/* 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.io.converters;
import java.io.DataInput;
import java.io.DataOutput;
import java.io.IOException;
import xxl.core.functions.Function;
import xxl.core.io.Convertable;
/**
* This class provides a converter that is able to read and write objects that
* implements the <code>Convertable</code> interface. The objects are read and
* written by calling their read and write methods. When reading an object a
* factory method can be used for creating the object instead of specifying it.
*
* <p>Example usage (1).
* <code><pre>
* // create a byte array output stream
*
* ByteArrayOutputStream output = new ByteArrayOutputStream();
*
* // create two convertable objects (bit sets)
*
* BitSet b1 = new BitSet(13572l);
* BitSet b2 = new BitSet(-1l);
*
* // create a factory method for bit sets
*
* Function<Object, BitSet> factory = new Function<Object, BitSet>() {
* public BitSet invoke() {
* return new BitSet();
* }
* };
*
* // create a new convertable converter that converts bit sets
*
* ConvertableConverter<BitSet> converter = new ConvertableConverter<BitSet>(factory);
*
* // write the bit sets to the output stream
*
* converter.write(new DataOutputStream(output), b1);
* converter.write(new DataOutputStream(output), b2);
*
* // create a byte array input stream on the output stream
*
* ByteArrayInputStream input = new ByteArrayInputStream(output.toByteArray());
*
* // read the bit sets from the input stream and compare it to
* // the original bit sets
*
* System.out.println(b1.compareTo(converter.read(new DataInputStream(input))));
* System.out.println(b2.compareTo(converter.read(new DataInputStream(input))));
*
* // close the streams after use
*
* input.close();
* output.close();
* </pre></code></p>
*
* @param <T> the type of the object that can be converted by using this
* converter (must be a subtype of <code>Convertable</code>).
* @see DataInput
* @see DataOutput
* @see Function
* @see IOException
*/
public class ConvertableConverter<T extends Convertable> extends Converter<T> {
/**
* This instance can be used for getting a default instance of a
* convertable converter. 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
* a convertable converter.
*/
public static final ConvertableConverter<Convertable> DEFAULT_INSTANCE = new ConvertableConverter<Convertable>();
/**
* A factory method that is used for initializing the object to read. This
* function will be invoked when the read method is called without
* specifying an object to restore.
*/
protected Function<?, ? extends T> function;
/**
* Constructs a new convertable converter and uses the specified function
* as factory method. For this reason the object to read must not be
* specified when calling the read method. In this case the object is
* initialized be invoking the specified function.
*
* @param function a factory method that is used for initializinge the
* object to read when it is not specified.
*/
public ConvertableConverter(Function<?, ? extends T> function) {
this.function = function;
}
/**
* Constructs a new convertable converter without a factory method. For
* this reason the objects to read must be explicitly specified when
* calling the read method.
*/
public ConvertableConverter() {
this(null);
}
/**
* Reads the state (the attributes) for the specified object from the
* specified data input and returns the restored object.
*
* <p>This implementation uses the specified object (that implements the
* convertable interface) to call its read method. When this object is
* <code>null</code> it is initialized by invoking the function (factory
* method).
*
* @param dataInput the stream to read data from in order to restore the
* object.
* @param object the object to be restored. If the object is null it is
* initialized by invoking the function (factory method).
* @return the restored object.
* @throws IOException if I/O errors occur.
* @throws NullPointerException when the given object is null and no
* factory method is specified.
*/
@Override
public T read(DataInput dataInput, T object) throws IOException {
if (object == null) {
if (function == null)
throw new NullPointerException("missing factory method");
object = function.invoke();
}
object.read(dataInput); //fill the object with data
return object;
}
/**
* Writes the state (the attributes) of the specified object to the
* specified data output.
*
* <p>This implementation calls the write method of the specified object
* (that implements the convertable interface).</p>
*
* @param dataOutput the stream to write the state (the attributes) of the
* object to.
* @param object the object whose state (attributes) should be written to
* the data output.
* @throws IOException includes any I/O exceptions that may occur.
*/
@Override
public void write(DataOutput dataOutput, T object) throws IOException {
object.write(dataOutput);
}
}