Jakarta Commons Cookbook读书笔记--Commons Collections(容器篇之一)

《Jakarta Commons cookbook读书笔记》系列文章

 

3.5 Iterator的扩展

3.5.1 循环迭代器LoopingIterator

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

List books=new ArrayList();
        books.add("EnglishBook");
        books.add("Commons Cookbook");
        books.add("Who Moved My Cheese");
        //当迭代到最后的元素后，再返回第一个元素重新循环，直至达到迭代次数为止
        LoopingIterator iterator=new LoopingIterator(books);
        for(int i=0;i<5;i++){
            String book=(String)iterator.next();
            System.out.print(book+";");
        }
->EnglishBook;Commons Cookbook;Who Moved My Cheese;EnglishBook;Commons Cookbook;

 

3.5.2 ArrayList迭代器ArrayListIterator
可以自定义范围地遍历

import org.apache.commons.collections.iterators.ArrayListIterator;
 
String[] arrays=new String[]{"a","b","c","d","f"};
        //遍历下标为1到4的元素
        Iterator iterator=new ArrayListIterator(arrays,1,4);
        while(iterator.hasNext()){
            System.out.print(iterator.next()+"; ");
        }
->b; c; d; 

 

3.5.3 筛选迭代器FilterIterator
使用Predicate筛选，关于Predicate的介绍，请看我的这系列的上一篇文章Jakarta Commons Cookbook读书笔记--Commons Collections(函子篇)

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

List list=new ArrayList(Arrays.asList(new Integer[]{7,9,35,67,88}));
       
        //过滤出大于30的元素
        Predicate predicate=new Predicate(){           
            public boolean evaluate(Object object){
                int num=((Integer)object).intValue();
                return num>30;
            }
        };
        Iterator iterator=new FilterIterator(list.iterator(),predicate);
        while(iterator.hasNext()){
            System.out.print(iterator.next()+"; ");
        }
->35; 67; 88; 

 

3.5.4 过滤重复的元素UniqueFilterIterator

List list=new ArrayList(Arrays.asList(new String[]{"a","b","c","b","a"}));
       
        Iterator iterator=new UniqueFilterIterator(list.iterator());
        while(iterator.hasNext()){
            System.out.print(iterator.next()+"; ");
        }
->a; b; c;

 

3.6 使用Bag

Bag是这样的一种容器，它能够存储多个逻辑相等（即equals()为true，而且hash()相等）的元素，并可以统计它们的个数。

3.6.1 高性能的HashBag

import org.apache.commons.collections.bag.HashBag;

Bag bag1=new HashBag();
        bag1.add("book1",10);
        bag1.add("book2",20);
       
        Bag bag2=new HashBag();
        bag2.add("book2",5);
        bag2.add("book3",10);
       
        bag1.add("book1");
        bag1.remove("book1",2);   
        //减去bag2内相应元素的数量
        bag1.removeAll(bag2);
       
        System.out.println("book1: "+bag1.getCount("book1"));
        System.out.println("book2: "+bag1.getCount("book2")+"\n");
       
        //bag1保留bag2内的元素，简单来说就是求交集
        bag1.retainAll(bag2);
        System.out.println("book1: "+bag1.getCount("book1"));
        System.out.println("book2: "+bag1.getCount("book2"));
        System.out.println("book3: "+bag1.getCount("book3"));
->
book1: 9
book2: 15

book1: 0
book2: 5
book3: 0

 

3.6.2 TreeBag可以保存加入元素的顺序

import org.apache.commons.collections.bag.TreeBag;

Bag bag1=new TreeBag();
        bag1.add("book1",2);
        bag1.add("book2",1);
        bag1.add("book3",2);
        bag1.add("book4",1);
        bag1.add("book5",1);
       
        Iterator iterator=bag1.iterator();
        while(iterator.hasNext()){
            System.out.println(iterator.next());
        }
->
book1
book1
book2
book3
book3
book4
book5

 

HashBag内部原理是使用一个HashMap当作内部容器，key为加入的对象，对应的value是对象的次数。同理TreeBag使用TreeMap作为内部容器。
需要注意的是虽然Bag继承了Collection，但是它的removeAll(),containsAll(),add(),remove()和retainAll()方法并不严格遵循Collection接口的规范。例如removeAll方法根据规范是移除所有的元素，而Bag的removeAll是带参数的，只移除参数包含的元素。

3.7 用于临时数据转移的Buffer

Buffer类似于java5.0中的Queue，是个先进先出（First-in First-out）的数据容器。

3.7.1 无尺寸缓冲区UnboundFifoBuffer和有尺寸缓冲区BoundedFifoBuffer

import org.apache.commons.collections.buffer.BoundedFifoBuffer;

Buffer bBuffer=new BoundedFifoBuffer(2);
        bBuffer.add("book1");
        bBuffer.add("book2");
        try{
            bBuffer.add("book3");
        }catch(BufferOverflowException e){
            System.out.println("Buffer is over flow");
        }
        //移除第一个加入的元素
        bBuffer.remove();
        Iterator iterator=bBuffer.iterator();
        while(iterator.hasNext()){
            System.out.println(iterator.next());
        }
->
Buffer is over flow
book2

 

 

import org.apache.commons.collections.buffer.UnboundedFifoBuffer;

Buffer bBuffer=new UnboundedFifoBuffer(2);
        bBuffer.add("book1");
        bBuffer.add("book2");
        try{
            bBuffer.add("book3");
        }catch(BufferOverflowException e){
            System.out.println("Buffer is over flow");
        }
        //移除第一个加入的元素
        bBuffer.remove();
        Iterator iterator=bBuffer.iterator();
        while(iterator.hasNext()){
            System.out.println(iterator.next());
        }
->
book2
book3

 

