NetRexx is a programming language designed as an alternative to Java. The idea behind NetRexx, which was created by IBM Fellow M.F. Cowlishaw (who also designed Rexx), is to make programming the Java Virtual Machine (JVM) easier than with Java. Although NetRexx syntax is derived from Rexx, NetRexx is strongly typed, making it first-rate for programming the JVM and allowing access to primitive (binary) datatypes. NetRexx compiles to Java bytecode, and NetRexx classes and Java classes are equivalent. In short, when your development requirements include web programming or other applications for the JVM, you may well find it easier and more productive to program in NetRexx than Java.
NetRexx is freely available under IBM's Employee Written Software agreement and can be downloaded from http://www2.hursley.ibm.com/netrexx/ as either a ZIP file or a UNIX tar.Z file. It comes with instructions for installation and use, and also includes extensive documentation on the language definition and keyword instructions. Cowlishaw's book, The NetRexx Language (Prentice Hall, 1997), provides a complete reference.
Because NetRexx programs run on the JVM, they are fully portable and can be executed on any platform where Java is present. This includes Windows, OS/2, many flavors of UNIX, OS/390, and (soon) VM.
A NetRexx program contains the following:
classinstruction defines a new class, including the name of the parent class and any behaviors that it implements.
propertiesinstruction is optional; it defines class and instance variables. Properties may be public or private, so there may be more than one
propertiesinstruction in a class.
methodinstruction identifies the beginning of a new method. There is no special syntax for the end of a method; a method ends when a new method begins.
As with Rexx, you don't need to declare variables before using them. NetRexx automatically determines the proper type of a variable from the context of its first use. Properties must be defined at the beginning of a class. Though it is not necessary to explicitly type them, it may be convenient to do so at this point. All other variables are local to the method that uses them.
Terms in NetRexx are evaluated in a simple and intuitive fashion that may be new to many programmers. NetRexx evaluates each term in the context in which it is first encountered. This allows new keywords to be added to the language without breaking existing programs. Simplified, the full search order is as follows:
- A local variable or method argument?
- A method in the current class?
- A property in the current class?
- A special word, such as
- A data type?
- If not, it must be a language keyword or instruction.
In Example 1, the
select in the first line of the
action method is recognized as a NetRexx keyword. However, if you insert
select = 'blotz' at the beginning of that method, then the
select appearing alone in a clause would have been detected as an error by the NetRexx compiler -- "unexpected use of local variable." Using
select as a local variable in another method in the class would not preclude its recognition as a keyword in this method.
Example 1: The action method.
NetRexx also uses compound terms, which are evaluated from left to right as in Java. Thus, the term
u.left(1) is the value of the term
u, to which the NetRexx method
left is applied with an argument of 1.
Recall that you don't have to declare variables in NetRexx. However, to be a first-class language for the JVM, NetRexx must be strongly typed. This can be largely transparent to the programmer, however, because NetRexx provides a default type for variables, the
Rexx type, which is equivalent to the strings of the classic Rexx language. The NetRexx compiler automatically performs the necessary type conversion when translating a program to Java bytecodes.
Using variables of type
Rexx significantly increases programmer productivity. You are freed from concern over the types of variables in a program, and the number of keystrokes required to type in the program is reduced.
NameList in Listing One are all of type
Rexx, but they came to this state from different directions:
- The most common way to use a
Rexxvariable is to simply assign it a value and use it in your code (this is illustrated by the variable
Validmethod). Within the context of the program,
registeredis used as a Boolean, but there is no need to declare it as such.
Rexxvariables that are assigned a value resulting from a Java method invocation (
getText()) in the
Registermethod; since the value returned by
getText()is of type
String, we explicitly cast this value to type
Rexxbefore assigning it.
NameListis a private property that must be defined at the beginning of the class. Although I've declared a type here, it would be equally correct to simply list it as a property with no type or initial value.
