/* XXL: The eXtensible and fleXible Library for data processing
Copyright (C) 2000-2011 Prof. Dr. Bernhard Seeger
Head of the Database Research Group
Department of Mathematics and Computer Science
University of Marburg
Germany
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 3 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, see <http://www.gnu.org/licenses/>.
http://code.google.com/p/xxl/
*/
package xxl.core.collections.bags;
import java.util.List;
import xxl.core.cursors.Cursor;
import xxl.core.cursors.sources.EmptyCursor;
import xxl.core.functions.AbstractFunction;
import xxl.core.functions.Function;
import xxl.core.predicates.Predicate;
/**
* This class provides an empty bag. In other words, this bag is not able
* to contain an element. Therefore, every method that inserts elements
* into the bag does nothing. This class is useful when you have to return
* a bag of some kind, but do not have any elements the bag points to.<p>
*
* Example usage (1).
* <pre>
* // create a new empty bag
*
* EmptyBag bag = EmptyBag.DEFAULT_INSTANCE;
*
* // println the number of elements contained by the bag
*
* System.out.println(bag.size());
*
* // create an iteration over 20 random Integers (between 0 and 100)
*
* Cursor cursor = new DiscreteRandomNumber(new JavaDiscreteRandomWrapper(100), 20);
*
* // catch UnsupportedOperationExceptions
*
* try {
*
* // insert all elements of the given iterator
*
* bag.insertAll(cursor);
* }
* catch (UnsupportedOperationException uoe) {
* System.out.println(uoe.getMessage());
* }
*
* // println the number of elements contained by the bag
*
* System.out.println(bag.size());
*
* // close the open bag and cursor after use
*
* bag.close();
* cursor.close();
* </pre>
*
* @see Cursor
* @see EmptyCursor
* @see Function
*/
public class EmptyBag extends AbstractBag {
/**
* Constant specifying that exceptions should <b>not</b> be thrown if
* an element is inserted into the EmptyBag.
*/
public static boolean NO_EXCEPTIONS=false;
/**
* Constant specifying that exceptions should be thrown if
* an element is inserted into the EmptyBag.
*/
public static boolean THROW_EXCEPTIONS=true;
/**
* Variable determining if exceptions are thrown.
*/
protected boolean throwExceptions=THROW_EXCEPTIONS;
/**
* A factory method to create a new EmptyBag (see contract for
* {@link Bag#FACTORY_METHOD FACTORY_METHOD} in interface Bag). It may
* be invoked with a <i>parameter list</i>(for further
* details see Function) of objects, an iterator or without any
* parameters. The factory method always returns a default instance of
* this class.
*
* @see Function
*/
public static final Function<Object,EmptyBag> FACTORY_METHOD = new AbstractFunction<Object,EmptyBag>() {
public EmptyBag invoke (List<? extends Object> list) {
return DEFAULT_INSTANCE;
}
};
/**
* This instance can be used for getting a default instance of
* EmptyBag. It is similar to the <i>Singleton Design Pattern</i> (for
* further details see Creational Patterns, Prototype in <i>Design
* Patterns: Elements of Reusable Object-Oriented Software</i> by
* Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides)
* except that there are no mechanisms to avoid the creation of other
* instances of EmptyBag.
*/
public static final EmptyBag DEFAULT_INSTANCE = new EmptyBag();
/**
* Constructs a new empty bag that is not able to contain any
* elements.
*
* @param throwExceptions Determining if exceptions are thrown when
* inserting elements. Possible values: NO_EXCEPTIONS, THROW_EXCEPTIONS.
*/
public EmptyBag(boolean throwExceptions)
{
this.throwExceptions = throwExceptions;
}
/**
* Constructs a new empty bag that is not able to contain any
* elements. Exceptions are thrown if elements are inserted.
*/
public EmptyBag()
{
this(THROW_EXCEPTIONS);
}
/**
* Returns a cursor to iterate over the elements in this bag without
* any predefined sequence. The cursor is specifying a <i>view</i> on
* the elements of this bag so that closing the cursor takes no
* effect on the bag (e.g., not closing the bag). The behavior
* of the cursor is unspecified if this bag is modified while the
* cursor is in progress in any way other than by calling the methods
* of the cursor.<br>
* This implementation always returns an <tt>EmptyCursor</tt> because
* the bag is empty.
*
* @return a cursor to iterate over the elements in this bag without
* any predefined sequence.
*/
public Cursor cursor () {
return EmptyCursor.DEFAULT_INSTANCE;
}
/**
* Adds the specified element to this bag. This method does not
* perform any kind of <i>duplicate detection</i>.<br>
* This implementation always throws an
* <tt>UnsupportedOperationException</tt>.
*
* @param object element to be added to this bag.
* @throws UnsupportedOperationException when an object should be
* inserted into this bag.
*/
public void insert (Object object) throws UnsupportedOperationException {
if (throwExceptions == THROW_EXCEPTIONS)
throw new UnsupportedOperationException("No modification allowed.");
}
/**
* Returns the number of elements in this bag (its cardinality). If
* this bag contains more than <tt>Integer.MAX_VALUE</tt> elements,
* <tt>Integer.MAX_VALUE</tt> is returned.<br>
* This implementation always returns <tt>0</tt> because the bag is
* empty.
*
* @return the number of elements in this bag (its cardinality).
*/
public int size () {
return 0;
}
/**
* Returns a cursor to iterate over all elements in this bag for which
* the given predicate returns <tt>true</tt>. This method is very
* similar to the cursor method except that its result is determined
* by a predicate. Like the cursor returned by the cursor method, this
* cursor's behavior is unspecified if this bag is modified while the
* cursor is in progress in any way other than by calling the methods
* of the cursor.<br>
* This implementation always returns an <tt>EmptyCursor</tt> because
* the bag is empty.
*
* @param predicate a predicate that determines whether an element of
* this bag should be returned or not.
* @return a cursor to iterate over all elements in this bag for which
* the given predicate returns <tt>true</tt>.
*/
public Cursor query (Predicate predicate) {
return EmptyCursor.DEFAULT_INSTANCE;
}
}