Java example source code file (Iterators.java)

This example Java source code file (Iterators.java) is included in the alvinalexander.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" ^TM.

Learn more about this Java project at its project page.

Java - Java tags/keywords

adapter, array, empty, filter, hashset, iterator, iterators, map, readonly, singleton, union, unique, util

The Iterators.java Java example source code

/*
 * Copyright (c) 1997, 2010, 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
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

package com.sun.xml.internal.xsom.impl.scd;

import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.Set;
import java.util.HashSet;
import java.util.Collections;

/**
 * Various convenient {@link Iterator} implementations.
 * @author Kohsuke Kawaguchi
 */
public class Iterators {

    static abstract class ReadOnly<T> implements Iterator {
        public final void remove() {
            throw new UnsupportedOperationException();
        }
    }

    // we need to run on JDK 1.4
    private static final Iterator EMPTY = Collections.EMPTY_LIST.iterator();

    public static <T> Iterator empty() {
        return EMPTY;
    }

    public static <T> Iterator singleton(T value) {
        return new Singleton<T>(value);
    }

    /**
     * {@link Iterator} that returns a single (or no) value.
     */
    static final class Singleton<T> extends ReadOnly {
        private T next;

        Singleton(T next) {
            this.next = next;
        }

        public boolean hasNext() {
            return next!=null;
        }

        public T next() {
            T r = next;
            next = null;
            return r;
        }
    }

    /**
     * {@link Iterator} that wraps another {@link Iterator} and changes its type.
     */
    public static abstract class Adapter<T,U> extends ReadOnly {
        private final Iterator<? extends U> core;

        public Adapter(Iterator<? extends U> core) {
            this.core = core;
        }

        public boolean hasNext() {
            return core.hasNext();
        }

        public T next() {
            return filter(core.next());
        }

        protected abstract T filter(U u);
    }

    /**
     * For each U, apply U->Iterator<T> function and then iterate all
     * the resulting T.
     */
    public static abstract class Map<T,U> extends ReadOnly {
        private final Iterator<? extends U> core;

        private Iterator<? extends T> current;

        protected Map(Iterator<? extends U> core) {
            this.core = core;
        }

        public boolean hasNext() {
            while(current==null || !current.hasNext()) {
                if(!core.hasNext())
                    return false;   // nothing more to enumerate
                current = apply(core.next());
            }
            return true;
        }

        public T next() {
            return current.next();
        }

        protected abstract Iterator<? extends T> apply(U u);
    }

    /**
     * Filter out objects from another iterator.
     */
    public static abstract class Filter<T> extends ReadOnly {
        private final Iterator<? extends T> core;
        private T next;

        protected Filter(Iterator<? extends T> core) {
            this.core = core;
        }

        /**
         * Return true to retain the value.
         */
        protected abstract boolean matches(T value);

        public boolean hasNext() {
            while(core.hasNext() && next==null) {
                next = core.next();
                if(!matches(next))
                    next = null;
            }

            return next!=null;
        }

        public T next() {
            if(next==null)      throw new NoSuchElementException();
            T r = next;
            next = null;
            return r;
        }
    }

    /**
     * Only return unique items.
     */
    static final class Unique<T> extends Filter {
        private Set<T> values = new HashSet();
        public Unique(Iterator<? extends T> core) {
            super(core);
        }

        protected boolean matches(T value) {
            return values.add(value);
        }
    }

    /**
     * Union of two iterators.
     */
    public static final class Union<T> extends ReadOnly {
        private final Iterator<? extends T> first,second;

        public Union(Iterator<? extends T> first, Iterator second) {
            this.first = first;
            this.second = second;
        }

        public boolean hasNext() {
            return first.hasNext() || second.hasNext();
        }

        public T next() {
            if(first.hasNext())     return first.next();
            else                    return second.next();
        }
    }

    /**
     * Array iterator.
     */
    public static final class Array<T> extends ReadOnly {
        private final T[] items;
        private int index=0;
        public Array(T[] items) {
            this.items = items;
        }

        public boolean hasNext() {
            return index<items.length;
        }

        public T next() {
            return items[index++];
        }
    }
}