Understanding an Enum Class Declaration

KevinSimonson <kvnsmnsn@hotmail.com>
Fri, 1 Oct 2010 10:30:00 -0700 (PDT)
I'm trying to understand how the open source Limewire project's code
works. I'm
taking a look at the source code for enumeration type
<SearchCategory>. It be-
gins like this:

package org.limewire.core.api.search;

import java.util.EnumMap;

import org.limewire.core.api.Category;

 * All categories that can be searched.
public enum SearchCategory {
    ALL(null, 0),
    AUDIO(Category.AUDIO, 1),
    VIDEO(Category.VIDEO, 2),
    IMAGE(Category.IMAGE, 3),
    DOCUMENT(Category.DOCUMENT, 4),
    PROGRAM(Category.PROGRAM, 5),
    OTHER(Category.OTHER, 6),
    TORRENT(Category.TORRENT, 7);

    private static final EnumMap<Category, SearchCategory> perCategory
= new EnumMap<Category, SearchCategory>(Category.class);

    static {
        for(SearchCategory searchCategory : values()) {
            if(searchCategory.category != null) {

    private final Category category;
    private final int id;

    private SearchCategory(Category category, int id) {
        this.id = id;
        this.category = category;

     * Returns the SearchCategory associated with the given Category.
     * This will never return {@link SearchCategory#ALL} because
     * are specific to types.
    public static SearchCategory forCategory(Category category) {
        return perCategory.get(category);

It goes on to add more methods. I have no idea what is going on
here. Does
anybody know where I can go to read about Java <enum> types so that I
can under-
stand what this code means? I understand enumeration types from Ada
enough, but what's throwing me for a loop is the first parameter in
the paren-
theses after each <enum> constant. For example, take the line that
"VIDEO(Category.VIDEO, 2),". What does <Category.VIDEO> do here?

Also, what's the deal with the class fields <EnumMap>, <category>, and
<id>? In
Ada, if you declare a variable to be of an enumeration type it just
takes one of
a list of values. I had understood that if a variable <sc> was
declared to be
of type <SearchCategory>, then <sc> could be either <ALL>, <AUDIO>,
<VIDEO>, or
any of the other listed values. Can I also get the value <category>
by refer-
ring to <cs.category>?

Finally, after the declaration of <EnumMap>, what's the section
enclosed by
<static {> and <}> all about?

If anybody could explain to me what's going on in this code, or point
me to a
place where this is all documented, I'd really appreciate it.

Kevin Simonson

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