This is a good post which tries to demostrate the power of closures and functors.

It also helps illustrate how noisy things can be in Java. e.g.

import org.apache.commons.functor.*;
import org.apache.commons.functor.core.composite.*;
import org.apache.commons.functor.adapter.*;
import org.apache.commons.functor.UnaryFunction;
import org.apache.commons.functor.core.Constant;
import org.apache.commons.functor.core.IsEqual;
import org.apache.commons.functor.core.comparator.IsGreaterThanOrEqual;
import org.apache.commons.functor.core.comparator.Min;
import org.apache.commons.functor.core.Identity;


     UnaryFunction getItemCat = 
        new UnaryFunction()
        public Object evaluate (Object obj)
                return ((SETLItem)obj).getCategory();
        UnaryFunction getItemPrice = 
        new UnaryFunction()
        public Object evaluate  (Object obj)
                return new Double(((SETLItem)obj).getPrice());
        Constant catA = new Constant("A");
        Constant usd200 = new Constant(new Double(200));
        BinaryPredicateUnaryPredicate belongsToCatA = new BinaryPredicateUnaryPredicate
          (new UnaryCompositeBinaryPredicate(new IsEqual(), getItemCat, catA));
        BinaryPredicateUnaryPredicate moreThanUSD200 = new BinaryPredicateUnaryPredicate
          (new UnaryCompositeBinaryPredicate(new IsGreaterThanOrEqual(), getItemPrice, usd200));
        UnaryOr isEligibleForDiscount = new UnaryOr(new UnaryAnd(belongsToCatA, moreThanUSD100), 
          new UnaryAnd(belongsToCatB, moreThanUSD200));

        if (isEligibleForDiscount.test(item1))
          System.out.println("Item #1 is eligible for discount!");
          System.out.println("Item #1 is not eligible for discount!");

whereas we could do something like this in Groovy

isEligibleForDiscount = { it.category == "A" && it.price > 200}
if (isEligibleForDiscount(item1) {
  println "Item #1 is eligible for discount!"
else {
  println "Item #1 is not eligible for discount!"

In IT you can usually use any tool to solve any problem. However sometimes switching tools makes things much easier & simpler.