Learning Java, 4th Edition

Scripted Methods and Objects

You can declare and use methods in BeanShell, just as you would inside a Java class:

    int addTwoNumbers( int a, int b ) {
        return a + b;
    }
    sum = addTwoNumbers( 5, 7 );  // 12

BeanShell methods may also have dynamic (loose) argument and return types.

    add( a, b ) {
        return a + b;
    }
    foo = add(1, 2);                // 3
    foo = add("Hello ", "Kitty");   // "Hello Kitty"

In BeanShell, as in JavaScript and Perl, method closures can take the place of classes for scripting objects (but in BeanShell you can also use the regular class syntax). You can turn the context of a method call into an object reference by having the method return the special value this. You can then use the this reference to refer to any variables that were set during the method call. To be useful, an object may also need methods; so in BeanShell, methods may also contain methods at any level. Here is a simple example:

    user( n ) {
        name = n;
        reset() {
            print( "Reset user:"+name );
        }
        return this;  // return user as object
    }
    bob = user("Bob" );

    print( bob.name ); // "Bob"
    bob.reset();      // prints "Reset user: Bob"

This example assigns the context of the user() method to the variable bob and refers to the field bob.name and the method bob.reset().

If you find this strange, don’t worry. The most common reason you’d want to script an object is to implement a Java interface, and you can do that using the standard Java anonymous inner class syntax, as we’ll discuss next, or just use a regular class. BeanShell gives you a lot ...