blocks|key|1269778|text|新风格的for-loop+("foreach")适用于数组和实现Iterable接口的东西。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1269779|它也更类似于Iterator，而不是Iterable，所以对于Enumeration来说，使用foreach是没有意义的，除非Iterator也这样做了(但它并没有)。支持Iterator的人也不鼓励使用Enumeration。|1269780|entityMap^0|W|8|0|6|8|I|8|V|B|1R|8|2E|8|2U|B|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|M|8|@$9|N|A|O|B|C]|$9|P|A|Q|B|C]|$9|R|A|S|B|C]|$9|T|A|U|B|C]|$9|V|A|W|B|C]|$9|X|A|Y|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|Z|8|@]|D|@]|E|$]]]|I|$]]

The new-style-for-loop ("foreach") works on arrays, and things that implement the <code>Iterable</code> interface.

It's also more analogous to <code>Iterator</code> than to <code>Iterable</code>, so it wouldn't make sense for <code>Enumeration</code> to work with foreach unless <code>Iterator</code> did too (and it doesn't). 
<code>Enumeration</code> is also discouraged in favor of <code>Iterator</code>.

blocks|key|1484439|text|因为枚举(以及从该接口派生的大多数类)不实现Iterable。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1484440|您可以尝试编写自己的包装器类。|1484441|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

Because an Enumeration (and most classes derived from this interface) does not implement Iterable.

You can try to write your own wrapper class.

blocks|key|1269899|text|我已经用两个非常简单的类解决了这个问题，一个用于Enumeration，另一个用于Iterator。枚举封装器如下：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1269900|static+class+IterableEnumeration<T>
extends+Object
implements+Iterable<T>,+Iterator<T>
{
private+final+Enumeration<T>++++++++enumeration;
private+boolean+++++++++++++++++++++used=false;

IterableEnumeration(final+Enumeration<T>+enm)+{
++++enumeration=enm;
++++}

public+Iterator<T>+iterator()+{
++++if(used)+{+throw+new+IllegalStateException("Cannot+use+iterator+from+asIterable+wrapper+more+than+once");+}
++++used=true;
++++return+this;
++++}

public+boolean+hasNext()+{+return+enumeration.hasMoreElements();+}
public+T+++++++next()++++{+return+enumeration.nextElement();+++++}
public+void++++remove()++{+throw+new+UnsupportedOperationException("Cannot+remove+elements+from+AsIterator+wrapper+around+Enumeration");+}
}|code-block|syntax|javascript|1269901|它可以与静态实用程序方法一起使用(这是我的首选)：|1269902|/**
+*+Convert+an+`Enumeration<T>`+to+an+`Iterable<T>`+for+a+once-off+use+in+an+enhanced+for+loop.
+*/
static+public+<T>+Iterable<T>+asIterable(final+Enumeration<T>+enm)+{
++++return+new+IterableEnumeration<T>(enm);
++++}

...

for(String+val:+Util.asIterable(enm))+{
++++...
++++}|1269903|或者通过实例化该类：|1269904|for(String+val:+new+IterableEnumeration<String>(enm))+{
++++...
++++}|1269905|entityMap^0|O|B|15|8|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@$9|V|A|W|B|C]|$9|X|A|Y|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|Z|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|10|8|@]|D|@]|E|$]]|$1|M|3|N|5|H|7|11|8|@]|D|@]|E|$I|J]]|$1|O|3|P|5|6|7|12|8|@]|D|@]|E|$]]|$1|Q|3|R|5|H|7|13|8|@]|D|@]|E|$I|J]]|$1|S|3|-4|5|6|7|14|8|@]|D|@]|E|$]]]|T|$]]

I have solved this problem with two very simple classes, one for <code>Enumeration</code> and one for <code>Iterator</code>. The enumeration wrapper is as follows:

<pre><code>static class IterableEnumeration&lt;T&gt;
extends Object
implements Iterable&lt;T&gt;, Iterator&lt;T&gt;
{
private final Enumeration&lt;T&gt; enumeration;
private boolean used=false;

IterableEnumeration(final Enumeration&lt;T&gt; enm) {
 enumeration=enm;
 }

public Iterator&lt;T&gt; iterator() {
 if(used) { throw new IllegalStateException("Cannot use iterator from asIterable wrapper more than once"); }
 used=true;
 return this;
 }

public boolean hasNext() { return enumeration.hasMoreElements(); }
public T next() { return enumeration.nextElement(); }
public void remove() { throw new UnsupportedOperationException("Cannot remove elements from AsIterator wrapper around Enumeration"); }
}
</code></pre>

Which can be used either with a static utility method (which is my preference):

<pre><code>/**
 * Convert an `Enumeration&lt;T&gt;` to an `Iterable&lt;T&gt;` for a once-off use in an enhanced for loop.
 */
static public &lt;T&gt; Iterable&lt;T&gt; asIterable(final Enumeration&lt;T&gt; enm) {
 return new IterableEnumeration&lt;T&gt;(enm);
 }

