Using XPath Expression in Loop

Question

I am using the following XPath expression to loop through a XML document:

/templatedatabase/datatypes/integers/integer[index]/@defaultvalue

Method header:

private static String getGetDefaultValueByIndex(Document doc, XPath xpath, int index)

"index" is a parameter in the method. The problem is the following: The query doesn´t return a value if I put a number in the method header as parameter.(e.g. getGetDefaultValueByIndex(doc, xpath, 1)).

But if I, for instance put a number like 1,2,... in place of index in the query I get the data I want from the XML file while executing the same method. The XPath query is correct and the method is working if I put in the number manually in the query like this:

/templatedatabase/datatypes/integers/integer[1]/@defaultvalue

--> This returns a value.

However:

getGetDefaultValueByIndex(doc, xpath, 1)
/templatedatabase/datatypes/integers/integer[index]/@defaultvalue

--> This doesn´t return a value.

I don´t really understand why this is not working. I can´t imagine that you can´t loop through a XPath Query using a variable.

The complete code of the method:

private static String getGetDefaultValueByIndex(Document doc, XPath xpath, int index) {
         String DefaultValue = null;
         try {
                String expression = "/templatedatabase/datatypes/integers/integer" + "[index]" + "/@defaultvalue";
                DefaultValue = (String) xPath.evaluate(expression, doc, XPathConstants.STRING);     
                System.out.println(DefaultValue);
     } catch(XPathExpressionException e) {
         e.printStackTrace();
     }
         return DefaultValue;
     }

}

Do you mean: "/templatedatabase/datatypes/integers/integer" + [index] + "/@defaultvalue"; ? — Helmer
– Helmer, Commented Apr 1, 2015 at 9:32
Yes, but the query still doesn´t return a value when using index. It is still working when I use a number instead of index though. The query: /templatedatabase/datatypes/integers/integer" + "[index]" + "/@defaultvalue --> Still returns no value.(Index is 1 in method header.) "/templatedatabase/datatypes/integers/integer" + "[1]" + "/@defaultvalue" --> Returns value. — OZ111
– OZ111, Commented Apr 1, 2015 at 9:48

Ian Roberts · Accepted Answer · 2015-04-01 10:11:04Z

1

You've got the quotes in the wrong place, try:

String expression = "/templatedatabase/datatypes/integers/integer[" + index + "]/@defaultvalue";

This will include the value of the index parameter into the XPath expression, rather than the literal string "index".

A word of warning though - constructing XPath expressions dynamically using string concatenation is fine when you're dealing with numeric variables like index, but if you want to provide arbitrary user-specified string parameters to the expression it's safer to use a static XPath expression including XPath variables like

"/templatedatabase/datatypes[@name=$targetName]"

and define an XPathVariableResolver to inject the actual values.

edited Apr 1, 2015 at 10:11

answered Apr 1, 2015 at 9:56

Ian Roberts

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