tomcat 7 源码分析-12 Enumeration枚举

tomcat 7 源码分析-12 Enumeration枚举

 

Enumeration枚举，就是要把内容没有重复的遍历，而且不破坏原有的存储空间。在http headers的实现中，tomcat实现了一些枚举类。

Enumerate the distinct header names. Each nextElement() is O(n)

 

 class  NamesEnumerator  implements  Enumeration<String> {
     int  pos;
     int  size;
     String next;
     MimeHeaders headers;

     public  NamesEnumerator(MimeHeaders headers) {
         this .headers=headers;
         pos=0 ;
         size = headers.size();
         findNext();
     }

     private   void  findNext() {
         next=null ;
         for (  ; pos< size; pos++ ) {
             next=headers.getName( pos ).toString();
             for (  int  j= 0 ; j<pos ; j++ ) {
                 if ( headers.getName( j ).equalsIgnoreCase( next )) {
                     // duplicate.
                     next=null ;
                     break ;
                 }
             }
             if ( next!= null  ) {
                 // it's not a duplicate
                 break ;
             }
         }
         // next time findNext is called it will try the
         // next element
         pos++;
     }

     public   boolean  hasMoreElements() {
         return  next!= null ;
     }

     public  String nextElement() {
         String current=next;
         findNext();
         return  current;
     }
 }

class NamesEnumerator implements Enumeration<String> {
    int pos;
    int size;
    String next;
    MimeHeaders headers;

    public NamesEnumerator(MimeHeaders headers) {
        this.headers=headers;
        pos=0;
        size = headers.size();
        findNext();
    }

    private void findNext() {
        next=null;
        for(  ; pos< size; pos++ ) {
            next=headers.getName( pos ).toString();
            for( int j=0; j<pos ; j++ ) {
                if( headers.getName( j ).equalsIgnoreCase( next )) {
                    // duplicate.
                    next=null;
                    break;
                }
            }
            if( next!=null ) {
                // it's not a duplicate
                break;
            }
        }
        // next time findNext is called it will try the
        // next element
        pos++;
    }
    
    public boolean hasMoreElements() {
        return next!=null;
    }

    public String nextElement() {
        String current=next;
        findNext();
        return current;
    }
}

 Enumeration的核心是findNext函数，实现了找到下一个元素的功能，保障与之前的不重复。

简单分析findNext的算法：

pos就表示next的位置，如果在循环pos前面的元素时，发现有重复的，就把next=null，跳出循环后，判断是否为null。

如果next为null，外边的循环仍然继续，pos后移，此举就是跳出有重复的元素。

如果next不等于null，说明此pos的元素与之前不重复，next即是下一个元素。整个循环结束。

最后pos++，为下一个next做准备。

 

类似的写了个队char的枚举类，验证了下算法

 package  com.test.Enumerator;

 import  java.util.Enumeration;

 public   class  CharEnumeration  implements  Enumeration<Character> {

     Character next;
     Character characters[];
     int  pos;
     int  size;

     CharEnumeration(Character a[]) {

         characters = a;
         size = a.length;
         pos = 0 ;
         findNext();
     }

     @Override
     public   boolean  hasMoreElements() {
         // TODO Auto-generated method stub
         return  next !=  null ;
     }

     private   void  findNext() {
         next = null ;

         for  (; pos < size; pos++) {

             next = characters[pos];
             for  ( int  i =  0 ; i < pos; i++) {

                 if  (next == characters[i]) {
                     next = null ;
                     break ;
                 }
             }

             if  (next !=  null )
                 break ;
         }
         pos++;
     }

     @Override
     public  Character nextElement() {
         // TODO Auto-generated method stub
         Character cur = next;
         findNext();
         return  cur;
     }

     public   void  PrintAll() {

         while  (hasMoreElements()) {

             System.out.println(nextElement());
         }
     }

 }

package com.test.Enumerator;

import java.util.Enumeration;

public class CharEnumeration implements Enumeration<Character> {

	Character next;
	Character characters[];
	int pos;
	int size;

	CharEnumeration(Character a[]) {

		characters = a;
		size = a.length;
		pos = 0;
		findNext();
	}

	@Override
	public boolean hasMoreElements() {
		// TODO Auto-generated method stub
		return next != null;
	}

	private void findNext() {
		next = null;

		for (; pos < size; pos++) {

			next = characters[pos];
			for (int i = 0; i < pos; i++) {

				if (next == characters[i]) {
					next = null;
					break;
				}
			}

			if (next != null)
				break;
		}
		pos++;
	}

	@Override
	public Character nextElement() {
		// TODO Auto-generated method stub		
		Character cur = next;
		findNext();
		return cur;
	}

	public void PrintAll() {

		while (hasMoreElements()) {
			
			System.out.println(nextElement());
		}
	}

}

 

 package  com.test.Enumerator;
 public   class  EnumeratorTest {

     /**
      * @param args
      */
     public   static   void  main(String[] args) {
         // TODO Auto-generated method stub

         Character [] sample = {'a' , 'b' , 'c' , 'd' , 'e' , 'a' , 'a' , 'b' };
         CharEnumeration che = new  CharEnumeration(sample);
         che.PrintAll();
     }

 }

package com.test.Enumerator;
public class EnumeratorTest {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		// TODO Auto-generated method stub

		Character [] sample = {'a','b','c','d','e','a','a','b'};
		CharEnumeration che = new CharEnumeration(sample);
		che.PrintAll();
	}

}