In my earlier post, I have shown demo of External Complete Reverse Iterator for Iterator Pattern.In this post, I am going to show demo of External and Internal Selective Iterator done in Java. Selective Iterator iterates through the collection but leaves the elements which do not meet criteria specified in predicate logic.
For this to implement using Iterator pattern, major requirements are : functor interface, concrete functor class, predicate interface, concrete predicate class, customized collection (like Vector, Arraylist), Object class, selective Iterator and main application.
Functor interface and class are used to implement certain function. The function is included in an object which executes the function based on input parameters. The functor call is like below.
Functor<Swimmer> functor = new SwimmerFunctor(JTextArea2);
functor.process(swimmer);
And its implementation is like below.
public void process(Swimmer swimmer){
textArea.append(swimmer.getFname()+” “+swimmer.getLname()+ ” \n”);
}
Using this technique to delegate work to another object is called functor pattern.
Predicate is also a special functor but it always return true or false. In my example, I have used predicate to check whether the age of swimmer is above or equal to 12 or not.
A selective Iterator uses predicate to check whether to iterate through a particular element or not.
The iteration loop becomes like this
Functor<Swimmer> functor = new SwimmerFunctor(JTextArea2);
Predicate<Swimmer> pred = new SwimmerPredicate();
Iterator<Swimmer> swIterator = slist.selectiveIterator(pred);
while(swIterator.hasNext()){
Swimmer swimmer = swIterator.next();
functor.process(swimmer);
}
If iteration is placed inside the collection, then it becomes Internal selective Iteration. If placed outside, it becomes External Selective Iteratoration.
In the demo, I have created Swimmer as object and added 9 swimmers of age 11 and 12 to customized concrete collection of type myList. An external complete built-in iterator is used to populate all the swimmers in first JTextArea of JFrame. The selective external iterator is used to populate swimmers that have age of 12 or greater in the second JTextArea of the JFrame. Also, I have created a method inside myList named doAll(Functor<T> functor, Predicate<T> pred) which does selective iteration inside the myList i.e. Internal Selective Iteration.
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First of all, I created a customized collection myList which extends built-in Vector. It has got two methods.
1. revIterator(): It returns IteratorReverse that I am going to create
2. doAll(): It processes all the elements of collection internally using functor.
doAll() implementation is simple. It is shown below.
//method to to internal reverse operation (phase 3)
public void doAll(Functor<T> functor){
Iterator<T> it = this.revIterator();
while(it.hasNext()){
functor.process(it.next());
}
}
But here, IteratorRevese (implementing Iterator and returned by revIterator) has to return element from reverse side of collection. The implementation of IteratorReverse goes like below.
public class IteratorReverse<T> implements Iterator<T>{
myList<T> myList;
int index; //to maintain state or to maintain position of it in collection
I have implemented next() and hasNext() method as follows.
@Override
public T next() {
if(index>= myList.size()){
return null;
}
else{
T t = myList.elementAt(myList.size()-index-1);
index = index +1;
return t;
}
}
@Override
public boolean hasNext() {
if(index>= myList.size()){
return false;
}
else
return true;
}
IteratorReverse use generics for type safety and reuse. The following summarizes the main logic of back reversal
T t = myList.elementAt(myList.size()-index-1);
When we forward ‘index’ keeps on increasing while myList.size()-index-1 keeps on decreasing.
I have implemented Functor pattern through interface. SwimmerFunctor is concrete implementation of Interface Functor which takes JTextArea2 from mainApp as argument and appends text into it as shown in picture above.
As shown in figure above, when ‘All Swimmers’ button is pressed, it will use built-in Iterator and iterates through the collection in forward direction. But when ‘All Swimmers Backward’ button is pressed, it will use IteratorReverse and iterates through the collection in backward direction.
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