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[Jakarta Commons笔记] Commons Collections - Iterator组

Posted on 2005-08-06 12:47 laogao 阅读(1732) 评论(0)  编辑  收藏 所属分类: On Java

 

来看最后一组 – Iterator

 

LoopingIterator

ArrayListIterator

FilterIterator

UniqueFilterIterator

IteratorUtils

 

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 #

# Friday #

# Monday #

# Tuesday #

========================================

 

有了这些实用的Iterator类,我们就可以轻松的实现可配置的遍历行为了。

 

 


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