Examples of Functors, Transformers, Predicates, and Closures in Java

One day, I found myself re-designing a procurement portal, and I kept re-writing the same for-loop over and over again (no pun intended). I had an epiphany; I could do better and I started using the Apache Commons Collection Utilities (Transformers, Predicates, and Closures). Now don’t think just because I started to use the Apache Commons Collection Utilities, the project was better. However, the result was a highly extensible framework…. Later it was dismantled by another team… but that is a different story (Grin).

Functors

I laugh every time I think of the word Functors, but that is because I’m immature, case in point, I still laugh at fart jokes. Anyway, Functors, or Function Objects, in the Apache or Jakarta Commons Collection Utilities are a set of interfaces designed specifically to be used against collections of objects. This framework embodies a balance between code reuse and behavioral specialization through composition as opposed to strict Object Oriented design. Composition is well suited for Creational patterns such as Factories, Structural patterns like Decorators, or Behavioral patterns like Strategies. The Apache Commons Collections framework defines three types of interfaces:

• Closures are functions that can alter the object and get a reference to each object in the collection.


• Transformers are responsible for transforming data from one format to another or from one object to another.


• Predicates simply execute a conditional test against each item in a collection and return true or false for each item.

NOTE:

My examples sometimes use Anonymous Inner Classes and Inner Classes. Some developers have strong feelings about defining classes in this manner. There are times when doing this is appropriate and times when it is inappropriate. As with any programming solutions, this technique may or may not suit your needs or environment. So, let’s get over it and move on.

Closure

Here is a typical problem statement which maybe resolved with the use of Closures. I want to execute a specific method or change the state on every object in a collection. For example, I might want to execute the toString method. Please note that these examples do not pull out a value and transform into another collection of objects. They simply iterate over the collection and do something to it or with it. For the purpose of this first example, I will send the results of the toString to system.out. In the second example, I will alter the state of each bean and change the name of every object. Both examples use the utility method CollectionUtils.forAllDo.

Lets take some baby steps, and look at this example.

The Code:

package com.blogspot.apachecommonstipsandtricks.transformersexamples;
import java.util.*;
import org.apache.commons.collections.*;
import org.apache.commons.lang.*;
import com.blogspot.apachecommonstipsandtricks.*;
public class SimpleClosure
{
public static void main(String[] args)
{
  System.out.println("\nTest Number One Results :");
  List<String> collectionOfWords = Arrays.asList("Java", "Example",
                                                 "Help", "Tips", "And",
                                                 "Tricks", "Apache",
                                                 "Commons", "Collections");
  // Lets call toString on every object and print it out.
  CollectionUtils.forAllDo(collectionOfWords, new Closure()
  {
    public void execute(Object o)
    {
      assert o != null;
      System.out.print(o.toString() + " ");
    }
  });
  System.out.println("\n\nTest Number Two Results :");
  int i = 1;
  List<DTO> collectionOfDTOs = Arrays.asList(new DTO(i++, "Java Tips and Tricks", Gender.Male, State.WI),
                                             new DTO(i++, "Apache Commons"      , Gender.Male, State.WI),
                                             new DTO(i++, "Jakarta Commons"     , Gender.Male, State.WI),
                                             new DTO(i++, "Collections"         , Gender.Male, State.WI),
                                             new DTO(i++, "Closures"            , Gender.Male, State.WI) );
  CollectionUtils.forAllDo(collectionOfDTOs, new Closure()
  {
    public void execute(Object o)
    {
      DTO dto = (DTO) o;
      assert dto != null;
      String s = StringUtils.defaultIfEmpty(dto.getName(), "null");
      dto.setName("Yoda says, " + s + " Rocks!");
    }
  });
  CollectionUtils.forAllDo(collectionOfDTOs,PrintIt.getInstance());
}
}

The Results:

Test Number One Results :
Java Example Help Tips And Tricks Apache Commons Collections

Test Number Two Results :
com.blogspot.apachecommonstipsandtricks.DTO{id=1, name='Yoda says, Java Tips and Tricks Rocks!', gender=Male, state=WI}
com.blogspot.apachecommonstipsandtricks.DTO{id=2, name='Yoda says, Apache Commons Rocks!', gender=Male, state=WI}
com.blogspot.apachecommonstipsandtricks.DTO{id=3, name='Yoda says, Jakarta Commons Rocks!', gender=Male, state=WI}
com.blogspot.apachecommonstipsandtricks.DTO{id=4, name='Yoda says, Collections Rocks!', gender=Male, state=WI}
com.blogspot.apachecommonstipsandtricks.DTO{id=5, name='Yoda says, Closures Rocks!', gender=Male, state=WI}

