code: @Override
  	public String getPrefixedKey(final String prefixString, final String key) {
  		lock.readLock().lock();
  		try {
  			if (prefixString != null && prefixes.contains(prefixString) && key != null) {
  				return new StringBuilder(prefixString).append(getPrefixDelimiter()).append(key).toString();
  			}
  		} finally {
  			lock.readLock().unlock();
  		}
  		throw new IllegalArgumentException("The given prefix is not part of this PrefixConfig or the key is null.");
  	}
docs: /*
  (non-Javadoc)
  
  @see
  net.sf.prefixedproperties.config.PrefixConfig#getPrefixedKey(java.lang.
  String, java.lang.String)