Programming Tools: Java Scripting Languages

Groovy's documentation provides an example that illustrates the benefits of using closures in place of anonymous classes. Using an anonymous class, we can write:

Button b = new Button ("Push Me");
  b.onClick (new Action() {
    public void execute (Object target)
    {
      buttonClicked();
    }
  });

Using a closure, this becomes:

Button b = new Button ("Push Me");
  b.onClick { buttonClicked() }

Another nice feature of Groovy is its "each" operator, which is used to iterate over a collection:

SomeCollection stuff = new SomeCollection();
  stuff.each() someClosure

// for example:

def myList = [1,2,4,5,6]
myList.each { println it }  // where 'it' is current collection item

// outputs:

1
2
3
4
5
6

Groovy also has built-in structured "builders" for languages such as XML, HTML and more. Again, a simple example illustrates how natural it is to build structured text:

xml = new groovy.xml.MarkupBuilder()
def myList = ['acct1':['id':123111, 'type':'savings', 'balance':1234.56],
              'acct2':['id':221212, 'type':'checking', 'balance':2010.02]]
doc = xml.accounts() {
  for (entry in myList)
    acct(id:entry.key) {
      for (thing in entry.value)
        item(name:thing.key, type=thing.value)
    }
}

This small code fragment outputs:

<accounts>
  <acct id='acct2'>
    <item name='type'>checking</item>
    <item name='id'>221212</item>
    <item name='balance'>2010.02</item>
  </acct>
  <acct id='acct1'>
    <item name='type'>savings</item>
    <item name='id'>123111</item>
    <item name='balance'>1234.56</item>
  </acct>

Bindings

In the following example, the use of variable c is caught at compile time in a statically defined language. Only when a is greater than b is the use of the undefined variable, c, detected in an interpreted language.

a = 1
b = 2
if a > b:
  print c
print a,b

Another major difference between a compiled language and an interpreted one is when things are bound to their references. In Groovy, the following code prints "Bound to local variable":

def varA = "Bound to global variable"
def closure = { varA }
public class C {
  def varA = "Bound to local variable"
  def closure = { varA }  // bound to local varA at definition time
  public def f = closure  // f bound to local closure
};
def c = new C();          // create instance of C using new
println c.f()             // invoke f in C

in Jython, the equivalent-looking code prints "Bound to global variable":

varA = "Bound to global variable"
closure = lambda: varA
class C:
  varA = "Bound to local variable"
  closure = lambda self: varA
  f = closure
c = C()
print c.f()

For those wanting more flexibility, the Jython/Python bindings are handier. For those wanting more stability, the Groovy implementation may be more desirable.

Currying

Currying function arguments is another difference. Groovy has a special "curry" mechanism to bind arguments to a function. In the following example, "foo bar" is printed:

def c = { arg1, arg2-> println "${arg1} ${arg2}" }
def d = c.curry("foo")
d("bar")

Jython inherits Python's natural ability to curry arguments using a number of techniques. One is:

def c(arg1, arg2): print arg1,arg2
def d(arg2): c("foo",arg2)
d("bar")

Maturity

Jython has been around a long time and is based on a mature language, Python. However, its development has stalled in recent years. Groovy is a relatively new language and thus still is developing. For example, its error diagnostics leave a lot to be desired. Also, at the moment, Groovy's following is much smaller than Jython's or Python's. However, both languages are picking up development activity, so you have a chance to influence both languages if you want to become involved.