To evaluate an attribute in an XML document using XPath in Java, you can use the javax.xml.xpath package. Here's an example that demonstrates how to evaluate an attribute in an XML document using XPath:

import java.io.StringReader;
import javax.xml.xpath.XPath;
import javax.xml.xpath.XPathExpression;
import javax.xml.xpath.XPathFactory;
import org.xml.sax.InputSource;

public class XPathAttributeEvaluator {

    public static void main(String[] args) throws Exception {
        // create an XML document string
        String xml = "<root><child attribute=\"value\">text</child></root>";

        // create an XPath expression to select the attribute
        XPathFactory xPathfactory = XPathFactory.newInstance();
        XPath xpath = xPathfactory.newXPath();
        XPathExpression expr = xpath.compile("/root/child/@attribute");

        // parse the XML document
        InputSource inputSource = new InputSource(new StringReader(xml));

        // evaluate the XPath expression
        String attribute = (String) expr.evaluate(inputSource);
        System.out.println("Attribute value: " + attribute);
    }

}

This code creates an XML document string, and then creates an XPath expression that selects the attribute attribute of the child element. The XPath expression is compiled and then evaluated against the XML document using the evaluate method of the XPathExpression object. The result of the evaluation is a String object that contains the value of the attribute.

Note that the evaluate method can return different types depending on the type of the selected node. For example, if the selected node is an element, the method will return a org.w3c.dom.Node object that represents the element. You can cast the result to the appropriate type based on the XPath expression and the XML document you are working with.