Commons Collections example source code file (BoundedFifoBuffer.java)

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Java - Commons Collections tags/keywords

boundedcollection, boundedfifobuffer, boundedfifobuffer, buffer, bufferoverflowexception, bufferunderflowexception, illegalstateexception, io, ioexception, iterator, iterator, object, object, the, the, util

The Commons Collections BoundedFifoBuffer.java source code

/*
 *  Licensed to the Apache Software Foundation (ASF) under one or more
 *  contributor license agreements.  See the NOTICE file distributed with
 *  this work for additional information regarding copyright ownership.
 *  The ASF licenses this file to You under the Apache License, Version 2.0
 *  (the "License"); you may not use this file except in compliance with
 *  the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 *  Unless required by applicable law or agreed to in writing, software
 *  distributed under the License is distributed on an "AS IS" BASIS,
 *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *  See the License for the specific language governing permissions and
 *  limitations under the License.
 */
package org.apache.commons.collections.buffer;

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.util.AbstractCollection;
import java.util.Arrays;
import java.util.Collection;
import java.util.Iterator;
import java.util.NoSuchElementException;

import org.apache.commons.collections.BoundedCollection;
import org.apache.commons.collections.Buffer;
import org.apache.commons.collections.BufferOverflowException;
import org.apache.commons.collections.BufferUnderflowException;

/**
 * The BoundedFifoBuffer is a very efficient implementation of
 * <code>Buffer that is of a fixed size.
 * <p>
 * The removal order of a <code>BoundedFifoBuffer is based on the 
 * insertion order; elements are removed in the same order in which they
 * were added.  The iteration order is the same as the removal order.
 * <p>
 * The {@link #add(Object)}, {@link #remove()} and {@link #get()} operations
 * all perform in constant time.  All other operations perform in linear
 * time or worse.
 * <p>
 * Note that this implementation is not synchronized.  The following can be
 * used to provide synchronized access to your <code>BoundedFifoBuffer:
 * <pre>
 *   Buffer fifo = BufferUtils.synchronizedBuffer(new BoundedFifoBuffer());
 * </pre>
 * <p>
 * This buffer prevents null objects from being added.
 * <p>
 * This class is Serializable from Commons Collections 3.1.
 *
 * @since Commons Collections 3.0 (previously in main package v2.1)
 * @version $Revision: 646777 $ $Date: 2008-04-10 13:33:15 +0100 (Thu, 10 Apr 2008) $
 * 
 * @author Avalon
 * @author Berin Loritsch
 * @author Paul Jack
 * @author Stephen Colebourne
 * @author Herve Quiroz
 */
public class BoundedFifoBuffer extends AbstractCollection
        implements Buffer, BoundedCollection, Serializable {

    /** Serialization version */
    private static final long serialVersionUID = 5603722811189451017L;

    /** Underlying storage array */
    private transient Object[] elements;
    
    /** Array index of first (oldest) buffer element */
    private transient int start = 0;
    
    /** 
     * Index mod maxElements of the array position following the last buffer
     * element.  Buffer elements start at elements[start] and "wrap around"
     * elements[maxElements-1], ending at elements[decrement(end)].  
     * For example, elements = {c,a,b}, start=1, end=1 corresponds to 
     * the buffer [a,b,c].
     */
    private transient int end = 0;
    
    /** Flag to indicate if the buffer is currently full. */
    private transient boolean full = false;
    
    /** Capacity of the buffer */
    private final int maxElements;

    /**
     * Constructs a new <code>BoundedFifoBuffer big enough to hold
     * 32 elements.
     */
    public BoundedFifoBuffer() {
        this(32);
    }

    /**
     * Constructs a new <code>BoundedFifoBuffer big enough to hold
     * the specified number of elements.
     *
     * @param size  the maximum number of elements for this fifo
     * @throws IllegalArgumentException  if the size is less than 1
     */
    public BoundedFifoBuffer(int size) {
        if (size <= 0) {
            throw new IllegalArgumentException("The size must be greater than 0");
        }
        elements = new Object[size];
        maxElements = elements.length;
    }

    /**
     * Constructs a new <code>BoundedFifoBuffer big enough to hold all
     * of the elements in the specified collection. That collection's
     * elements will also be added to the buffer.
     *
     * @param coll  the collection whose elements to add, may not be null
     * @throws NullPointerException if the collection is null
     */
    public BoundedFifoBuffer(Collection coll) {
        this(coll.size());
        addAll(coll);
    }

    //-----------------------------------------------------------------------
    /**
     * Write the buffer out using a custom routine.
     * 
     * @param out  the output stream
     * @throws IOException
     */
    private void writeObject(ObjectOutputStream out) throws IOException {
        out.defaultWriteObject();
        out.writeInt(size());
        for (Iterator it = iterator(); it.hasNext();) {
            out.writeObject(it.next());
        }
    }

