throwing exception inside the java 8 stream foreach - error-handling

Question

I am using java 8 stream and I can not throw the exceptions inside the foreach of stream.
stream.forEach(m -> {
try {
if (isInitial) {
isInitial = false;
String outputName = new SimpleDateFormat(Constants.HMDBConstants.HMDB_SDF_FILE_NAME).format(new Date());
if (location.endsWith(Constants.LOCATION_SEPARATOR)) {
savedPath = location + outputName;
} else {
savedPath = location + Constants.LOCATION_SEPARATOR + outputName;
}
File output = new File(savedPath);
FileWriter fileWriter = null;
fileWriter = new FileWriter(output);
writer = new SDFWriter(fileWriter);
}
writer.write(m);
} catch (IOException e) {
throw new ChemIDException(e.getMessage(),e);
}
});
and this is my exception class
public class ChemIDException extends Exception {
public ChemIDException(String message, Exception e) {
super(message, e);
}
}
I am using loggers to log the errors in upper level. So I want to throw the exception to top. Thanks

Answer 1

Try extending RuntimeException instead. The method that is created to feed to the foreach does not have that type as throwable, so you need something that is runtime throwable.
WARNING: THIS IS PROBABLY NOT A VERY GOOD IDEA
But it will probably work.

Answer 2

Why are you using forEach, a method designed to process every element, when all you want to do, is to process the first element? Instead of realizing that forEach is the wrong method for the job (or that there are more methods in the Stream API than forEach), you are kludging this with an isInitial flag.
Just consider:
Optional<String> o = stream.findFirst();
if(o.isPresent()) try {
String outputName = new SimpleDateFormat(Constants.HMDBConstants.HMDB_SDF_FILE_NAME)
.format(new Date());
if (location.endsWith(Constants.LOCATION_SEPARATOR)) {
savedPath = location + outputName;
} else {
savedPath = location + Constants.LOCATION_SEPARATOR + outputName;
}
File output = new File(savedPath);
FileWriter fileWriter = null;
fileWriter = new FileWriter(output);
writer = new SDFWriter(fileWriter);
writer.write(o.get());
} catch (IOException e) {
throw new ChemIDException(e.getMessage(),e);
}
which has no issues with exception handling. This example assumes that the Stream’s element type is String. Otherwise, you have to adapt the Optional<String> type.
If, however, your isInitial flag is supposed to change more than once during the stream processing, you are definitely using the wrong tool for your job. You should have read and understood the “Stateless behaviors” and “Side-effects” sections of the Stream API documentation, as well as the “Non-interference” section, before using Streams. Just converting loops to forEach invocations on a Stream doesn’t improve the code.