When opening a stdhep file with the org.lcsim classes the file is immediately converted into an org.lcsim "EventHeader" object, after which it is hard to write the event back out as a stdhep file.
However, org.lcsim contains underlying libraries (freehep-io-stdhep and freehep-physics) which can be used to read and write stdhep files. There are two representations of stdhep events available, a true Object-Oriented approach using Particle and Event objects, and a lower level interface which reflects the Stdhep "common-block" format. If you want to read stdhep files, apply a simple filter, and write them out again, the "common-block" style interface, while a little less pretty, is probably considerably more efficient.
import hep.io.stdhep.StdhepEvent;
import hep.io.stdhep.StdhepReader;
import hep.io.stdhep.StdhepRecord;
import hep.io.stdhep.StdhepWriter;
import java.io.EOFException;
import java.io.IOException;
/**
* A simple class for filtering stdhep events
*/
public class StdhepFilter
{
public static void main( String[] args ) throws IOException
{
int nRecord = 0;
int nIn = 0;
int nOut = 0;
StdhepReader reader = new StdhepReader("inputfile.stdhep");
StdhepWriter writer = new StdhepWriter("outputfile.stdhep",reader.getTitle(),reader.getComment(),0);
try
{
for (;;)
{
StdhepRecord record = reader.nextRecord();
nRecord++;
if (record instanceof StdhepEvent)
{
StdhepEvent event = (StdhepEvent) record;
nIn++;
// Insert your filter code here!
if (event.getNHEP() < 100) continue;
}
writer.writeRecord(record);
nOut++;
}
}
catch (EOFException x)
{
// No problem
}
finally
{
writer.close();
reader.close();
System.out.println("Records: "+nRecord+" Events: "+nIn+" Out: "+nOut);
}
}
}
Documentation on the freehep-stdhep classes is available here:
http://java.freehep.org/freehep-stdhep/apidocs/index.html
Documentation on the freehep-physics classes (only necessary if you want the more OO access to stdhep events is here:
http://java.freehep.org/freehep-physics/apidocs/index.html
Note in particular the StdhepConverter class.