java design pattern - Iterator pattern

1.这是个比较古老的例子，用了4个函数来实现Iterator，现在Java 的Iterator 应该只要实现 hasNext(), next(), remove() 这三个函数就好了。

package iterator;

import java.util.LinkedList;

interface ISubject {
    public IIterator CreateIterator();
}

interface IIterator {
    void First();        //Reset to first element
    String Next();        //get next element
    Boolean IsDone();        //End of collection check
    String CurrentItem();    //Retrieve Current Item
}

class Science implements ISubject {
    private LinkedList<String> subjects;

    public Science(){
        subjects = new LinkedList<String>();
        subjects.addLast("Math");
        subjects.addLast("Comp. Sc.");
        subjects.addLast("Physics");
    }

    @Override
    public IIterator CreateIterator(){
        return new ScienceIterator(subjects);
    }

    //Containing the ScienceIterator
    public class ScienceIterator implements IIterator{
        private LinkedList<String> subjects;
        private int position;

        public ScienceIterator(LinkedList<String> subjects){
            this.subjects = subjects;
            position = 0;
        }

        public void First(){
            position = 0;
        }

        public String Next(){
            return subjects.get(position++);
        }

        public Boolean IsDone(){
            return position >= subjects.size();
        }

        public String CurrentItem(){
            return subjects.get(position);
        }
    }
}


class Arts implements ISubject {
    private String[] subjects;

    public Arts(){
        subjects = new String[2];
        subjects[0] = "Bengali";
        subjects[1] = "English";
    }

    public IIterator CreateIterator(){
        return new ArtsIterator(subjects);
    }


    //Containing the ArtsIterator
    public class ArtsIterator implements IIterator{
        private String[] subjects;
        private int position;

        public ArtsIterator(String[] subjects){
            this.subjects = subjects;
            position = 0;
        }

        public void First(){
            position = 0;
        }

        public String Next(){
            return subjects[position++];
        }

        public Boolean IsDone(){
            return position >= subjects.length;
        }

        public String CurrentItem(){
            return subjects[position];
        }
    }
}


public class IteratorPattern {
    public static void main(String[] args){
        System.out.println("*** Iterator Pattern Demo ***");
        ISubject Sc_subject = new Science();
        ISubject Ar_subjects = new Arts();

        IIterator Sc_iterator = Sc_subject.CreateIterator();
        IIterator Ar_iterator = Ar_subjects.CreateIterator();

        System.out.println("\nScience Subjects :");
        Print(Sc_iterator);

        System.out.println("\nArts subjects: ");
        Print(Ar_iterator);
    }

    public static void Print(IIterator iterator){
        while(!iterator.IsDone()){
            System.out.println(iterator.Next());
        }
    }
}

2.  这个例子比较好，从这里抄了一个例子，使用了一个private inner class 来实现的Iterator，我在某一本书上看说用private inner class 比public inner class 要好。
http://www.journaldev.com/1716/iterator-design-pattern-java

package iterator;

import java.util.ArrayList;
import java.util.List;

enum ChannelTypeEnum{
    ENGLISH,HINDI,FRENCH,ALL;
}

class Channel{
    private double frequency;
    private ChannelTypeEnum TYPE;

    public Channel(double freq, ChannelTypeEnum TYPE){
        this.TYPE= TYPE;
        this.frequency = freq;
    }

    public double getFrequency(){
        return frequency;
    }

    public ChannelTypeEnum getType(){
        return TYPE;
    }

    @Override
    public String toString(){
        return "Frequery ="+this.frequency+", Type="+this.TYPE;
    }
}

interface ChannelIterator{
    public boolean hasNext();
    public Channel next();
}

interface ChannelCollection{
    public void addChannel(Channel c);
    public void removeChannel(Channel c);
    public ChannelIterator iterator(ChannelTypeEnum type);
}

class ChannelCollectionImpl implements ChannelCollection{
    private List<Channel> channelList;

