Question 4.1.1: Some code to get you started. First, please visit
the following Website
http://davin.50webs.com/J.T.W/download-links.html for the
programs that you need to have installed before you can do any coding
in J.T.W. You should then download a tarball (also known as a
compressed archive file):
http://davinpearson.com/binaries/java-training-wheels.tar.gz
containing the code you need to get started. Then unzip the tarball
and change directory to java-training-wheels
and issue the
following command on GNU/Linux systems: ./configure
or on MS
Windows systems: bash configure
. If you are using
M.S. Windows and your HOME
variable is unset, then you will
need to set it to a sensible value. Examples of sensible values for
your HOME
variable include, c:\
or c:\home
or d:\home
if your d
drive is a hard drive. To set the HOME
variable in
windows, press Windows E and right click on My Computer
(Windows XP) or This Computer (Windows 10) and click on
Properties, then click on Advanced system settings, then click
on Advanced, then click on New environment variable to set the
HOME
variable.
When you run the configure
script you will be prompted for the
location of prefix directory and the location of the place to keep
your *.jtw
files. You will also be asked if you want to install
just Davin’s jtw-mode
or Davin’s full version of Emacs.
NOTE: If are reading this file on your local filesystem then you would have already completed this question.
Question 4.1.2: Your first J.T.W. program. Traditionally in
computer science the first program that you write in any programming
language is a program that does nothing else but prints out "Hello,
World". The following code does just that. In the following code, note
the use of the class
construct. In Java and J.T.W., every piece of
program code that does some real computational work resides in a class
of some description.
class
MyFirstProgrambegin
beginMain
System.out.println("Hello, World!");endMain
end
The code for any class
X
in these tutorials should reside in a
file called X.jtw
. Therefore the above code should be put into a
file called MyFirstProgram.jtw
. If two classes X
and Y
use
each other and X
contains the main
function
then it is convenient
to place them both in a file called X.jtw
. To build and run some
code, you first need to be in the ~/jtw-tutorials
folder and
secondly you need to issue the following shell command: make X.run
where X
is the name of the class
that you want to run, so it is
make MyFirstProgram.run
in this case. For all the questions that follow this one, it will be assumed that you know how to do this. See How to build a collection of class files or an entire package for more information about building classes that use other classes in different files or building entire packages.
Question 4.1.3: Multiple calls to System.out.println()
. Change
the above code from printing the string "Hello, World!" to printing
out the following messages. Please note that it will be easiest to use
multiple calls to System.out.println()
which sends text to the
screen for the purpose of viewing.
Hello, Anne! How are you doing? Hello, Brian! How are you doing? Hello, Clare! How are you doing?
Question 4.1.4: Functions,parameters and arguments. A function
is
a piece of code that does some computational work and optionally
returns a value. Notice how the hello
function
below takes a value
of whose name to say hello to. This value who is called a
parameter. The values passed to the parameter by the call to the
function
is called an argument. For the purposes of this question, add
two more calls to the hello function
in the main
function
to get the
same result as the code for the previous question. The keyword
void
indicates that this function
does not return a value. See the
next question for a function
that does return a value.
class
MySecondProgrambegin
function
void
hello(String who)begin
System.out.println("Hello " + who + ", how are you doing?");end
beginMain
hello("Anne");endMain
end
Question 4.1.5: Return values. Notice how the following hello
function
returns a string rather than printing out the string. Add two
more calls to the hello
function
below to get the same result as
for Question 4.1.4.
class
MyThirdProgrambegin
function
String hello(String who)begin
return "Hello " + who + ", how are you doing?";end
beginMain
System.out.println(hello("Anne"));endMain
end
Question 4.1.6: Ignoring return values. In J.T.W. and Java, it is
not necessary to use a value that is returned by a function
. Sometimes
this wastes computational resources since the value that is computed
by the function
is not used but other times when the function
whose
value is to be ignored does some additional work by setting the
value(s) of some variable(s) to different values then the function
call is not a waste of resources. To ignore the value returned by the
hello function
, simply call the function
without using the value like
so: hello("Ignored");
For the purposes of this question, try
calling the hello function
without using the return value by adding a
line of code to the main
function
.
Question 4.1.7: Comments. Study the following code. Note the use
of comments. Comments are used to disable code for debugging purposes
and also to help explain how a program works. The most useful comment
in J.T.W. and Java is /** until the first */. This type of
comment is harvested by Javadoc to produce documentation on how a
class
works. The second and third most useful comments are
(respectively) // until the end of the line and /* until
the first */. The third type of comment is not very useful because in
Java you are not allowed to have one comment inside another, so if you
use this type of comment you will constantly need to search for and
remove */ closing comments. In the tutorials that follow you will see
many comments, although mainly the first and second types of comments.
/** This comment is harvested by Javadoc to document the MyFourthProgram class */class
MyFourthProgrambegin
// I am a single line comment /* I am a multi-line comment */ /** This comment is harvested by Javadoc to document the hello function */function
String hello(String who)begin
return "Hello " + who + ", how are you doing?";end
) /** This comment is harvested by Javadoc to document the main function */beginMain
System.out.println(hello("Anne"));endMain
end