You might have noticed I created a class called PrintIt and it has a static method called getInstance. This class is an implementation of the singleton design pattern. In a large system where Closures (Predicates and Transformers) are being created by the hundreds, it makes sense to keep the memory foot print to a minimum. This is accomplished by using a singleton pattern. Word of caution, there is only one instance of this class per Java Virtual Machine (jvm), so using static properties could cause big problems, if you don’t understand how they work.

The Code:

package com.blogspot.apachecommonstipsandtricks.transformersexamples;
import org.apache.commons.collections.*;
public class PrintIt implements Closure
{
// This class implements a Singleton Pattern
private static PrintIt ourInstance = new PrintIt();
/**
 * Get a singleton instance of PrintIt
 */
public static PrintIt getInstance()
{
  return ourInstance;
}
private PrintIt() // This is a singleton, dont change this!
{
}
public void execute(Object o)
{
  System.out.println( o.toString() );
}
}

Once again, in the first example, all we did was iterate through a collection and call toString on every element in the collection. In the second example, we are actually modifying the state of the bean by rewriting the name of the DTO.. I love Yoda! Lets talk about transformers now.

Transformer

I’m a little older than the Transformer cartoons, but I cant help but think of the movie that was released not long ago. So, here is a problem statement that would best be resolved by transformers. You just have a whole bunch of string values from a Http Request object and you need to convert them to Integers.

The Code:

package com.blogspot.apachecommonstipsandtricks.transformersexamples;
import java.util.*;
import org.apache.commons.collections.*;
public class SimpleTransformer
{
public static void main(String[] args)
{
  Collection<String> stringOfNumbers = Arrays.asList("1", "2", "3", "4");
  Collection<Integer> intNums = CollectionUtils.collect(stringOfNumbers, new Transformer() {
      public Object transform(Object o) {
          return Integer.valueOf((String) o);
      }
  });
  CollectionUtils.forAllDo(intNums, PrintIt.getInstance() );
}
}

The Results:

1
2
3
4

Again, I used the PrintIt class to print the results, but you get the idea with this example. I converted a collection of Strings to Integers. Of course there are many ways to skin a cat, this is just an example. You might have noticed by now, that the interface is public Object transform(Object o)… It is not a Java 1.5 Generics implementation. I’m not entirely sure why the Apache team hasn’t released a Java 1.5 Generics version of these utilities, but if you are in a pinch and you have to have it, someone posted a link to an adaptation of the library that has the Generics. http://larvalabs.com/collections

Here is a more practical problem statement. You have a collection of plain old java beans and in each bean a method that returns a String, part of the String represents the id into another system, environment or sub identity. For example, the String might be prefixed with three letters, for example "PAS". You might find some legacy ERP systems that do this to the PO number. Maybe the billing department puts the initials of the sales person or team at the front of the number. In this example we will transform an Array of Strings that have ids in them into an array of ids in the form of numbers only.

The Code:

package com.blogspot.apachecommonstipsandtricks.transformersexamples;
import java.util.*;
import org.apache.commons.collections.*;
import org.apache.commons.lang.*;
public class SimpleTransformer
{
public static void main(String[] args)
{
  Collection<String> stringOfNumbers = Arrays.asList("ABC0001", "BCD0002", "CDF0003", "BFA0004");
  Collection<Integer> intNums = CollectionUtils.collect(stringOfNumbers, new Transformer()
  {
    public Object transform(Object o)
    {
      String s = ((String) o);
      return Integer.valueOf(s.substring(3, s.length()));
    }
  });
  CollectionUtils.forAllDo(intNums, PrintIt.getInstance());
}
}

The Results:

1
2
3
4

Now let’s look at something a little more practical. As a problem statement, let’s say we have a billing object from the old system called OldBill, and we need to identify it in the new system, called NewBill. The ids in the old system started with “A” and a number. In the new system they will start with “Z” and the number from the old system plus 500. In the next example we will break apart the concerns of the transformers into two different transformers and glue them together with the utility class TransformerUtils.chainedTransformer. Creating a transformer in this manner allows us to plug in a different behavior or add and subtract behaviors, even on the fly.

