Jakarta Commons Cookbook讀書筆記系列
3.5 Iterator的擴展
3.5.1 循環迭代器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");
//當迭代到最后的元素后，再返回第一個元素重新循環，直至達到迭代次數為止
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
可以自定義范圍地遍歷
import org.apache.commons.collections.iterators.ArrayListIterator;
String[] arrays=new String[]{"a","b","c","d","f"};
//遍歷下標為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 篩選迭代器FilterIterator
使用Predicate篩選，關于Predicate的介紹，請看我的這系列的上一篇文章Jakarta Commons Cookbook讀書筆記--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}));
//過濾出大于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 過濾重復的元素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是這樣的一種容器，它能夠存儲多個邏輯相等（即equals()為true，而且hash()相等）的元素，并可以統計它們的個數。
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);
//減去bag2內相應元素的數量
bag1.removeAll(bag2);
System.out.println("book1: "+bag1.getCount("book1"));
System.out.println("book2: "+bag1.getCount("book2")+"\n");
//bag1保留bag2內的元素，簡單來說就是求交集
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順序
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內部原理是使用一個HashMap當作內部容器，key為加入的對象，對應的value是對象的次數。同理TreeBag使用TreeMap作為內部容器。
需
要注意的是雖然Bag繼承了Collection，但是它的removeAll(),containsAll(),add(),remove()和
retainAll()方法并不嚴格遵循Collection接口的規范。例如removeAll方法根據規范是移除所有的元素，而Bag的
removeAll是帶參數的，只移除參數包含的元素。
3.7 用于臨時數據轉移的Buffer
Buffer類似于java5.0中的Queue，是個先進先出（First-in First-out）的數據容器。
3.7.1 無尺寸緩沖區UnboundFifoBuffer和有尺寸緩沖區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");
}
//移除第一個加入的元素
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");
}
//移除第一個加入的元素
bBuffer.remove();
Iterator iterator=bBuffer.iterator();
while(iterator.hasNext()){
System.out.println(iterator.next());
}
->
book2
book3
3.7.2 帶優先級的緩沖區PriorityBuffer
按照數值從小到大排列
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允許使用Comparator來排列優先順序，關于Comparator，請看我的這系列的上一篇文章Jakarta Commons Cookbook讀書筆記--Commons Collections(函子篇)
import java.util.Comparator;
//按照銷售數量從大到小排列
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 使用阻塞式緩沖區BlockingBuffer
BlokingBuffer裝飾一個Buffer實例，并使其處于阻塞狀態，只要有對象加入則馬上處理。當一個進程調用以BlockingBuffer的get()和remove()方法時，將不返回任何值，直到它有一個對象返回。
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的擴展
3.8.1 使用MultiMap實現一鍵存儲多個值
MultiValueMap會使用一個ArrayList來保存同一個鍵的所有值
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實現根據值檢索鍵
DualHashBidiMap使用兩個HashMap來保存鍵值對，其中一個正常保存鍵值對，另一個反過來保存值對應的鍵。
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是可以記住加入順序的BidiMap，其內部使用TreeMap來保存鍵值對。
3.8.3 大小寫不敏感的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 指定鍵和值類型（JDK1.4或以下版本）
如果你使用JDK1.4或以下的版本，你如果需要指定鍵或值的類型，可以使用TypeMap來裝飾。
import org.apache.commons.collections.map.TypedMap;
Map map=TypedMap.decorate(new HashMap(), String.class, String.class);
map.put("key", "value");
//加入非指定的類型會拋出IllegalArgumentException,程序會停止運行
map.put("key2", new Integer(12));
3.8.6 根據鍵自動生成值的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
Jakarta Commons Cookbook讀書筆記系列
程序員的一生其實可短暫了，這電腦一開一關，一天過去了，嚎；電腦一開不關，那就成服務器了，嚎……