression i+1 gives us the value stored in i plus 1. What does i*2 produce?

• Create a float variable and give it a value:

The expression 1.7E7 is a way of saying 1.7×107: that is, seventeen million.

• Now key

and note the result. Keying

says "f is equal to f+1." Can you explain the answer? What result do you get when you key

• What happens if we attempt to store the value of f into i?

The definitions pane

• Click Open on the DrJava tool bar, and navigate to your lab2 subdirectory. The file Counter.java should be located in this directory. It is one of the files you copied in the previous step. Select this file and open it. the contents of the file will be displayed in the definitions pane.

There are several things you should notice. First, the text is displayed in several different colors. Each color denotes a different kind of "lexical component." For instance, words like "public" and "class" are key words. Key words are displayed in a unique color. The color coding will help you recognize keying mistakes when you key a program.

Second, the text is indented in a uniform way. This indention reflects the structure of the program and makes it easier to read. When you key in a program, DrJava automatically indents for you. If the indention is not what you expect, it is often a sign that you have made a syntactic mistake.

• Click at the beginning of the line that reads "* Create a new Counter..." and enter a few spaces. Do the same with the next two or three lines.
• Select these lines by clicking and dragging the mouse. (Portions of the lines should be highlighted.)
• Press the Tab key. Notice that the original indenting is restored. In DrJava, you can use the Tab key to properly indent the current line or a selected group of lines.

Finally, DrJava will show matching parentheses, braces, and brackets by highlighting. Again, this will help you find and avoid mistakes when you key a program.

• Click in the definitions pane immediately after a closing brace, "}". Note that the code up to and including the matching open brace is highlighted. Do the same with a closing parenthesis, ")"

Navigating

• Open the Preferences window again. (Choose Preferences from the Edit pull-down menu.)
• Click the Display Options category, and activate the Show All Line Numbers check box in the Display Options pane. Click Apply and OK.

You will notice that each line in the display pane is now numbered. Error messages often mention a line number. You can also navigate to line by number by choosing the Go To Line... option from the Edit pull-down menu, or by keying Control-G. (The line numbers do not need to be displayed to do this.)

• Type Control-G, enter 30 in the filed labeled `What line would you like to go to?" and click OK.

You can also navigate the display pane by choosing Find/Replace from the Edit pull-down, or by keying Control-F.

• Go to the Preferences window once move, and deactivate Show All Line Numbers.

Compiling

You will notice that the document name "Counter" is followed by an asterisk "*" in the open document list. This means that the document has been modified in the definitions pane but the changes have not been saved to disk. (You modified the document when you added and removes the spaces.)

• Click the Save button on the tool bar to write the changes to disk.

Next, you will compile the class definition: that is, invoke a program called a compiler that will translate the source into "machine level" code that can be interpreted by the Java interpreted. (The interpreter is called the "Java virtual machine.") The compilation process will produce a new file named Counter.class in your lab2 directory.

• Click the Compile button on the tool bar.

Something is wrong. Notice that the Compiler Output tabbed pane at the bottom of the window is selected, and it indicates that five errors were found. The first error is highlighted, as well as the line in the definitions pane that caused the error.

• Click on the next error in the compiler output pane. Notice that it is now highlighted, with a different line.

If you read the error messages and look at the highlighted lines, you will notice that they all have something to do with the symbol "count". The problem is that we have forgotten to define this variable. (Don't be concerned if you don't understand this yet. This exercise is just to get you familiar with using DrJava!)

• Click in the definitions pane, at the start of the blank line following the line reading "public class..."
• Press Return/Enter to create a new line, and key in the following

followed by Return/Enter to insert another blank line. Note that DrJava automatically indents what you have keyed.

The beginning of the document should now look like this:

• Click Save and Compile. This time you should see the message "Last compilation completed successfully." in the compiler output pane.