The Code:

package com.blogspot.apachecommonstipsandtricks.transformersexamples;
import java.util.*;
import org.apache.commons.collections.*;
public class ChainedTransformer
{
public static void main(String[] args)
{
  List<OldBill> aList = Arrays.asList(new OldBill("A1"), new OldBill("A2"),
                                      new OldBill("A3"), new OldBill("A4"));
  Transformer[] chainedTransformer = new Transformer[]{
          new Transformer() {
              public Object transform(Object o) {
                  return ((OldBill )o).getId().replace('A', 'Z');
              }
          },
          new Transformer() {
              public Object transform(Object o) {
                  char[] c = ((String) o).toCharArray();
                  int x = Integer.parseInt(String.valueOf(c[1])) + 500;
                  return new NewBill( String.valueOf(c[0]) + x );
              }
          }
  };
  System.out.println("The aList");
  CollectionUtils.forAllDo(aList, PrintIt.getInstance());
  List<NewBill> bList = (List<NewBill>) CollectionUtils.collect(aList, TransformerUtils.chainedTransformer(chainedTransformer));
  System.out.println("\nThe bList");
  CollectionUtils.forAllDo(bList, PrintIt.getInstance());
}
}
package com.blogspot.apachecommonstipsandtricks.transformersexamples;
public class OldBill
{
private String id;
public OldBill(String id)
{
  this.id = id;
}
public String getId()
{
  return id;
}
public void setId(String id)
{
  this.id = id;
}
@Override public String toString()
{
  return "OldBill{id='" + id + "\'}";
}
}
package com.blogspot.apachecommonstipsandtricks.transformersexamples;
public class NewBill
{
private String id;
public NewBill(String id)
{
  this.id = id;
}
public String getId()
{
  return id;
}
public void setId(String id)
{
  this.id = id;
}
@Override public String toString()
{
  return "NewBill{id='" + id + "\'}";
}
}

The Results:

The aList
OldBill{id='A1'}
OldBill{id='A2'}
OldBill{id='A3'}
OldBill{id='A4'}

The bList
NewBill{id='Z501'}
NewBill{id='Z502'}
NewBill{id='Z503'}
NewBill{id='Z504'}

Predicate

Predicates do one thing and one thing only, they return either true or false. As a problem statement, let’s say we have a collection of Strings and we want keep out values that can not be converted to numbers.

The Code:

package com.blogspot.apachecommonstipsandtricks.transformersexamples;
import java.util.*;
import org.apache.commons.collections.*;
public class SimplePredicate
{
public static void main(String[] args)
{
  List<String> mixedup = Arrays.asList("A", "0", "B", "C", "1", "D", "F", "3");
  Collection numbersOnlyList = CollectionUtils.predicatedCollection(new ArrayList(),
      new Predicate() {
          public boolean evaluate(Object o) {
              try {
                  Integer.valueOf((String) o);
                  return true;
              } catch (NumberFormatException e) {
                return false;
              }
          }
      });
  for (String s : mixedup) {
      try {
          numbersOnlyList.add(s);
      } catch (IllegalArgumentException e) {
          System.out.println("I love CollectionUtils!");
      }
  }
  System.out.println("\nResults of the predicatedCollection List:");
  CollectionUtils.forAllDo(numbersOnlyList, PrintIt.getInstance() );
}
}

The Results:

I love CollectionUtils!
I love CollectionUtils!
I love CollectionUtils!
I love CollectionUtils!
I love CollectionUtils!

Results of the predicatedCollection List:
0
1
3

Ok here is something more useful. Have you ever had a need to do something like SQL but in Java? For example, you wanted to select beans from a collection based on some conditional blocks? Here are some great examples of SQL commands that can be used in Java and they behave just like SQL complex where clauses, distinct, like, and group by. In this example, we will be using the folloing utlitly classes and maps

• PredicateUtils.allPredicate
• PredicateUtils.anyPredicate
• PredicateUtils.notPredicate
• PredicateUtils.uniquePredicate
• TransformedMap.decorate
• MultiValueMap

