Bug 156338 [Acceptance]Exception thrown when set prefix interval to 2
Description:Exception thrown when set prefix interval to 2 Steps to reproduce: preview the attached reports and set prefix interval of group to 2 Expected result: It works properly. Actula result: When setting the prefix interval to 2 exception thrown but it worked properly while the prefix interval was 1 Error log: org.eclipse.birt.report.service.api.ReportServiceException: Error happened while running the report
data/org.eclipse.birt.data/src/org/eclipse/birt/data/engine/impl/group/StringGroupCalculator.java
{
	/**
	 * 
	 * @param intervalStart
	 * @param intervalRange
	 * @throws BirtException
	 */
	public StringGroupCalculator( Object intervalStart, double intervalRange )
			throws BirtException
	{
		super( intervalStart, intervalRange );
	}
	/*
	 * (non-Javadoc)
	 * 
	 * @see org.eclipse.birt.data.engine.impl.group.GroupCalculator#calculate(java.lang.Object)
	 */
	public Object calculate( Object value ) throws BirtException
	{
		if ( value == null || value.equals( "" ) )
		{
			return value;
		}
		return value.toString( ).substring( 0,
				(int) ( Math.round( intervalRange ) ) );
	}
}