code: @Override
  	public void setDefaultPrefix(final String prefixString) {
  		lock.writeLock().lock();
  		try {
  			if (prefixString == null || StringUtils.isBlank(prefixString)) {
  				defaultPrefix = null;
  			} else {
  				if (prefixes != null) {
  					if (!prefixes.contains(prefixString)) {
  						throw new IllegalArgumentException("The given prefix is not part of the prefixes.");
  					}
  				}
  				defaultPrefix = prefixString;
  			}
  		} finally {
  			lock.writeLock().unlock();
  		}
  	}
docs: /*
  (non-Javadoc)
  
  @see
  net.sf.prefixedproperties.config.PrefixConfig#setDefaultPrefix(java.lang.
  String)