Step 3: How to edit a project

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This step will show you how to edit a source file in Eclipse.

  1. Launch the Eclipse IDE if it is not currently running. Return to Step 1 of this tutorial if you need help completing this step.

  2. We need to create a source file where we will write our program. Right-click on the "HelloWorld" project in the Package Explorer tabbed pane on the left side of the screen and navigate to New → Class, as in the following example:
    New File

  3. Another window will be displayed. Then click in the text box labeled "Name:" and type in HelloWorld, as in the following example:
    New Class Wizard with HelloWorld for the Name
    Click Finish. This step will create a new file named and add it to the project for you. It will also open a tabbed window where you can edit the source file.

  4. Now that we've completed the setup we can write the program. Enter the following text into the window.
    Hello World source code
    As you edit the file, you may notice red underlines appearing. This is Eclipse's attempt to correct you, the same way a word processor attempts to correct spelling. If you follow the above and type the code correctly, the red lines will eventually disappear, though it is normal for some to appear as you are writing programs. If there are still some left, compare the above image with the file you wrote until you find the difference. You can align the braces as shown in the code examples in the textbook or align them as shown in the code above. The difference with the code above will not cause problems.

    Also, note that even if the red lines disappear, this doesn't mean that your program will work correctly! Just as your word processor only knows how to spell and the rudiments of grammar, it does not how to write an essay. Eclipse knows what a valid Java program looks like, but doesn't understand whether or not the program you wrote does what you want it to. For a simple program like the above, it may very well work as soon as it is recognized as a valid Java program, but for more complicated programs, validity is only the tip of the iceberg.

    It's also worth mentioning that Eclipse will fill things in for you. For example, if you put a left parenthesis (, it will follow it up with a right parenthesis ). You can keep on typing and fill in the space between the parentheses. When you're done, you can either manually type the end parenthesis ), which will type over the old one, or you can press the right arrow key to skip past the old one.

  5. Adding the following file header to the top of the file and fill in the fields appropriately. The Title is the name of the overall program (HelloWorld in this case) and Files is the list of source files you created (just in this case). A file header must be included with all of the files you hand in. Instead of typing the header yourself you can select (click and drag over the text) the text below and choose "Copy" from the "Edit" menu, or hold Ctrl and press C on your keyboard. Then put the insertion point at the top of your file and choose "Paste" from the "Edit" menu, or hold Ctrl and press V on your keyboard.
  6. ///////////////////////////////////////////////////////////////////////////////
    // Title:            (program's title)
    // Files:            (list of source files)
    // Semester:         Fall 2014
    // Author:           (your name and email address)
    // CS Login:         (your login name)
    // Lecturer's Name:  (name of your lecturer)
    // Lab Section:      (your lab section number)
    // Pair Partner:     (name of your pair programming partner and their email address)
    // CS Login:         (partner's login name)
    // Lecturer's Name:  (name of your partner's lecturer)
    // Lab Section:      (your partner's lab section number)
    // Credits:          (list anyone who helped you write your program)
    //////////////////////////// 80 columns wide //////////////////////////////////

    While you are editing the source code, it is a good idea to save your work frequently. The more often you save your work, the less work you will have to recreate in the event of a system crash or other interruption in your work.

    You save the file by clicking on the disk icon. Files that have been modified since last save will have an asterisk (*) before the file name, as in the above image.

  7. COMMENTS: Now, is a good time to have you add a class comment (after the file header and before the start of the class construct) and a method comment (before the start of the main method definition) to your source code. This image shows includes the file header, a class comment, and a method comment.
File header comment, then class header comment, then start of class construct, then method header comment, then method, then end of method, then end of class.

Find the commenting guide and style guide on the course website's menu. You're almost there! The next step will be to run the program.

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Original version created by Deb Deppeler and Sue Hazlett
Updated for Eclipse by Martin Hock
Updated for lab section use by Beck Hasti
2010-2012 Updated by Jim Skrentny
2014 Updated for v4.4 and added Editor Preferences by Deb Deppeler