Downloading, Compiling and Executing Java programs

Overview

In this module, you will learn how to:

Background needed:

Note: We will describe how to get things done in Windows (Win-XP or Win-2000). Linux users already know how to issue commands from the commandline. Mc-OS users may know, but if not, can run a Unix-like commandline. Luckily, for compiling and running Java programs, the actual commands are very similar in all three operating systems.

Getting used to the commandline

There are two ways to compile and run programs:

  • Method 1: Use a development environment. These are sophisicated software packages entirely targeted at helping programmers write programs. Examples include: JGrasp, Eclipse, SunOne and JBuilder. These combine an editor, compiler and project-manager all into one along with some program development tools beginners don't need.
  • Method 2: Install a compiler, use your own editor and compile/execute from the command-line.
It's worth trying both, just to see how they work. We'll focus on the more elemental command-line approach here.
  • Step 1: First, lfire up a command-line window:
    • Go to "Start", then "Run".
    • This will bring up the "Run" window. Type cmd in the "open" field, and hit return or "OK".

    • This should bring up a command-line window that looks this:

  • Step 2: Create a new directory in which to play:
    • Use cd .. to go up in the directory structure, mkdir to make new directory, and dir to examine the contents.
    • For example, let's create a directory called myproject off of the c: drive. Notice that we backed up from a directory called Administrator (which happens to be the username of this user), then applied mkdir myproject to create the directory, and then typed cd myproject to enter the directory itself. Finally, we examine the list of files (none) in the directory itself using dir.

Editing

Now, we will use a simple text editor like Notepad or Wordpad and create a Java program:

  • Step 1: Fire up the editor. Go to "Start", then "Programs", then "Accessories", then click on either Notepad or Wordpad. This will bring up the editor.
  • Step 2: In your browser, go to this link.
  • Step 3: Cut and paste from the browser into Notepad:

  • Step 4: Save the file into the new directory that you created above.
  • Important: Don't make any changes to the text that you pasted into Notepad.

Installing Java

  • First, check whether Java is installed on your computer. Type javac in the command-line window. If it's NOT installed, you'll get an error like:

    If it's installed, you'll see a list of options show up. For now, let's ignore those options. You can skip the rest of this section.

  • Install Java. Follow Step 1 of these instructions to install Java.

Compiling and executing

Now that Java has been installed, you should be able to compile our test program called HelloWorld.java.

  • Follow Step 2 of these instructions to compile and execute.
  • When you typed javac, that part is called compilation, which prepares a Java program for execution.
  • When you typed java, that actually causes a Java program to execute.

Compiling and executing a more complex program

  • Download this Java program: HelloWorldWindow.java.
  • Compile at the commandline by typing javac HelloWorldWindow.java
  • Execute by typing java HelloWorldWindow
  • This is what you should see:

How do you get rid of a Java program when you're done? Two ways:
  • The easiest is to type Control-C (control key and "C" held together) in the commandline window from which you executed the program.
  • If your program is set up for "quitting", then you can close the window by clicking on the "X" at the top-right. Note: This won't always work because not all programs have been set up for it, in which case you need to use the Control-C option.

Exercise: Instead of printing "HelloWorld!" to the screen, can you make the program print your name instead?

  • Bring up the program in an editor.
  • Go to the line where it says drawString ... etc.
  • Change the string inside the double-quotes to your name.
  • Compile and execute.
Congrats, you've just edited (slightly!) a program, compiled it and executed it.

Exercise: Sometimes programs come in multiple files. For example, the same HelloWorld program can be written in three files. To see how this works, download the three files HelloWorldWindow2.java, TestFrame2.java, and TestPanel2.java, then compile the first one and execute by typing java HelloWorldWindow2 at the command-line.

Exercise: You need to try one more thing: unpacking an archive. Create a fresh directory, then download this ZIP file into the new directory and unpack it. Then follow the steps in the exercise above.

What does it all mean?

OK, thus far we've learned how to compile and execute a program, possibly without understanding what we're doing. So, let's explain.

Let's start by looking at compilation and execution:

  • First, software vs. hardware. Hardware, as the word sounds, is all the solid stuff a computer is made of, that will make a sound if you drop it on the floor. Software, on the other hand, lives entirely in electronic form, and consists of programs, each of which are a collection of instructions that "run" or "execute" on the hardware. All it is, really, is a collection of detailed instructions telling the hardware what to do and when.
  • Now, the hardware is only able to execute programs written in "hardware" language, consisting of instructions that are so low-level that nobody wants to write programs directly for hardware. A program that's written in low-level instructions that is ready to execute is called an executable program.
  • Software developers instead write programs in higher-level languages that have names you may have heard of: C, C++, Java and so on.
  • Since the hardware can't directly execute these high-level programs, we use a special tool (itself a piece of software) called a compiler that translates the high-level program into a low-level program suitable for the hardware. Thus, when you run a compiler, it (the compiler) examines a high-level program (sometimes called the source-code), and translates that into an executable.
  • All software that you download and install consists of executable programs. You very rarely (as you did today), download high-level programs to first compile and then execute.
  • If you work with the source-code, then you can make changes to the program, compile and execute (as you did in the exercise above).
  • If you wish, you can create your own programs from scratch in a high-level language. This is the task called programming, or software development when it's done at a larger scale. To do this, however, you need to learn a little computer science.

Now, let's say a few words about Java:

  • Java is one among many high-level languages.
  • One of the simplest examples of a Java program is the first HelloWorld.java program you saw earlier.
  • Let's examine this program: 
      public class HelloWorld {

    public static void main (String[] argv)
    {
      System.out.println ("Hello World!");
    }
  }
  • Note that, like English has words, this language has words like static and argv (How can you not like a word like argv?!) as well as syntactic elements like braces, parentheses and semi-colons.
  • Generally, while English is somewhat forgiving in that a writing course might forgive spelling errors, a compiler will refuse to compile a program that isn't 100% syntactically correct. This fussiness causes beginning programmers to pull their teeth initially, but all programmers eventually get good at avoiding syntax errors.

    Exercise: Deliberately create a syntax error and see what happens. For example, misspell the word class.

  • If you like, you can learn more about a simple Java program by reading through this material, which is taken from an intro course (no background needed).
  • Do not expect to understand programs without taking courses or spending time on this matter. It's like learning a foreign language - it's not going to happen in a week.

What next

To learn more about programming and computer science:

  • See this article: Why Scientists Should Learn Computer Science. It's written for faculty, but should make sense to students too.
  • Learn yourself: You can use this intro course on Java that's based on Java applets. This will take time, and you won't become an expert anytime soon.
  • There are a gazillion books that proclaim to teach you Java or C or whatever in 21 days. There are equally many websites with the same claim. For a contrasting view, see this article by Peter Norvig.
  • Take courses. At GW, you would take CSci-53 and CSci-133 to get started. Or talk to an advisor.