Java tutorial
/** * * JFreeReport : a free Java reporting library * * * Project Info: http://reporting.pentaho.org/ * * (C) Copyright 2001-2007, by Object Refinery Ltd, Pentaho Corporation and Contributors. * * This library is free software; you can redistribute it and/or modify it under the terms * of the GNU Lesser General Public License as published by the Free Software Foundation; * either version 2.1 of the License, or (at your option) any later version. * * This library 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 Lesser General Public License for more details. * * You should have received a copy of the GNU Lesser General Public License along with this * library; if not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, * Boston, MA 02111-1307, USA. * * [Java is a trademark or registered trademark of Sun Microsystems, Inc. * in the United States and other countries.] * * ------------ * WeakReferenceList.java * ------------ * (C) Copyright 2001-2007, by Object Refinery Ltd, Pentaho Corporation and Contributors. */ import java.io.IOException; import java.io.ObjectInputStream; import java.io.Serializable; import java.lang.ref.Reference; import java.lang.ref.WeakReference; /** * The WeakReference list uses <code>java.lang.ref.WeakReference</code>s to store its contents. In contrast to the * WeakHashtable, this list knows how to restore missing content, so that garbage collected elements can be restored * when they are accessed. * <p/> * By default this list can contain 25 elements, where the first element is stored using a strong reference, which is * not garbage collected. * <p/> * Restoring the elements is not implemented, concrete implementations will have to override the * <code>restoreChild(int)</code> method. The <code>getMaxChildCount</code> method defines the maxmimum number of * children in the list. When more than <code>maxChildCount</code> elements are contained in this list, add will always * return false to indicate that adding the element failed. * <p/> * To customize the list, override createReference to create a different kind of reference. * <p/> * This list is able to add or replace elements, but inserting or removing of elements is not possible. * * @author Thomas Morgner */ public abstract class WeakReferenceList implements Serializable, Cloneable { /** * The master element. */ private Object master; /** * Storage for the references. */ private transient Reference[] childs; /** * The current number of elements. */ private int size; /** * The maximum number of elements. */ private final int maxChilds; /** * Creates a new weak reference list. The storage of the list is limited to getMaxChildCount() elements. * * @param maxChildCount the maximum number of elements. */ protected WeakReferenceList(final int maxChildCount) { this.maxChilds = maxChildCount; this.childs = new Reference[maxChildCount - 1]; } /** * Returns the maximum number of children in this list. * * @return the maximum number of elements in this list. */ protected final int getMaxChildCount() { return maxChilds; } /** * Returns the master element of this list. The master element is the element stored by a strong reference and cannot * be garbage collected. * * @return the master element */ protected Object getMaster() { return master; } /** * Attempts to restore the child stored on the given index. * * @param index the index. * @return null if the child could not be restored or the restored child. */ protected abstract Object restoreChild(int index); /** * Returns the child stored at the given index. If the child has been garbage collected, it gets restored using the * restoreChild function. * * @param index the index. * @return the object. */ public Object get(final int index) { if (isMaster(index)) { return master; } else { final Reference ref = childs[getChildPos(index)]; if (ref == null) { throw new IllegalStateException("State: " + index); } Object ob = ref.get(); if (ob == null) { ob = restoreChild(index); childs[getChildPos(index)] = createReference(ob); } return ob; } } /** * Replaces the child stored at the given index with the new child which can be null. * * @param report the object. * @param index the index. */ public void set(final Object report, final int index) { if (isMaster(index)) { master = report; } else { childs[getChildPos(index)] = createReference(report); } } /** * Creates a new reference for the given object. * * @param o the object. * @return a WeakReference for the object o without any ReferenceQueue attached. */ private Reference createReference(final Object o) { return new WeakReference(o); } /** * Adds the element to the list. If the maximum size of the list is exceeded, this function returns false to indicate * that adding failed. * * @param rs the object. * @return true, if the object was successfully added to the list, false otherwise */ public boolean add(final Object rs) { if (size == 0) { master = rs; size = 1; return true; } else { if (size < getMaxChildCount()) { childs[size - 1] = createReference(rs); size++; return true; } else { // was not able to add this to this list, maximum number of entries reached. return false; } } } /** * Returns true, if the given index denotes a master index of this list. * * @param index the index. * @return true if the index is a master index. */ protected boolean isMaster(final int index) { return index % getMaxChildCount() == 0; } /** * Returns the internal storage position for the child. * * @param index the index. * @return the internal storage index. */ protected int getChildPos(final int index) { return index % getMaxChildCount() - 1; } /** * Returns the size of the list. * * @return the size. */ public int getSize() { return size; } /** * Serialisation support. The transient child elements were not saved. * * @param in the input stream. * @throws IOException if there is an I/O error. * @throws ClassNotFoundException if a serialized class is not defined on this system. */ private void readObject(final ObjectInputStream in) throws IOException, ClassNotFoundException { in.defaultReadObject(); childs = new Reference[getMaxChildCount() - 1]; for (int i = 0; i < childs.length; i++) { childs[i] = createReference(null); } } /** * Creates and returns a copy of this object. The precise meaning of "copy" may depend on the class of the object. * The general intent is that, for any object <tt>x</tt>, the expression: <blockquote> * <pre> * x.clone() != x</pre></blockquote> * will be true, and that the expression: <blockquote> * <pre> * x.clone().getClass() == x.getClass()</pre></blockquote> * will be <tt>true</tt>, but these are not absolute requirements. While it is typically the case that: <blockquote> * <pre> * x.clone().equals(x)</pre></blockquote> * will be <tt>true</tt>, this is not an absolute requirement. * <p/> * By convention, the returned object should be obtained by calling <tt>super.clone</tt>. If a class and all of its * superclasses (except <tt>Object</tt>) obey this convention, it will be the case that <tt>x.clone().getClass() == * x.getClass()</tt>. * <p/> * By convention, the object returned by this method should be independent of this object (which is being cloned). To * achieve this independence, it may be necessary to modify one or more fields of the object returned by * <tt>super.clone</tt> before returning it. Typically, this means copying any mutable objects that comprise the * internal "deep structure" of the object being cloned and replacing the references to these objects with references * to the copies. If a class contains only primitive fields or references to immutable objects, then it is usually * the case that no fields in the object returned by <tt>super.clone</tt> need to be modified. * <p/> * The method <tt>clone</tt> for class <tt>Object</tt> performs a specific cloning operation. First, if the class of * this object does not implement the interface <tt>Cloneable</tt>, then a <tt>CloneNotSupportedException</tt> is * thrown. Note that all arrays are considered to implement the interface <tt>Cloneable</tt>. Otherwise, this method * creates a new instance of the class of this object and initializes all its fields with exactly the contents of the * corresponding fields of this object, as if by assignment; the contents of the fields are not themselves cloned. * Thus, this method performs a "shallow copy" of this object, not a "deep copy" operation. * <p/> * The class <tt>Object</tt> does not itself implement the interface <tt>Cloneable</tt>, so calling the <tt>clone</tt> * method on an object whose class is <tt>Object</tt> will result in throwing an exception at run time. * * @return a clone of this instance. * @throws CloneNotSupportedException if the object's class does not support the <code>Cloneable</code> interface. * Subclasses that override the <code>clone</code> method can also throw this * exception to indicate that an instance cannot be cloned. * @see Cloneable */ protected Object clone() throws CloneNotSupportedException { final WeakReferenceList list = (WeakReferenceList) super.clone(); list.childs = (Reference[]) childs.clone(); return list; } }