In Java, the Document Object Model (DOM) API provides a way to represent an XML document as a tree of nodes, which can be manipulated programmatically. The root of the tree is represented by a Document object, which can be obtained by parsing an XML document using a parser such as DocumentBuilderFactory.

Here's an example of how to create a Document object in Java using the DOM API:

DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
DocumentBuilder builder = factory.newDocumentBuilder();
Document document = builder.newDocument();

Element root = document.createElement("root");
document.appendChild(root);

Element child = document.createElement("child");
child.setTextContent("Hello, world!");
root.appendChild(child);

In this example, a new Document object is created using the DocumentBuilder class, which is obtained from a DocumentBuilderFactory. The Document object is initially empty, so a new Element is created to serve as the root of the tree. A child element is also created and added to the root element, and some text content is added to the child element.

Once you have created a Document object, you can manipulate its tree of nodes using various methods provided by the DOM API. For example, you can add and remove nodes, change their attributes and text content, and navigate the tree using methods such as getElementsByTagName() and getParentNode().

The DOM API is a powerful tool for manipulating XML documents in Java, but it can also be relatively complex and memory-intensive for large documents. If you are working with very large XML documents, you may want to consider using a different API such as StAX or SAX, which provide a more streaming-based approach to parsing and processing XML.