...

for(String val: Util.asIterable(enm)) {
 ...
 }
</code></pre>

or by instantiating the class:

<pre><code>for(String val: new IterableEnumeration&lt;String&gt;(enm)) {
 ...
 }
</code></pre>

blocks|key|1269961|text|使用Collections实用程序类，可以迭代枚举，如下所示：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1269962|Enumeration+headerValues=request.getHeaders("mycustomheader");
List+headerValuesList=Collections.list(headerValues);

for(Object+headerValueObj:headerValuesList){
+...+do+whatever+you+want+to+do+with+headerValueObj
}|code-block|syntax|javascript|1269963|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Using Collections utility class, Enumeration can be made iterable like:

<pre><code>Enumeration headerValues=request.getHeaders("mycustomheader");
List headerValuesList=Collections.list(headerValues);

for(Object headerValueObj:headerValuesList){
 ... do whatever you want to do with headerValueObj
}
</code></pre>

blocks|key|1484643|text|Enumeration不实现Iterable，因此不能直接在foreach循环中使用。但是，使用Apache+Commons+Collections可以使用以下命令遍历枚举：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1484644|for+(Object+o+:+new+IteratorIterable(new+EnumerationIterator(e)))+{
++++...
}|code-block|syntax|javascript|1484645|您也可以在Collections.list()中使用更短的语法，但效率较低(对元素进行两次迭代，占用更多内存)：|1484646|for+(Object+o+:+Collections.list(e)))+{
++++...
}|1484647|entityMap|0|LINK|mutability|MUTABLE|url|http://commons.apache.org/collections^0|0|B|E|8|1C|Q|0|0|0|5|I|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@$9|X|A|Y|B|C]|$9|Z|A|10|B|C]]|D|@$9|11|A|12|1|13]]|E|$]]|$1|F|3|G|5|H|7|14|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|15|8|@$9|16|A|17|B|C]]|D|@]|E|$]]|$1|M|3|N|5|H|7|18|8|@]|D|@]|E|$I|J]]|$1|O|3|-4|5|6|7|19|8|@]|D|@]|E|$]]]|P|$Q|$5|R|S|T|E|$U|V]]]]

<code>Enumeration</code> doesn't implement <code>Iterable</code> and as such can't be used directly in a foreach loop. However using <a href="http://commons.apache.org/collections" rel="nofollow noreferrer">Apache Commons Collections</a> it's possible to iterate over an enumeration with:

<pre><code>for (Object o : new IteratorIterable(new EnumerationIterator(e))) {
 ...
}
</code></pre>

You could also use a shorter syntax with <code>Collections.list()</code> but this is less efficient (two iterations over the elements and more memory usage) :

<pre><code>for (Object o : Collections.list(e))) {
 ...
}
</code></pre>

blocks|key|1484724|text|在java+8及更高版本中，这是可能的：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1484725|import+java.util.Collections;
import+java.util.Enumeration;

Enumeration+e+=+...;
Collections.list(e).forEach(o+->+{
++++...+//+use+item+"o"
});|code-block|syntax|javascript|1484726|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

With java 8 and beyond this is possible:

<pre><code>import java.util.Collections;
import java.util.Enumeration;

Enumeration e = ...;
Collections.list(e).forEach(o -&gt; {
 ... // use item "o"
});
</code></pre>

blocks|key|1270196|text|我们可以使用for循环通过.values​​()遍历枚举|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1270197|method+which+returns+all+the+elements+contained+in+the+enumeration.|1270198|举个例子：|1270199|for+(USERS_ROLES+userRole:+USERS_ROLES+.values+​​())+{
++++++++++++System.out.println+("This+is+one+user+role:"+%2B+userRole.toString+());
}|code-block|syntax|javascript|1270200|我在java+10中这样做了。|1270201|entityMap^0|D|B|0|0|6|7|1O|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@$9|T|A|U|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|V|8|@$9|W|A|X|B|C]|$9|Y|A|Z|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|10|8|@]|D|@]|E|$]]|$1|J|3|K|5|L|7|11|8|@]|D|@]|E|$M|N]]|$1|O|3|P|5|6|7|12|8|@]|D|@]|E|$]]|$1|Q|3|-4|5|6|7|13|8|@]|D|@]|E|$]]]|R|$]]

We can use the for loop to iterate over an enumeration using the <code>.values​​()</code>
<code>method</code> <code>which returns all the elements contained in the enumeration.</code>
An example :
<pre><code>for (USERS_ROLES userRole: USERS_ROLES .values ​​()) {
 System.out.println (&quot;This is one user role:&quot; + userRole.toString ());
}
</code></pre>
I did it in java 10

Why can't I do:

<pre><code>Enumeration e = ...
for (Object o : e)
 ...
</code></pre>

Why can't I use foreach on Java Enumeration?

为什么我不能这样做：Enumeration e = ...for (Object o : e)  ...

问为什么我不能使用foreach on Java枚举？
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