The Code:

package com.blogspot.apachecommonstipsandtricks.transformersexamples;
import java.util.*;
import org.apache.commons.collections.*;
import org.apache.commons.collections.map.*;
import com.blogspot.apachecommonstipsandtricks.*;
public class PredicatesSQLSample
{
public static void main(String[] args)
{
  List<DTO> list = Arrays.asList(new DTO(1,"Bob",  Gender.Male, State.WI), new DTO(2,"Larry",Gender.Male,   State.WI),
                                 new DTO(3,"Bill", Gender.Male, State.WI), new DTO(4,"Sue",  Gender.Female, State.AZ),
                                 new DTO(3,"Bill", Gender.Male, State.WI), new DTO(4,"Sue",  Gender.Female, State.AZ),
                                 new DTO(5,"Joe",  Gender.Male, State.AZ), new DTO(6,"Zoe",  Gender.Female, State.MI));
  Predicate sqlOrQueryPredicate = PredicateUtils.anyPredicate(new Predicate[]{
    new Predicate()
    {
      public boolean evaluate(Object o)
      {
        return State.WI.equals(((DTO) o).getState());
      }
    }, new Predicate()
  {
    public boolean evaluate(Object o)
    {
      return Gender.Female.equals(((DTO) o).getGender());
    }
  }
  });
  Predicate sqlAndQueryPredicate = PredicateUtils.allPredicate(new Predicate[]{
    new Predicate()
    {
      public boolean evaluate(Object o)
      {
        return State.AZ.equals(((DTO) o).getState());
      }
    }, new Predicate()
  {
    public boolean evaluate(Object o)
    {
      return Gender.Male.equals(((DTO) o).getGender());
    }
  }
  });
  Predicate likeNameStartsWithB = new Predicate(){
    public boolean evaluate(Object o)
    {
      return ((DTO) o).getName().startsWith("B");
    }
  };

  Collection aList = CollectionUtils.select(list, sqlOrQueryPredicate);
  Collection bList = CollectionUtils.select(list, PredicateUtils.notPredicate( sqlOrQueryPredicate ));
  Collection cList = CollectionUtils.select(list, sqlAndQueryPredicate);
  Collection dList = CollectionUtils.select(list, PredicateUtils.allPredicate(new Predicate[]{PredicateUtils.uniquePredicate(), sqlOrQueryPredicate} ));
  Collection eList = CollectionUtils.select(list, PredicateUtils.allPredicate(new Predicate[]{PredicateUtils.uniquePredicate() ,likeNameStartsWithB} ));
  Collection fList = CollectionUtils.select(list, PredicateUtils.uniquePredicate() );

  Map aGroupByStateMap = TransformedMap.decorate(new MultiValueMap(),new Transformer(){
    public Object transform(Object o)
    {
      return ((DTO) o).getState();
    }
  }, TransformerUtils.nopTransformer() );
  for (Object o : fList)
  {
    aGroupByStateMap.put( o, o );
  }

  System.out.println("\nAll the people :\nselect * from list");
  CollectionUtils.forAllDo(list,PrintIt.getInstance());
  System.out.println("\nAll the people in Wisconsin OR Female :\nselect * from list where ( state = WI or gender = female );");
  CollectionUtils.forAllDo(aList, PrintIt.getInstance());
  System.out.println("\nAll the people NOT ( Wisconsin OR Female ) :\nselect * from list where ! ( state = WI or gender = female );");
  CollectionUtils.forAllDo(bList, PrintIt.getInstance());
  System.out.println("\nAll the people in Arizona AND Male :\nselect * from list where ( state = AZ and gender = male );");
  CollectionUtils.forAllDo(cList, PrintIt.getInstance());
  System.out.println("\nAll the distinct people in Arizona AND Male :\nselect distinct * from list where ( state = WI or gender = female );");
  CollectionUtils.forAllDo(dList, PrintIt.getInstance());
  System.out.println("\nAll the distinc people with the name that starts with B :\nselect distinct * from list where name like \"B%\";");
  CollectionUtils.forAllDo(eList, PrintIt.getInstance());
  System.out.println("\nAll the distinct people grouped by state :\nselect distinct * from list group by state;");
  Set states = aGroupByStateMap.keySet();
  for (Object state : states)
  {
    System.out.println(((State)state).getFullyQualifiedName());
    CollectionUtils.forAllDo((Collection) aGroupByStateMap.get(state), PrintIt.getInstance());
  }
}
}

