Iterator

java.util.Iterator接口定义了标准的Collection遍历方法，但是如果不做改变的使用它，我们得到的是从头到尾一次性的遍历。假如我们需要循环遍历，假如我们需要遍历某一段，假如我们需要遍历满足某些条件的元素，等等等等，我们就不能完全依赖于这个Iterator的标准实现了。除非我们宁可在此基础上在调用的代码中多加一些判断，不过这样的话代码就会显得混乱，时间长了就容易变得难以维护。Commons Collections的这一组Iterator为我们带来了便利。




这些Iterator使用都很一目了然，直接看例子吧：




package sean.study.commons.collections;




import java.util.Arrays;


import java.util.Iterator;


import java.util.List;




import org.apache.commons.collections.Predicate;


import org.apache.commons.collections.iterators.ArrayListIterator;


import org.apache.commons.collections.iterators.FilterIterator;


import org.apache.commons.collections.iterators.LoopingIterator;


import org.apache.commons.lang.StringUtils;




public class IteratorUsage {




public static void main(String[] args) {


demoIteratorUsage();


}


public static void demoIteratorUsage() {




System.out.println(StringUtils.center(" demoClosureUsage ", 40, "="));


// data setup


String[] weekDays = {


"Monday", "Tuesday", "Wednesday",


"Thursday", "Friday", "Saturday", "Sunday"


};


List weekDayList = Arrays.asList(weekDays);


// workdays


Iterator iter1 = new ArrayListIterator(weekDays, 0, 5);


printColl("Partial:", iter1, 5);


// loop


Iterator iter2 = new LoopingIterator(weekDayList);


printColl("Loop:", iter2, 10);


// looping workdays


Predicate notWeekendPredicate = new Predicate() {


public boolean evaluate(Object obj) {


String str = (String) obj;


if ("Saturday".equalsIgnoreCase(str)) {


return false;


}


if ("Sunday".equalsIgnoreCase(str)) {


return false;


}


return true;


}


};


Iterator iter3 = new FilterIterator(


new LoopingIterator(weekDayList),


notWeekendPredicate


);


printColl("No Weekends loop:", iter3, 12);


System.out.println(StringUtils.repeat("=", 40));




}


public static void printColl(String label, Iterator iter, int maxCount) {


if (StringUtils.isNotBlank(label)) {


System.out.println(label);


}


int i = 0;


while (iter.hasNext() && i < maxCount) {


System.out.println("# " + iter.next() + " #");


i++;


}


}




}




运行结果如下：




=========== demoClosureUsage ===========


Partial:


# Monday #


# Tuesday #


# Wednesday #


# Thursday #


# Friday #


Loop:


# Monday #


# Tuesday #


# Wednesday #


# Thursday #


# Friday #


# Saturday #


# Sunday #


# Monday #


# Tuesday #


# Wednesday #


No Weekends loop:


# Monday #


# Tuesday #


# Wednesday #


# Thursday #


# Friday #


# Monday #


# Tuesday #


# Wednesday #


# Thursday