Macros.message(view, "Hello world!");
Running this one line script causes jEdit to display a message box
(more precisely, a JOptionPane object) with the
traditional beginner's message and an OK button.
Let's see what is happening here.
This statement calls a static method (or function) named
message in jEdit's Macros class. If you
don't know anything about classes or static methods or Java (or C++,
which employs the same concept), you will need to gain some
understanding of a few terms. Obviously this is not the place for
academic precision, but if you are entirely new to object-oriented
programming, here are a few skeleton ideas to help you with
BeanShell.
An object is a collection of data that can be initialized, accessed and manipulated in certain defined ways.
A class is a specification of what data an object contains and what methods can be used to work with the data. A Java application consists of one or more classes (in the case of jEdit ,over 600 classes) written by the programmer that defines the application's behavior. A BeanShell macro uses these classes, along with built-in classes that are supplied with the Java platform, to define its own behavior.
A subclass (or child class) is a class which uses (or “inherits”) the data and methods of its parent class along with additions or modifications that alter the subclass's behavior. Classes are typically organized in hierarchies of parent and child classes to organize program code, to define common behavior in shared parent class code, and to specify the types of similar behavior that child classes will perform in their own specific ways.
A method (or function) is a procedure that works with data in a particular object, other data (including other objects) supplied as parameters, or both. Methods typically are applied to a particular object which is an instance of the class to which the method belongs.
A static method differs from other methods in that it does not deal with the data in a particular object but is included within a class for the sake of convenience.
Java has a rich set of classes defined as part of the Java platform. Like all Java applications, jEdit is organized as a set of classes that are themselves derived from the Java platform's classes. We will refer to Java classes and jEdit classes to make this distinction. Some of jEdit's classes (such as those dealing with regular expressions and XML) are derived from or make use of classes in other open-source Java packages. Except for BeanShell itself, we won't be discussing them in this guide.
In our one line script, the static method
Macros.message() has two parameters because that is
the way the method is defined in the Macros class. You must
specify both parameters when you call the function. The first parameter,
view, is a variable naming the current, active
View object.
Information about pre-defined variables can be found in the section called “Predefined Variables in BeanShell”.
The second parameter, which appears to be quoted text, is a
string literal - a sequence of characters of
fixed length and content. Behind the scenes, BeanShell and Java take
this string literal and use it to create a String
object. Normally, if you want to create an object in Java or BeanShell,
you must construct the object using the new keyword
and a constructor method that is part of the
object's class. We'll show an example of that later. However, both Java
and BeanShell let you use a string literal anytime a method's parameter
calls for a String.
If you are a Java programmer, you might wonder about a few things
missing from this one line program. There is no class definition, for
example. You can think of a BeanShell script as an implicit definition
of a main() method in an anonymous class. That is
in fact how BeanShell is implemented; the class is derived from a
BeanShell class called XThis.
If you don't find that helpful, just think of a script as one or more
blocks of procedural statements conforming to Java syntax rules. You
will also get along fine (for the most part) with C or C++ syntax if you
leave out anything to do with pointers or memory management - Java and
BeanShell do not have pointers and deal with memory management
automatically.
Another missing item from a Java perspective is a
package statement. In Java, such a statement is
used to bundle together a number of files so that their classes become
visible to one another. Packages are not part of BeanShell, and you
don't need to know anything about them to write BeanShell macros.
Finally, there are no import statements in
this script. In Java, an import statement makes
public classes from other packages visible within the file in which the
statement occurs without having to specify a fully qualified class name.
Without an import statement or a fully qualified name, Java cannot
identify most classes using a single name as an identifier.
jEdit automatically imports a number of commonly-used packages
into the namespace of every BeanShell script. Because of this, the
script output of a recorded macro does not contain
import statements. For the same reason, most
BeanShell scripts you write will not require import
statements.
Java requires import statement to be located at
the beginning of a source file. BeanShell allows you to place
import statements anywhere in a script, including
inside a block of statements. The import statement
will cover all names used following the statement in the enclosing
block.
If you try to use a class that is not imported without its fully-qualified name, the BeanShell interpreter will complain with an error message relating to the offending line of code.