code: public String evaluate(InputSource source)
          throws XPathExpressionException {
          return (String)this.evaluate( source, XPathConstants.STRING );
      }
docs: <p>Evaluate the compiled XPath expression in the context of the specified <code>InputSource</code> and return the result as a
  <code>String</code>.</p>
  
  <p>This method calls {@link #evaluate(InputSource source, QName returnType)} with a <code>returnType</code> of
  {@link XPathConstants#STRING}.</p>
  
  <p>See "Evaluation of XPath Expressions" section of JAXP 1.3 spec
  for context item evaluation,
  variable, function and QName resolution and return type conversion.</p>
  
  <p>If <code>source</code> is <code>null</code>, then a <code>NullPointerException</code> is thrown.</p>
  
  @param source The <code>InputSource</code> of the document to evaluate over.
  
  @return The <code>String</code> that is the result of evaluating the expression and converting the result to a
  <code>String</code>.
  
  @throws XPathExpressionException If the expression cannot be evaluated.
  @throws NullPointerException If  <code>source</code> is <code>null</code>.