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Commons Collections example source code file (BoundedFifoBuffer.java)

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

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);
            }

        };
    }

}

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