Generating a specification document

• Click the Javadoc button on the tool bar. Javadoc is a program that creates HTML specification documents.
• You will be asked to "Select a destination directory for the Javadoc files." The default will (probably) be a directory named doc in your home directory. Click the button containing the ellipses (...), and select a directory named doc in your lab2 directory. If the directory does not exist, you will asked if you would like to create it. Respond Yes.
• In a few seconds, a Javadoc Viewer will open, displaying the specifications of the classes defined in your lab2 directory.
• If Counter is not displayed in the right pane of the Javadoc Viewer, click Counter in the list of classes shown in the left pane.
• Scroll down and look at the specification shown in the Javadoc Viewer. Compare the specification in the Javadoc Viewer with the definition of the class in the definition pane. The specification as shown in the Viewer is what a programmer who wants to make use of a class will see. The specification must give a complete and clear description of the features of the class.
• Close the Javadoc Viewer.

As mentioned above, Javadoc produces a collection of HTML documents. These are usually viewed with a browser.

• Open a browser on your desktop (Firefox, Opera ...), and navigate (under the Open or Open File selection) to the doc directory in your lab2 directory. Open the file index.html. Click on Counter in the left browser frame. Note that your browser displays the same contents as the Javadoc Viewer.
• Your instructor might require you to print the document from your browser.

Testing a class

JUnit is a framework we will use to test our classes. Unfortunately, as this is written DrJava does not yet support the latest version, JUnit 4. The Test button on the tool bar will run an earlier version which we will not use.

You can obtain the latest version of JUnit for your personal system from http://www.junit.org.

• Open the Preferences window again. (Choose Preferences from the Edit pull-down menu.)
• Click the Resource Locations category, and make sure that the JUnit jar file appears in the Extra Classpath window. This will be a file named something like /usr/junit/junit4-4.jar. If it does not, consult your instructor.
• Close the Preferences window by clicking OK or Cancel.
• Open the file CounterTest.java. This should be in your lab2 directory. This file contains the definition of a class used to test the Counter class. Note that the open document list includes both open files.
• Click Compile.
• In the Interactions pane, key

• If the Interactions pane is reset, click on the Console tab to see the results of the test. At the end of the test output, you will see the message

• Scroll up to the start of the test output. You will see a message something like this

The problem is a Counter should start at 0, but it starts at 1 as written.

• Display the class Counter in the display pane by clicking on Counter in the open document list.
• You will see the following code about a dozen lines or so lines from the top of the document:

Change the "1" to "0" in the line "count = 1". Save and Compile.

• Run the test again. This time you will see the message

in the Console pane. (You can clear the Console pane by choosing Clear Console from the Tools pull down menu, or by right-clicking in the Console pane.)

• Close CounterTest. (Click CounterTest in the open documents list, and then click Close on the tool bar.)
• Your instructor might require you to print the document Counter. Display the class Counter in the display pane by clicking on Counter in the open document list, choose the Print option form the File pull-down menu.

Running a program from DrJava

• Open the files CounterUI.java and Count.java. These should be in your lab2 directory. These modules along with Counter constitute a complete program.
• Click Compile.
• Click Count in the open document list to display this file in the display pane. Click the Interactions tab to display the interactions pane.
• Click the Run button on the toll bar. Output from the program will appear in the interactions pane. Input is keyed into the boxes displayed in the interactions pane. (Terminate your input line by pressing Return/Enter.)
• Exercise the program a bit to see how it behaves.

You might want to spend a few minutes experimenting with some of the options available in the Edit and Tools pull-down menus.

What to submit

If directed by your instructor, turn in output from the browser, and listings of the modified Counter class and the interactions history for the session. Be sure everything you turn in is clearly labeled with your name and section number.

You can print the interactions history as follows.

• Select the Save Interactions History... option from the Tools pull-down menu. (You can also get this option by right-clicking in the interactions pane.)
• Save in a file named temp.history in your lab2 directory.
• Print this file from a terminal window. Use the following command or one provided by your instructor: