[Chapter 17] Enumeration
Author: Mark Grand and Jonathan Knudsen
Date: Fri Aug 8 17:42:15 1997

#

| Chapter 17 The java.util Package |

# Enumeration

## Name

Enumeration

## Synopsis

Interface Name:

  `java.util.Enumeration`

Super-interface:

  None

Immediate Sub-interfaces:

  None

Implemented By:

  `java.util.StringTokenizer`

Availability:

  JDK 1.0 or later

## Description

An object that implements the `Enumeration` interface provides a way to access a set of objects sequentially. The `Enumeration` object hides the actual organization of the set of objects from the code that is using it. An `Enumeration` can iterate through, or enumerate, its set of objects one at a time. A specific implementation of the interface controls the order in which the objects are presented.

The following is an example of how an `Enumeration` is used. The example shows a method for printing the values in an `Enumeration`:

    void printAll(Enumeration e) {
        while ( e.hasMoreElements() ) {
            System.out.println(e.nextElement());
        }
    }

Note that an `Enumeration` can be used only once: it iterates through its collection of objects in one direction and cannot be reset or rewound.

Normally, an `Enumeration` is not instantiated directly, but instead returned by a method that needs to enumerate a set of values. For example, the `elements()` method of the `Vector` class returns an `Enumeration` of the elements in the `Vector`. By the same token, the `elements()` and `keys()` methods of the `Hashtable` class return `Enumeration` objects for the keys and values in the `Hashtable`.

## Interface Declaration

    public abstract interface java.util.Enumeration {
      // Methods
      public abstract boolean hasMoreElements();
      public abstract Object nextElement() throws NoSuchElementException;
    }

## Methods

### hasMoreElements

#### public abstract boolean hasMoreElements()

Returns

  `true` if the there are more objects to retrieve; `false` otherwise.

Description

  This method returns `true` if the `nextElement()` method of this `Enumeration` returns an object the next time it is called.

### nextElement

#### public abstract Object nextElement()

Returns

  The next object in this `Enumeration`.

Throws

  `NoSuchElementException`

    If there are no more objects to return.

Description

  This method returns the next object in the set of objects encapsulated by this `Enumeration`.

## See Also

`Hashtable`, `StringTokenizer`, `Vector`

EmptyStackException |  | EventListener