getting exception while doing xslt transformation through java [duplicate]

Answer 1

The problem is in the constructor of StreamResult, you are passing null there. And this is what it expects

public StreamResult(String systemId) 
Construct a StreamResult from a URL.
Parameters:
    systemId - Must be a String that conforms to the URI syntax.

You should use another constructor. This should work, if your xml and xsl is correct (and t contains your xml). Note that StringBufferInputStream is deprecated but works as a quick-and-dirty solution):

TransformerFactory factory = TransformerFactory.newInstance();
     //File object with path to your xsl file
StreamSource xslStream = new StreamSource(new File("C:\\xsl\\rok.xsl"));
Transformer transformer = factory.newTransformer(xslStream);
    //Read xml from string
StreamSource in = new StreamSource(new StringBufferInputStream(t));
    //Prints to stdout
StreamResult out = new StreamResult(System.out);

Answer 2

That XML formed by

String t =abc+def; //generated xml

isn't well-formed. In particular you've created a document from concatenating two existing XML documents, and consequently it doesn't have a single root node. I don't quite know what you want to achieve (perhaps transform each XML document and then concatenate), but your concatenation of the two docs is certainly not valid.