3.7.2 带优先级的缓冲区PriorityBuffer
按照数值从小到大排列

import org.apache.commons.collections.buffer.PriorityBuffer;

Buffer pBuffer=new PriorityBuffer();
        pBuffer.add(new Long(2));
        pBuffer.add(new Long(20));
        pBuffer.add(new Long(12));
        pBuffer.add(new Long(4));
       
        Iterator iterator=pBuffer.iterator();
        while(iterator.hasNext()){
            System.out.println(iterator.next());
        }
->
2
4
12
20

 

PriorityBuffer允许使用Comparator来排列优先顺序，关于Comparator，请看我的这系列的上一篇文章Jakarta Commons Cookbook读书笔记--Commons Collections(函子篇)
//按照销售数量从大到小排列

import java.util.Comparator;

public class RecommandComparator implements Comparator {

    public int compare(Object o1, Object o2) {
        int result=-1;
        if(o1 instanceof Book && o2 instanceof Book){
            Book book1=(Book)o1;
            Book book2=(Book)o2;
           
            result=book1.getSalsNum().compareTo(book2.getSalsNum());           
        }
        return result;
    }
}

 

 

import org.apache.commons.collections.comparators.ReverseComparator;
import org.apache.commons.collections.buffer.PriorityBuffer;

Buffer pBuffer=new PriorityBuffer(new ReverseComparator(new RecommandComparator()));
        pBuffer.add(new Book("book1",Long.valueOf(200)));
        pBuffer.add(new Book("book2",Long.valueOf(7200)));
        pBuffer.add(new Book("book3",Long.valueOf(163)));
        pBuffer.add(new Book("book4",Long.valueOf(569)));
       
        Iterator iterator=pBuffer.iterator();
        while(iterator.hasNext()){
            Book book=(Book)iterator.next();
            System.out.println(book.getName()+":"+book.getSalsNum());
        }
->
book2:7200
book4:569
book3:163
book1:200

 

3.7.3 使用阻塞式缓冲区BlockingBuffer
BlokingBuffer装饰一个Buffer实例，并使其处于阻塞状态，只要有对象加入则马上处理。当一个进程调用以BlockingBuffer的get()和remove()方法时，将不返回任何值，直到它有一个对象返回。

import org.apache.commons.collections.Buffer;

public class BufferListener implements Runnable{
    private Buffer buffer;
   
    public BufferListener(Buffer buffer){
        this.buffer=buffer;
    }
   
    public void run() {
        while(true){
            String msg=(String)buffer.remove();
            System.out.println(msg);
        }
    }

   
}

 

 

import org.apache.commons.collections.buffer.BlockingBuffer;

Buffer buffer=BlockingBuffer.decorate(new UnboundedFifoBuffer());
        BufferListener listener=new BufferListener(buffer);
        Thread listenerThread=new Thread(listener);
        listenerThread.start();
       
        buffer.add("book1");
        buffer.add("book2");

->
book1
book2

 

3.8 Map的扩展

3.8.1 使用MultiMap实现一键存储多个值
MultiValueMap会使用一个ArrayList来保存同一个键的所有值

import org.apache.commons.collections.MultiMap;
import org.apache.commons.collections.map.MultiValueMap;

MultiMap map=new MultiValueMap();
        map.put("key", "value1");
        map.put("key", "value2");
        map.put("key", "value2");
        System.out.println((Collection)map.get("key"));
->[value1, value2, value2]

 

3.8.2 使用BidiMap实现根据值检索键
DualHashBidiMap使用两个HashMap来保存键值对，其中一个正常保存键值对，另一个反过来保

存值对应的键。

import org.apache.commons.collections.BidiMap;
import org.apache.commons.collections.bidimap.DualHashBidiMap;

BidiMap map=new DualHashBidiMap();
        map.put("key1", "value");       
        System.out.println(map.get("key1"));
        System.out.println(map.inverseBidiMap().get("value"));
->
value
key1

 

DualTreeBidiMap是可以记住加入顺序的BidiMap，其内部使用TreeMap来保存键值对。

3.8.3 大小写不敏感的CaseInsensitiveMap

import org.apache.commons.collections.map.CaseInsensitiveMap;
CaseInsensitiveMap map=new CaseInsensitiveMap();
        map.put("KEY", "value");.
                map.put("key", "value2");
        System.out.println(map.get("key"));
->value2

 

3.8.4 指定键和值类型（JDK1.4或以下版本）
如果你使用JDK1.4或以下的版本，你如果需要指定键或值的类型，可以使用TypeMap来装饰。

import org.apache.commons.collections.map.TypedMap;

Map map=TypedMap.decorate(new HashMap(), String.class, String.class);
        map.put("key", "value");
        //加入非指定的类型会抛出IllegalArgumentException,程序会停止运行
        map.put("key2", new Integer(12));

 

3.8.5 限制加入Map的键或值

import org.apache.commons.collections.map.PredicatedMap;
//只允许实例类型是String
        Predicate stringOnly=new InstanceofPredicate(String.class);
        Map map=PredicatedMap.decorate(new HashMap(), stringOnly,

stringOnly);
        //如果加入的键或值不符合条件，会抛出IllegalArgumentException,程序会

停止运行
        map.put("key2", new Integer(12));

 

3.8.6 根据键自动生成值的Map

import org.apache.commons.collections.map.LazyMap;
Transformer upperFirstLetter=new Transformer(){
            public Object transform(Object object){
                String name=(String)object;
                String result=name;
                if(name!=null&&!"".equals(name)){
                    result=name.substring(0,1).toUpperCase()

+name.substring(1);
                }
                return result;
            }
        };
       
        Map map=LazyMap.decorate(new HashMap(), upperFirstLetter);
        System.out.println(map.get("heis"));
->Heis

 

 

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