Properties::storeToXML method declares that it throws ClassCastException when Properties object contains any keys or values that are not Strings:
https://docs.oracle.com/en/java/javase/14/docs/api/java.base/java/util/Properties.html#storeToXML(java.io.OutputStream,java.lang.String,java.lang.String)
The behavior is not matching spec.

Sample Code:

private void storeToXML_01() throws IOException {
        ByteArrayOutputStream os = new ByteArrayOutputStream();
        Properties pr = new Properties();
        pr.put("one", "\u7532");
        pr.put(new Integer(8), 238923);
        pr.put(new Object(), new Double(1.1));
        pr.storeToXML(os, "Test", "UTF-8");
    }

Not throwing ClassCastException.