Filters some elements out from the input enumeration. : Enumeration Interface « Collections « Java Tutorial






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import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.Enumeration;
import java.util.HashSet;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.Set;

/**
 * @since 4.37
 * @author Jaroslav Tulach
 */
final class Enumerations extends Object {


    /**
     * Filters some elements out from the input enumeration.
     * Just make the
     * {@link Processor} return <code>null</code>. Please notice the <code>toAdd</code>
     * argument of the processor is always <code>null</code>.
     * 
     * Example to remove all objects that are not strings:
     * <pre>
     * Processor onlyString = new Processor() {
     *     public Object process(Object obj, Collection alwaysNull) {
     *         if (obj instanceof String) {
     *             return obj;
     *         } else {
     *             return null;
     *         }
     *     }
     * };
     * Enumeration strings = Enumerations.filter(elems, onlyString);
     * </pre>
     *
     * @param en enumeration of any objects
     * @param filter a callback processor for the elements (its toAdd arguments is always null)
     * @return new enumeration which does not include non-processed (returned null from processor) elements
     * @see NbCollections#checkedEnumerationByFilter
     */
    public static <T,R> Enumeration<R> filter(Enumeration<? extends T> en, Processor<T,R> filter) {
        return new FilEn<T,R>(en, filter);
    }

    /**
     * Support for breadth-first enumerating.
     * Before any element is returned
     * for the resulting enumeration it is processed in the {@link Processor} and
     * the processor is allowed to modify it and also add additional elements
     * at the (current) end of the <q>queue</q> by calling <code>toAdd.add</code>
     * or <code>toAdd.addAll</code>. No other methods can be called on the
     * provided <code>toAdd</code> collection.
     * 
     * Example of doing breadth-first walk through a tree:
     * <pre>
     * Processor queueSubnodes = new Processor() {
     *     public Object process(Object obj, Collection toAdd) {
     *         Node n = (Node)obj;
     *         toAdd.addAll (n.getChildrenList());
     *         return n;
     *     }
     * };
     * Enumeration strings = Enumerations.queue(elems, queueSubnodes);
     * </pre>
     *
     * @param en initial content of the resulting enumeration
     * @param filter the processor that is called for each element and can
     *        add and addAll elements to its toAdd Collection argument and
     *        also change the value to be returned
     * @return enumeration with the initial and queued content (it can contain
     *       <code>null</code> if the filter returned <code>null</code> from its
     *       {@link Processor#process} method.
     */
    public static <T,R> Enumeration<R> queue(Enumeration<? extends T> en, Processor<T,R> filter) {
        QEn<T,R> q = new QEn<T,R>(filter);

        while (en.hasMoreElements()) {
            q.put(en.nextElement());
        }

        return q;
    }

    /**
     * Processor interface that can filter out objects from the enumeration,
     * change them or add aditional objects to the end of the current enumeration.
     */
    public static interface Processor<T,R> {
        /** @param original the object that is going to be returned from the enumeration right now
         * @return a replacement for this object
         * @param toAdd can be non-null if one can add new objects at the end of the enumeration
         */
        public R process(T original, Collection<T> toAdd);
    }

    /** Altering enumeration implementation */
    private static final class AltEn<T,R> extends Object implements Enumeration<R> {
        /** enumeration to filter */
        private Enumeration<? extends T> en;

        /** map to alter */
        private Processor<T,R> process;

        /**
        * @param en enumeration to filter
        */
        public AltEn(Enumeration<? extends T> en, Processor<T,R> process) {
            this.en = en;
            this.process = process;
        }

        /** @return true if there is more elements in the enumeration
        */
        public boolean hasMoreElements() {
            return en.hasMoreElements();
        }

        /** @return next object in the enumeration
        * @exception NoSuchElementException can be thrown if there is no next object
        *   in the enumeration
        */
        public R nextElement() {
            return process.process(en.nextElement(), null);
        }
    }
     // end of AltEn

    /** QueueEnumeration
     */
    private static class QEn<T,R> extends Object implements Enumeration<R> {
        /** next object to be returned */
        private ListItem<T> next = null;

        /** last object in the queue */
        private ListItem<T> last = null;

        /** processor to use */
        private Processor<T,R> processor;

        public QEn(Processor<T,R> p) {
            this.processor = p;
        }

        /** Put adds new object to the end of queue.
        * @param o the object to add
        */
        public void put(T o) {
            if (last != null) {
                ListItem<T> li = new ListItem<T>(o);
                last.next = li;
                last = li;
            } else {
                next = last = new ListItem<T>(o);
            }
        }

        /** Adds array of objects into the queue.
        * @param arr array of objects to put into the queue
        */
        public void put(Collection<? extends T> arr) {
            for (T e : arr) {
                put(e);
            }
        }

