Spring Semester 2005
The environment used in this class is Windows XP. Your desktop might look like this:
In this class we will be writing computer programs.
A computer program is just another file.
We use text editors to create files.
In this class we will be using
Notepad to create and edit our programs.
Notepad you might have to go through something like this:
Once you start
Notepad a blank editor window will show up on your screen:
In this class we will be using the Java programming language to write programs.
You will find Java as peculiar as any other foreign language.
Java uses some English words but also curly braces, square brackets, semicolons, parens and such.
They all have a meaning of their own. It is the purpose of this class to demystify this language for you.
Notepad to create the simplest possible Java program. Type exactly as indicated below:
When you're done you need to save the file:
You never know where
Notepad wants to save the file you just created.
In the picture below we're in some folder called
But we really don't want to be there.
Instead we want to save the file we just created on the
Desktop was selected, we're ready to give the file a name and save it.
Java programs need to be saved as files with extension
That means the default extension
Notepad is offering (
not what we need.
So we change it to... whatever else is available.
In this case only
All Files is available.
All files on the
Desktop are listed.
We need to give our file a name and
Hello.java is the only name we can use in this case.
.java is mandatory and the name (
Hello) is the same as the identifier that follows the keyword
class in the code.
Once we save the file an icon should indicate that on the
Can you see it at the top? (Go by the name listed under the icon, since the icon is arbitrary).
Programs need to be compiled before the computer can make any sense of them.
The Java programming language (arcane as it may look to you) is still readable.
The language that the computer really understands (in this case bytecode) is not readable by normal human beings.
If we compile the program we wrote into Java bytecode the computer will be able to understand it.
We need to use the
javac utility to compile the program.
To run the
javac utility we need to get a command prompt window.
To obtain one you might need to go through a menu like this:
The command prompt window looks like the one shown below:
Notice there is a working directory (folder) you're in.
Since your program is on the desktop you should move there (with
And once you move the window will be updated.
You can list the files in the directory, so you can also check on your
Hell* indicates all the files whose names start with
There's only one and some information about the file is listed (size, date of creation).
To compile you invoke the
javac utility on the file name (
This simply means you need to type:
javac Hello.java at the command prompt.
If you make no mistakes in typing the file exactly as shown the compilation succeeds and a new prompt is given to you.
But most of the times we make mistakes. In that case you have to go back and correct them.
Finding and correcting errors is what a programmer typically does.
But eventually no errors remain and the compilation succeeds.
When that happens additional files will get created.
In this case one additional file (
Hello.class) which contains the bytecode.
Compilation of a program is only the first step.
Through it the computer is provided with a complete set of instructions (for a specific task) in its own language.
For the computer to perform the task the program instructs it to, we need to execute the program.
Execution of the program is accomplished with the help of the
Our program is the simplest possible program: one that does nothing.
But even the requirement of doing nothing has to be expressed in Java.
And it has the form you just saw.
Let's change the program and make it do something.
We need to change the source code (the file we developed earlier).
Creating or editing files is done with a text editor.
And then make your intentions of opening an existing file known.
Notepad shows you the contents of the desktop because that's where you last saved a file.
But it assumes that you want to look at a text file (one that has a
So change this to
All Files and then you will see even the file that
you want to open.
Hello.java and push the
And the contents of what we created just a few minutes ago will be loaded in the
Change the source code (the contents of the file) by typing a new line, as shown.
Save the file.
No questions will be asked this time around, and
Since the file has been updated you can compile it.
And if there are no compilation errors you can execute (or run) the program.
Now the program works as expected, printing one line of text.
Tue Jan 11 09:34:52 EST 2005