src/java.base/share/classes/sun/util/ResourceBundleEnumeration.java

/*
 * Copyright (c) 2001, 2023, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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package sun.util;

import java.util.Enumeration;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.Set;

/**
 * Implements an Enumeration that combines elements from a Set and
 * an Enumeration. Used by ListResourceBundle and PropertyResourceBundle.
 */
public class ResourceBundleEnumeration implements Enumeration<String> {

    private final Set<String> set;
    private final Iterator<String> iterator;
    private final Enumeration<String> enumeration; // may remain null

    /**
     * Constructs a resource bundle enumeration.
     * @param set an set providing some elements of the enumeration
     * @param enumeration an enumeration providing more elements of the enumeration.
     *        enumeration may be null.
     */
    public ResourceBundleEnumeration(Set<String> set, Enumeration<String> enumeration) {
        this.set = set;
        this.iterator = set.iterator();
        this.enumeration = enumeration;
    }

    String next = null;

    public boolean hasMoreElements() {
        if (next == null) {
            if (iterator.hasNext()) {
                next = iterator.next();
            } else if (enumeration != null) {
                while (next == null && enumeration.hasMoreElements()) {
                    next = enumeration.nextElement();
                    if (set.contains(next)) {
                        next = null;
                    }
                }
            }
        }
        return next != null;
    }

    public String nextElement() {
        if (hasMoreElements()) {
            String result = next;
            next = null;
            return result;
        } else {
            throw new NoSuchElementException();
        }
    }
}