Post on functional programming in Java - maybe a tad verbose

Added by James Strachan, last edited by James Strachan on Jul 15, 2004

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!");
        else
          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.

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Comments (1)

 

  1. Jul 23, 2004

    Tom Sorgie says:

    While closures in groovy are really cool. This example is doing a really bad j...

    While closures in groovy are really cool. This example is doing a really bad job of making the point about java and the fuctors library. The following (much shorter) java code has the same result.

       UnaryPredicate isEligibleForDiscount = new UnaryPredicate() {
        public boolean test(Object obj) {
            SETLItem item = (SETLItem)obj;
            return "A".equals(item.getCategory()) && item.getPrice() > 200;
        }
   };   

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