    //Constructor
    public ChannelCollectionImpl(){
        channelList = new ArrayList<Channel>();
    }

    public void addChannel(Channel c){
        this.channelList.add(c);
    }

    public void removeChannel(Channel c){
        this.channelList.remove(c);
    }

    @Override
    public ChannelIterator iterator(ChannelTypeEnum type){
        return new ChannelIteratorImpl(type, this.channelList);
    }

    private class ChannelIteratorImpl implements ChannelIterator{
        private ChannelTypeEnum type;
        private List<Channel> channels;
        private int position;

        public ChannelIteratorImpl(ChannelTypeEnum ty, List<Channel> channelList){
            this.type = ty;
            this.channels = channelList;
        }

        @Override
        public boolean hasNext(){
            while(position < channels.size()){
                Channel c = channels.get(position);
                if(c.getType().equals(type) || type.equals(ChannelTypeEnum.ALL)){
                    return true;
                }else
                    position ++;
            }
            return false;
        }

        @Override
        public Channel next(){
            Channel c = channels.get(position);
            position ++;
            return c;
        }
    }
}


public class IteratorDemo {
    public static void main(String[] args){
        ChannelCollection channels = populateChannels();
        ChannelIterator baseIterator = channels.iterator(ChannelTypeEnum.ALL);
        while(baseIterator.hasNext()){
            Channel c = baseIterator.next();
            System.out.println(c.toString());
        }
        System.out.println("********");
        //Channel Type Iterator
        ChannelIterator englishIterator = channels.iterator(ChannelTypeEnum.ENGLISH);
        while(englishIterator.hasNext()){
            Channel c = englishIterator.next();
            System.out.println(c.toString());
        }
    }

    private static ChannelCollection populateChannels(){
        ChannelCollection channels = new ChannelCollectionImpl();
        channels.addChannel(new Channel(98.5, ChannelTypeEnum.ENGLISH));
        channels.addChannel(new Channel(99.5, ChannelTypeEnum.HINDI));
        channels.addChannel(new Channel(100.5, ChannelTypeEnum.FRENCH));
        channels.addChannel(new Channel(101.5, ChannelTypeEnum.ENGLISH));
        channels.addChannel(new Channel(102.5, ChannelTypeEnum.HINDI));
        channels.addChannel(new Channel(103.5, ChannelTypeEnum.FRENCH));
        channels.addChannel(new Channel(104.5, ChannelTypeEnum.ENGLISH));
        channels.addChannel(new Channel(105.5, ChannelTypeEnum.HINDI));
        channels.addChannel(new Channel(106.5, ChannelTypeEnum.FRENCH));
        return channels;
    }
}

3. 记录下最近碰到的一个shell脚本出错的问题。
$1 在一个脚本调用其他脚本的过程中，如果有set 语句，会改变调用脚本的位置参数。
这是我要运行的脚本

#! /bin/bash -x

echo "\$1 is $1"
if [ -x "/etc/profile.d/sinit.sh" ];then
  source /etc/profile.d/sinit.sh
fi
echo "\$1 is $1"
  case "$1" in
       start) echo "$1 is start";;
       stop) echo "$1 is stop";;
       *) echo "wrong coice";;
  esac

这是另外一个被调用的脚本，当然其实这个应该算是个文本文件，而不能算是脚本，因为并没有#！(shebang), 当然在我实验的过程中，即使在第一行加上#！(shebang) ,
调用脚本的$1 仍然会被污染。不用.用source 也是一样。

/etc/profile.d/sinit.sh
TMOUT=1800
export TMOUT
set autologout=30

请看执行, source 后$1就被污染了。

./echo_env2.sh start
+ echo '$1 is start'
$1 is start
+ '[' -x /etc/profile.d/sinit.sh ']'
+ source /etc/profile.d/sinit.sh
++ TMOUT=1800
++ export TMOUT
++ set autologout=30
+ echo '$1 is autologout=30'
$1 is autologout=30
+ case "$1" in
+ echo 'wrong coice'
wrong coice