Implementation of Factory Method pattern for XML and Excel documents
Although your implementation is superficially correct, my guess is that it is missing some of the finer points of factory method implementation . 

A good example that you can look at is URLStreamHandlerFactory in java.

Some minor changes that I would make to your code would be to Introduce an Enum such as 

public enum TYPE {
    EXCEL,
    XML

}


Modify my DocumentCreator to 

public abstract class DocumentCreator {
    public static Document createDocument(TYPE type) {

        if (type.equals(TYPE.EXCEL)) {
            return new ExcelDoc();
        } else if (type.equals(TYPE.XML)) {
            return new XMLDoc();
        } else {
            // Default Implementation
            return new XMLDoc();
        }

    }

    public static Document createDocument() {

        return new XMLDoc();

    }
}


And my main Class would invoke as follows : 

public class MainClass {

    public static void main(String [] args){
       // DocumentCreator docCreator = new XMLDocumentCreator();
        System.out.println(DocumentCreator.createDocument().parse("SOMETHING"));
        System.out.println(DocumentCreator.createDocument(TYPE.XML).parse("SAX"));
        System.out.println(DocumentCreator.createDocument(TYPE.EXCEL).parse("DOM"));
        System.out.println(DocumentCreator.createDocument(TYPE.XML).parse("MSDoc"));
        System.out.println(DocumentCreator.createDocument(TYPE.EXCEL).parse("OpenOffice"));


      }
}


These are just a few minor modifications to your code . I would suggest you to go though this Factory Pattern Explained to understand the finer aspects  of 1) Instantiation control 2) loose coupling 3) improving readability by encapsulation  which is feel is the primary need of using the factory pattern.