    /**
     * Read the buffer in using a custom routine.
     * 
     * @param in  the input stream
     * @throws IOException
     * @throws ClassNotFoundException
     */
    private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
        in.defaultReadObject();
        elements = new Object[maxElements];
        int size = in.readInt();
        for (int i = 0; i < size; i++) {
            elements[i] = in.readObject();
        }
        start = 0;
        full = (size == maxElements);
        if (full) {
            end = 0;
        } else {
            end = size;
        }
    }

    //-----------------------------------------------------------------------
    /**
     * Returns the number of elements stored in the buffer.
     *
     * @return this buffer's size
     */
    public int size() {
        int size = 0;

        if (end < start) {
            size = maxElements - start + end;
        } else if (end == start) {
            size = (full ? maxElements : 0);
        } else {
            size = end - start;
        }

        return size;
    }

    /**
     * Returns true if this buffer is empty; false otherwise.
     *
     * @return true if this buffer is empty
     */
    public boolean isEmpty() {
        return size() == 0;
    }

    /**
     * Returns true if this collection is full and no new elements can be added.
     *
     * @return <code>true if the collection is full
     */
    public boolean isFull() {
        return size() == maxElements;
    }
    
    /**
     * Gets the maximum size of the collection (the bound).
     *
     * @return the maximum number of elements the collection can hold
     */
    public int maxSize() {
        return maxElements;
    }
    
    /**
     * Clears this buffer.
     */
    public void clear() {
        full = false;
        start = 0;
        end = 0;
        Arrays.fill(elements, null);
    }

    /**
     * Adds the given element to this buffer.
     *
     * @param element  the element to add
     * @return true, always
     * @throws NullPointerException  if the given element is null
     * @throws BufferOverflowException  if this buffer is full
     */
    public boolean add(Object element) {
        if (null == element) {
            throw new NullPointerException("Attempted to add null object to buffer");
        }

        if (full) {
            throw new BufferOverflowException("The buffer cannot hold more than " + maxElements + " objects.");
        }

        elements[end++] = element;

        if (end >= maxElements) {
            end = 0;
        }

        if (end == start) {
            full = true;
        }

        return true;
    }

    /**
     * Returns the least recently inserted element in this buffer.
     *
     * @return the least recently inserted element
     * @throws BufferUnderflowException  if the buffer is empty
     */
    public Object get() {
        if (isEmpty()) {
            throw new BufferUnderflowException("The buffer is already empty");
        }

        return elements[start];
    }

    /**
     * Removes the least recently inserted element from this buffer.
     *
     * @return the least recently inserted element
     * @throws BufferUnderflowException  if the buffer is empty
     */
    public Object remove() {
        if (isEmpty()) {
            throw new BufferUnderflowException("The buffer is already empty");
        }

        Object element = elements[start];

        if (null != element) {
            elements[start++] = null;

            if (start >= maxElements) {
                start = 0;
            }

            full = false;
        }

        return element;
    }

    /**
     * Increments the internal index.
     * 
     * @param index  the index to increment
     * @return the updated index
     */
    private int increment(int index) {
        index++; 
        if (index >= maxElements) {
            index = 0;
        }
        return index;
    }

    /**
     * Decrements the internal index.
     * 
     * @param index  the index to decrement
     * @return the updated index
     */
    private int decrement(int index) {
        index--;
        if (index < 0) {
            index = maxElements - 1;
        }
        return index;
    }

    /**
     * Returns an iterator over this buffer's elements.
     *
     * @return an iterator over this buffer's elements
     */
    public Iterator iterator() {
        return new Iterator() {

            private int index = start;
            private int lastReturnedIndex = -1;
            private boolean isFirst = full;

            public boolean hasNext() {
                return isFirst || (index != end);
                
            }

            public Object next() {
                if (!hasNext()) {
                    throw new NoSuchElementException();
                }
                isFirst = false;
                lastReturnedIndex = index;
                index = increment(index);
                return elements[lastReturnedIndex];
            }

            public void remove() {
                if (lastReturnedIndex == -1) {
                    throw new IllegalStateException();
                }

                // First element can be removed quickly
                if (lastReturnedIndex == start) {
                    BoundedFifoBuffer.this.remove();
                    lastReturnedIndex = -1;
                    return;
                }

                int pos = lastReturnedIndex + 1;
                if (start < lastReturnedIndex && pos < end) {
                    // shift in one part
                    System.arraycopy(elements, pos, elements,
                            lastReturnedIndex, end - pos);
                } else {
                    // Other elements require us to shift the subsequent elements
                    while (pos != end) {
                        if (pos >= maxElements) {
                            elements[pos - 1] = elements[0];
                            pos = 0;
                        } else {
                            elements[decrement(pos)] = elements[pos];
                            pos = increment(pos);
                        }
                    }
                }

                lastReturnedIndex = -1;
                end = decrement(end);
                elements[end] = null;
                full = false;
                index = decrement(index);
            }

        };
    }

}