/* Registry.nrx -- Applet for users to access private information */class Registry extends Applet -- inherit from & extend the Applet class properties private -- class and/or instance variables bclear = Button(" Clear ") -- Java Button class breg = Button(" Register ") bacc = Button(" Access ") name = TextField(15) -- Java TextField class pw = TextField(15) statlbl = Label NameList = Rexx method init -- initialization method LoadList -- initialize the names array method start setLayout(BorderLayout()) -- our frame's layout (Java methods) setBackground(Color.white) setForeground(Color.blue) p1 = Panel() -- a different layout for the buttons p1.add(bclear) p1.add(breg) p1.add(bacc) add("Center", p1) stat = Panel() statlbl = Label(" ".copies(100), Label.CENTER) stat.setBackground(Color.white) stat.setForeground(Color.red) stat.setFont(Font("Helvetica", Font.BOLD+Font.ITALIC, 22)) stat.add(statlbl) add("South", stat) -- add the panel to the frame p2 = Panel() -- a container for the "center" stuff p2.setLayout(BorderLayout()) -- but we want some control p2.setBackground(Color.blue) -- set fg and bg colors for this area p2.setForeground(Color.white) p3 = Panel() -- one piece of the "center" stuff ulabel = Label("Enter Username:") -- some label text name.setBackground(Color.white) -- fg and bg colors for the text field name.setForeground(Color.black) p3.add(ulabel) -- populate this piece p3.add(name) p4 = Panel() -- another piece of the "center" stuff plabel = Label("Enter Password:") -- some label text pw.setBackground(Color.white) -- fg and bg colors for the text field pw.setForeground(Color.black) pw.setEchoCharacter(char "*") -- echo character for sensitive data p4.add(plabel) -- populate this piece p4.add(pw) p2.add("North", p3) -- now put the two pieces into the p2.add("South", p4) -- "center stuff" container add("North", p2) -- and add that container to the frame /* Handle an action (button press) */ method action(e=Event, o=Object) returns boolean select when e.target = bclear then do -- if Clear button, name.setText('') -- set contents of text fields pw.setText('') -- to null statlbl.setText(String " ") -- and clear message area return 1 end when e.target = breg then do -- if Register button, if Register then Status("RegOK") -- and successful, return 1 -- put up nice dialog end when e.target = bacc then do -- if Access button, if Valid then AccessPrivate -- if registered, let them in else Status("BadID") -- otherwise, an error dialog return 1 end otherwise return super.action(e, o) -- other action we don't handle end /* Sign up a new user */ method Register u = Rexx name.getText -- get contents of text fields p = Rexx pw.getText if u = '' then do -- if username missing, Status("NOUSER") -- put up message to tell them return 0 -- that's a no-no; and return end -- failure if p = '' then do -- if password missing, Status("NOPW") -- put up message to tell them return 0 -- that's a no-no; and return end -- failure NameList[u] = p -- otherwise, add to list return 1 -- and return success /* Validate the current user */ method Valid returns boolean u = Rexx name.getText if u = '' | u.left(1) = ' ' then return 0 registered = 0 loop thisname over NameList if u == thisname then do registered = 1 leave end end if \registered then return 0 p = Rexx pw.getText if NameList[u] \== p then return 0 return 1 /* Put some initial users into the list */ method LoadList NameList = '' NameList['mickey'] = 'ClubLeader' NameList['minnie'] = 'mouseketeer' NameList['donald'] = 'aDuck' /* Status Reporting */ method Status(reason='') select when reason = "RegOK" then msg = "Registration Complete!" when reason = "NoUser" then msg = "Error: Username Required" when reason = "NoPW" then msg = "Error: Password Required" when reason = "BadID" then msg = "Error: Userid and/or Password Missing or Invalid" otherwise msg = "Error: Unknown error; get help" end statlbl.setText(String msg) method AccessPrivate statlbl.setText(string "Welcome to the Private Area") /* addr = -- specify URL address here url = URL(addr) statlbl.setText(String url.getContent) */