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();
	}

}