It can be done in many different ways: getElementsByTagName,
XPath etc., but just for the fun of it I tried using one
of the less known features of W3C DOM - the Walker. See code
below.

Arne

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import java.io.File;
import java.io.FileOutputStream;

import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;

import org.apache.xml.serialize.OutputFormat;
import org.apache.xml.serialize.XMLSerializer;
import org.w3c.dom.Document;
import org.w3c.dom.Node;
import org.w3c.dom.traversal.DocumentTraversal;
import org.w3c.dom.traversal.NodeFilter;
import org.w3c.dom.traversal.TreeWalker;

public class Selection {
    public static void main(String[] args) throws Exception {
        DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
        DocumentBuilder db = dbf.newDocumentBuilder();
        Document doc = db.parse(new File("C:\\stud.xml"));
        Document newdoc = db.newDocument();
        newdoc.appendChild(newdoc.createElement("Students"));
        TreeWalker walk =
((DocumentTraversal)doc).createTreeWalker(doc.getDocumentElement(),
NodeFilter.SHOW_ELEMENT, new Filter(), false);
        Node n;
        while((n = walk.nextNode()) != null) {

newdoc.getDocumentElement().appendChild(newdoc.importNode(n, true));
        }
        OutputFormat fmt = new OutputFormat();
        fmt.setIndenting(true);
        XMLSerializer ser = new XMLSerializer(new
FileOutputStream("C:\\stud2.xml"), fmt);
        ser.serialize(newdoc);
    }
}

class Filter implements NodeFilter {
    public short acceptNode(Node n) {
        if(n.getNodeName().equals("Student")) {
            int num =
Integer.parseInt(n.getAttributes().getNamedItem("number").getNodeValue());
            if(num >= 7 && num <= 8) {
               return NodeFilter.FILTER_ACCEPT;
            } else {
                return NodeFilter.FILTER_REJECT;
            }
        } else {
            return NodeFilter.FILTER_REJECT;
        }
    }
}