java collections读书笔记（5） Enumeration接口

Enumeration，提供了遍历collection的一个方法，这是一个接口，其遵循的是Iterator设计模式。

其源代码实现是：

package java.util;

/**
 * An object that implements the Enumeration interface generates a
 * series of elements, one at a time. Successive calls to the
 * <code>nextElement</code> method return successive elements of the
 * series.
 * <p>
 * For example, to print all elements of a <tt>Vector&lt;E&gt;</tt> <i>v</i>:
 * <pre>
 *   for (Enumeration&lt;E&gt; e = v.elements(); e.hasMoreElements();)
 *       System.out.println(e.nextElement());</pre>
 * <p>
 * Methods are provided to enumerate through the elements of a
 * vector, the keys of a hashtable, and the values in a hashtable.
 * Enumerations are also used to specify the input streams to a
 * <code>SequenceInputStream</code>.
 * <p>
 * NOTE: The functionality of this interface is duplicated by the Iterator
 * interface.  In addition, Iterator adds an optional remove operation, and
 * has shorter method names.  New implementations should consider using
 * Iterator in preference to Enumeration.
 *
 * @see     java.util.Iterator
 * @see     java.io.SequenceInputStream
 * @see     java.util.Enumeration#nextElement()
 * @see     java.util.Hashtable
 * @see     java.util.Hashtable#elements()
 * @see     java.util.Hashtable#keys()
 * @see     java.util.Vector
 * @see     java.util.Vector#elements()
 *
 * @author  Lee Boynton
 * @since   JDK1.0
 */
public interface Enumeration<E> {
    /**
     * Tests if this enumeration contains more elements.
     *
     * @return  <code>true</code> if and only if this enumeration object
     *           contains at least one more element to provide;
     *          <code>false</code> otherwise.
     */
    boolean hasMoreElements();

    /**
     * Returns the next element of this enumeration if this enumeration
     * object has at least one more element to provide.
     *
     * @return     the next element of this enumeration.
     * @exception  NoSuchElementException  if no more elements exist.
     */
    E nextElement();
}

最常见的处理方式是：

Enumeration enum = . . .;
while (enum.hasMoreElements()) {
Object o = enum.nextElement();
processObject(o);
}

或者：

for (Enumeration enum = . . .; enum.hasMoreElements(); ) {
Object o = enum.nextElement();
processObject(o);
}

Vector v = new Vector(3);
v.add(new FileInputStream("/etc/motd"));
v.add(new FileInputStream("foo.bar"));
v.add(new FileInputStream("/temp/john.txt"));

Enumeration enum = v.elements();
SequenceInputStream sis = new SequenceInputStream(enum);
InputStreamReader isr = new InputStreamReader(sis);
BufferedReader br = new BufferedReader(isr);
String line;
while ((line = br.readLine()) != null) {
System.out.println(line);
}
br.close();