Java tutorial
/* * Copyright 2002-2004 The Apache Software Foundation * * Licensed 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.Iterator; import java.util.NoSuchElementException; import org.apache.commons.collections.Buffer; import org.apache.commons.collections.BufferUnderflowException; /** * UnboundedFifoBuffer is a very efficient buffer implementation. * According to performance testing, it exhibits a constant access time, but it * also outperforms ArrayList when used for the same purpose. * <p> * The removal order of an <code>UnboundedFifoBuffer</code> 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 #remove()} and {@link #get()} operations perform in constant time. * The {@link #add(Object)} operation performs in amortized 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>UnboundedFifoBuffer</code>: * <pre> * Buffer fifo = BufferUtils.synchronizedBuffer(new UnboundedFifoBuffer()); * </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: 1.9 $ $Date: 2004/06/01 22:57:18 $ * * @author Avalon * @author Federico Barbieri * @author Berin Loritsch * @author Paul Jack * @author Stephen Colebourne */ public class UnboundedFifoBuffer extends AbstractCollection implements Buffer, Serializable { /** Serialization vesrion */ private static final long serialVersionUID = -3482960336579541419L; /** The array of objects in the buffer. */ protected transient Object[] buffer; /** The current head index. */ protected transient int head; /** The current tail index. */ protected transient int tail; /** * Constructs an UnboundedFifoBuffer with the default number of elements. * It is exactly the same as performing the following: * * <pre> * new UnboundedFifoBuffer(32); * </pre> */ public UnboundedFifoBuffer() { this(32); } /** * Constructs an UnboundedFifoBuffer with the specified number of elements. * The integer must be a positive integer. * * @param initialSize the initial size of the buffer * @throws IllegalArgumentException if the size is less than 1 */ public UnboundedFifoBuffer(int initialSize) { if (initialSize <= 0) { throw new IllegalArgumentException("The size must be greater than 0"); } buffer = new Object[initialSize + 1]; head = 0; tail = 0; } //----------------------------------------------------------------------- /** * 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(); int size = in.readInt(); buffer = new Object[size]; for (int i = 0; i < size; i++) { buffer[i] = in.readObject(); } head = 0; tail = size; } //----------------------------------------------------------------------- /** * Returns the number of elements stored in the buffer. * * @return this buffer's size */ public int size() { int size = 0; if (tail < head) { size = buffer.length - head + tail; } else { size = tail - head; } return size; } /** * Returns true if this buffer is empty; false otherwise. * * @return true if this buffer is empty */ public boolean isEmpty() { return (size() == 0); } /** * Adds the given element to this buffer. * * @param obj the element to add * @return true, always * @throws NullPointerException if the given element is null */ public boolean add(final Object obj) { if (obj == null) { throw new NullPointerException("Attempted to add null object to buffer"); } if (size() + 1 >= buffer.length) { Object[] tmp = new Object[((buffer.length - 1) * 2) + 1]; int j = 0; for (int i = head; i != tail;) { tmp[j] = buffer[i]; buffer[i] = null; j++; i++; if (i == buffer.length) { i = 0; } } buffer = tmp; head = 0; tail = j; } buffer[tail] = obj; tail++; if (tail >= buffer.length) { tail = 0; } return true; } /** * Returns the next object in the buffer. * * @return the next object in the buffer * @throws BufferUnderflowException if this buffer is empty */ public Object get() { if (isEmpty()) { throw new BufferUnderflowException("The buffer is already empty"); } return buffer[head]; } /** * Removes the next object from the buffer * * @return the removed object * @throws BufferUnderflowException if this buffer is empty */ public Object remove() { if (isEmpty()) { throw new BufferUnderflowException("The buffer is already empty"); } Object element = buffer[head]; if (null != element) { buffer[head] = null; head++; if (head >= buffer.length) { head = 0; } } return element; } /** * Increments the internal index. * * @param index the index to increment * @return the updated index */ private int increment(int index) { index++; if (index >= buffer.length) { 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 = buffer.length - 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 = head; private int lastReturnedIndex = -1; public boolean hasNext() { return index != tail; } public Object next() { if (!hasNext()) { throw new NoSuchElementException(); } lastReturnedIndex = index; index = increment(index); return buffer[lastReturnedIndex]; } public void remove() { if (lastReturnedIndex == -1) { throw new IllegalStateException(); } // First element can be removed quickly if (lastReturnedIndex == head) { UnboundedFifoBuffer.this.remove(); lastReturnedIndex = -1; return; } // Other elements require us to shift the subsequent elements int i = lastReturnedIndex + 1; while (i != tail) { if (i >= buffer.length) { buffer[i - 1] = buffer[0]; i = 0; } else { buffer[i - 1] = buffer[i]; i++; } } lastReturnedIndex = -1; tail = decrement(tail); buffer[tail] = null; index = decrement(index); } }; } }