The Results:

All the people :
select * from list
com.blogspot.apachecommonstipsandtricks.DTO{id=1, name='Bob', gender=Male, state=WI}
com.blogspot.apachecommonstipsandtricks.DTO{id=2, name='Larry', gender=Male, state=WI}
com.blogspot.apachecommonstipsandtricks.DTO{id=3, name='Bill', gender=Male, state=WI}
com.blogspot.apachecommonstipsandtricks.DTO{id=4, name='Sue', gender=Female, state=AZ}
com.blogspot.apachecommonstipsandtricks.DTO{id=3, name='Bill', gender=Male, state=WI}
com.blogspot.apachecommonstipsandtricks.DTO{id=4, name='Sue', gender=Female, state=AZ}
com.blogspot.apachecommonstipsandtricks.DTO{id=5, name='Joe', gender=Male, state=AZ}
com.blogspot.apachecommonstipsandtricks.DTO{id=6, name='Zoe', gender=Female, state=MI}

All the people in Wisconsin OR Female :
select * from list where ( state = WI or gender = female );
com.blogspot.apachecommonstipsandtricks.DTO{id=1, name='Bob', gender=Male, state=WI}
com.blogspot.apachecommonstipsandtricks.DTO{id=2, name='Larry', gender=Male, state=WI}
com.blogspot.apachecommonstipsandtricks.DTO{id=3, name='Bill', gender=Male, state=WI}
com.blogspot.apachecommonstipsandtricks.DTO{id=4, name='Sue', gender=Female, state=AZ}
com.blogspot.apachecommonstipsandtricks.DTO{id=3, name='Bill', gender=Male, state=WI}
com.blogspot.apachecommonstipsandtricks.DTO{id=4, name='Sue', gender=Female, state=AZ}
com.blogspot.apachecommonstipsandtricks.DTO{id=6, name='Zoe', gender=Female, state=MI}

All the people NOT ( Wisconsin OR Female ) :
select * from list where ! ( state = WI or gender = female );
com.blogspot.apachecommonstipsandtricks.DTO{id=5, name='Joe', gender=Male, state=AZ}

All the people in Arizona AND Male :
select * from list where ( state = AZ and gender = male );
com.blogspot.apachecommonstipsandtricks.DTO{id=5, name='Joe', gender=Male, state=AZ}

All the distinct people in Arizona AND Male :
select distinct * from list where ( state = WI or gender = female );
com.blogspot.apachecommonstipsandtricks.DTO{id=1, name='Bob', gender=Male, state=WI}
com.blogspot.apachecommonstipsandtricks.DTO{id=2, name='Larry', gender=Male, state=WI}
com.blogspot.apachecommonstipsandtricks.DTO{id=3, name='Bill', gender=Male, state=WI}
com.blogspot.apachecommonstipsandtricks.DTO{id=4, name='Sue', gender=Female, state=AZ}
com.blogspot.apachecommonstipsandtricks.DTO{id=6, name='Zoe', gender=Female, state=MI}

All the distinc people with the name that starts with B :
select distinct * from list where name like "B%";
com.blogspot.apachecommonstipsandtricks.DTO{id=1, name='Bob', gender=Male, state=WI}
com.blogspot.apachecommonstipsandtricks.DTO{id=3, name='Bill', gender=Male, state=WI}

All the distinct people grouped by state :
select distinct * from list group by state;
ARIZONA
com.blogspot.apachecommonstipsandtricks.DTO{id=4, name='Sue', gender=Female, state=AZ}
com.blogspot.apachecommonstipsandtricks.DTO{id=5, name='Joe', gender=Male, state=AZ}
MICHIGAN
com.blogspot.apachecommonstipsandtricks.DTO{id=6, name='Zoe', gender=Female, state=MI}
WISCONSIN
com.blogspot.apachecommonstipsandtricks.DTO{id=1, name='Bob', gender=Male, state=WI}
com.blogspot.apachecommonstipsandtricks.DTO{id=2, name='Larry', gender=Male, state=WI}
com.blogspot.apachecommonstipsandtricks.DTO{id=3, name='Bill', gender=Male, state=WI}

Well, that wraps it up. Next week I plan on showing you some more examples of this fantastic api called Apache Commons.

Author: Philip A Senger Google+