        /** Is there any next object?
        * @return true if there is next object, false otherwise
        */
        public boolean hasMoreElements() {
            return next != null;
        }

        /** @return next object in enumeration
        * @exception NoSuchElementException if there is no next object
        */
        public R nextElement() {
            if (next == null) {
                throw new NoSuchElementException();
            }

            T res = next.object;

            if ((next = next.next) == null) {
                last = null;
            }

            ;

            ToAdd<T,R> toAdd = new ToAdd<T,R>(this);
            R out = processor.process(res, toAdd);
            toAdd.finish();

            return out;
        }

        /** item in linked list of Objects */
        private static final class ListItem<T> {
            T object;
            ListItem<T> next;

            /** @param o the object for this item */
            ListItem(T o) {
                object = o;
            }
        }

        /** Temporary collection that supports only add and addAll operations*/
        private static final class ToAdd<T,R> extends Object implements Collection<T> {
            private QEn<T,R> q;

            public ToAdd(QEn<T,R> q) {
                this.q = q;
            }

            public void finish() {
                this.q = null;
            }

            public boolean add(T o) {
                q.put(o);

                return true;
            }

            public boolean addAll(Collection<? extends T> c) {
                q.put(c);

                return true;
            }

            private String msg() {
                return "Only add and addAll are implemented"; // NOI18N
            }

            public void clear() {
                throw new UnsupportedOperationException(msg());
            }

            public boolean contains(Object o) {
                throw new UnsupportedOperationException(msg());
            }

            public boolean containsAll(Collection c) {
                throw new UnsupportedOperationException(msg());
            }

            public boolean isEmpty() {
                throw new UnsupportedOperationException(msg());
            }

            public Iterator<T> iterator() {
                throw new UnsupportedOperationException(msg());
            }

            public boolean remove(Object o) {
                throw new UnsupportedOperationException(msg());
            }

            public boolean removeAll(Collection c) {
                throw new UnsupportedOperationException(msg());
            }

            public boolean retainAll(Collection c) {
                throw new UnsupportedOperationException(msg());
            }

            public int size() {
                throw new UnsupportedOperationException(msg());
            }

            public Object[] toArray() {
                throw new UnsupportedOperationException(msg());
            }

            public<X> X[] toArray(X[] a) {
                throw new UnsupportedOperationException(msg());
            }
        }
         // end of ToAdd
    }
     // end of QEn

    /** Filtering enumeration */
    private static final class FilEn<T,R> extends Object implements Enumeration<R> {
        /** marker object stating there is no nexte element prepared */
        private static final Object EMPTY = new Object();

        /** enumeration to filter */
        private Enumeration<? extends T> en;

        /** element to be returned next time or {@link #EMPTY} if there is
        * no such element prepared */
        private R next = empty();

        /** the set to use as filter */
        private Processor<T,R> filter;

        /**
        * @param en enumeration to filter
        */
        public FilEn(Enumeration<? extends T> en, Processor<T,R> filter) {
            this.en = en;
            this.filter = filter;
        }

        /** @return true if there is more elements in the enumeration
        */
        public boolean hasMoreElements() {
            if (next != empty()) {
                // there is a object already prepared
                return true;
            }

            while (en.hasMoreElements()) {
                // read next
                next = filter.process(en.nextElement(), null);

                if (next != null) {
                    // if the object is accepted
                    return true;
                }

                ;
            }

            next = empty();

            return false;
        }

        /** @return next object in the enumeration
        * @exception NoSuchElementException can be thrown if there is no next object
        *   in the enumeration
        */
        public R nextElement() {
            if ((next == EMPTY) && !hasMoreElements()) {
                throw new NoSuchElementException();
            }

            R res = next;
            next = empty();

            return res;
        }

        @SuppressWarnings("unchecked")
        private R empty() {
            return (R)EMPTY;
        }
    }
     // end of FilEn

    /** Returns true from contains if object is not null */
    private static class RNulls<T> implements Processor<T,T> {
        public T process(T original, Collection<T> toAdd) {
            return original;
        }
    }
     // end of RNulls
}








9.35.Enumeration Interface
9.35.1.The Enumeration Interface
9.35.2.If you prefer a for-loop
9.35.3.Where do we get the enumeration from?
9.35.4.The SequenceInputStream Class
9.35.5.Concatenates the content of two enumerations into one.
9.35.6.Filters enumeration to contain each of the provided elements just once.
9.35.7.Filters some elements out from the input enumeration.
9.35.8.For each element of the input enumeration asks the Processor to provide a replacement
9.35.9.Removes all nulls from the input enumeration.
9.35.10.Returns an enumeration that iterates over provided array.
9.35.11.Support for breadth-first enumerating.
9.35.12.Serializable Enumeration
9.35.13.An enumeration that iterates over an array.
9.35.14.Creating Custom Enumerations