Java程序辅导

1Lecture 1
Elementary Applications
By tradition, the first program in many programming
language is named HelloWorld. It is designed to show the user the
elements of syntax, compiling, and displaying a very simple
program. In addition there are various issues about word
processors and directories that need to be considered.
The program we shall write will output the string
Hello World!!!!!
The directory structure on the computer that is being used
to prepare these notes is on my Z drive with subdirectory 11Pic20
which, in turn will have subdirectories Lecture_1, Lecture_2,
etc. The structure is shown below:
You need to create a directory structure on your Z or C
drive; the particular one does not matter. For the sake of
simplicity, mimic the one being used here. Create a directory
11Pic20 on your Z drive and then create a subdirectory of 11Pic20
called Lecture_1.
Our program is:
public class HelloWorld
{
public static void main(String args[])
{
System.out.println("           Hello World!!!!!");
}
}
Using a simple editor such as Notepad type this into a file
called HelloWorld.java. Do not use Word or any other fancy word
processor. Such processors have various commands imbedded in
2their files which confuse the compiler. When you save it to a
directory use the "All files" option of the editor
The complete address of the program is
Z:\11Pic20\Lecture_1\HelloWorld.java
The most important point of syntax at this time is that the name
of the class HelloWorld, in the first line, be identical to the
name used in the .java file HelloWorld.java.
There is only one instruction in this program
System.out.println(" Hello World!!!!!");
Note that it ends with a semi-colon. All instructions in java end
with a semi-colon.
As for the remainder, just type in what is here.
Explanations for the code will come later.
To compile the program, assuming that the HelloWorld.java
file is in the directory mentioned above, open the DOS command
window change, your directory to Z:\11Pic20\Lecture_1\HelloWorld,
and then execute the command javac HelloWorld.java. See below:
After you have typed in "javac HelloWorld.java" and pressed the
Enter key you will get something similar to the above screen. The
"Symantic Java.." message may or may not appear; it depends on your
enviroment. If you have not made any errors the appearance of the
last line,Z:\11Pic20\Lecture_1>_, means that the program has
compiled. If you have made an error, the compiler will print out
a message, giving you a clue as to where the error might be.
To run the program execute the command java HelloWorld. The
string " Hello World!!!!!", will appear:
3If you look at your final directory at this point (see
below) you will see two files of importance with respect to java.
One is the text file HelloWorld.java. The other,
HelloWorld.class, was produced at compile time when you executed
the java HelloWorld command. The other files listed are connected
to the Lecture_1.doc file.
Some Generalities
Several generalities are in order at this point. Java
programs are classes; a program has to be defined within a class.
The program itself is typed in as a NAME.java text file, using
a simple word processor, such as Notepad. The program is compiled
using the command javac NAME.java. It is run with the command
java NAME. The compiler is case sensitive.
4Obtaining the Compiler
If you wish to obtain the free java compiler for use on
your own computer go to the location indicated below, scroll down
to the "Download Java 2 SDK .." part, and then press the
"continue" button.
Once you download and "unpack" it, the jdk system will be
placed, by default, in C:\jdk1.3. The compiler,javac, and similar
programs will be in C:\jdk1.3\bin.
You will need to add to PATH variable the autoexec.bat file
to use the programs javac and java in any directory on your
computer.
I find that working with autoexec.bat files is a gift given
to us by Bill Gates. Therefore, I do not guarantee that the
following will work for you. It did work for me.
I did this on my computer as follows: First, the original
autoexec.bat file was saved as backupautoexec.bat; make sure you
make a backup file. Next, the autoexec.bat file, which can be
opened with Notepad, contained the line
PATH=C:\DOS
Notice that there is no semi-colon at the end of the line. Change
this line to
PATH=C:\DOS;C:\jdk1.3\bin
5Note again that there is no semi-colon at the end of the line.
Save and close the file. Then go to the DOS command line and run
the command autoexec.bat from the C drive. If the prompt line
C:\>_ appears as the last line on your screen, all is well. If it
doesn't you will be glad you made the backup file earlier.
Exercises
The exercises below are designed to give you practice in reading
and interpreting the error messages that arise at compile time.
You are to type in each class as given (as a Notepad NAME.java
file), compile (using javac NAME.java), go back and correct your
text file, and then run the program (using java NAME).
1)
public class First
{
public static void main(String, args[])
{
System.out.println("Now is the time..");
}
}
2)
class Second
{
public void main(String args[])
{
System.out.println("What is wrong now?");
}
3)
public class Third
{
public void static main(String args[])
{
System.out.println();
System.out.println(" Adair Plumbing");
System.out.println()
System.out.println("Don't sleep with a drip tonight");
System.out.println("Call Adair);
}
}
6Lab 1
Write a program called Design that will produce the following
output:
The program should produce the above array of X's. There is one blank
line between the command line and the first line of Xs. There are 10
blank spaces before the first X in the first line with Xs.
The text file for your program must be called Design.java. Keep
in mind that the java compiler is case senstive.
DETAILS ON FILE NAMES DIRECTORIES
When your account in the student lab is created it is created
with a directory called SUBMIT. I suggest you create a second directory
whose name can be anything you please, say Mystuff. Do all your initial
work in Mystuff. Then when your program is working correctly, copy and
paste the files of the lab assignment (Design.java and Design.class for
this assignment) to your SUBMIT directory.
The name for the .java text file will be specified for each lab
assignment. The grader will run another program that will run your
program. If your SUBMIT file does not contain the files with the exact
specifed name you will not be credited for that assignment.