https://jsoup.org/: jsoup is a Java library for working with real-world HTML. It provides a very convenient API for extracting and manipulating data, using the best of DOM, CSS, and jquery-like methods. jsoup implements the WHATWG HTML5 specification, and parses HTML to the same DOM as modern browsers do. (Jonathan Hedley)
  • Jonathan Hedley 
package org.jsoup.select;

import org.jsoup.nodes.Node;


Node filter interface. Provide an implementing class to NodeTraversor to iterate through nodes. 
 This interface provides two methods, head and tail. The head method is called when the node is first seen, and the tail method when all of the node's children have been visited. As an example, head can be used to create a start tag for a node, and tail to create the end tag. 



For every node, the filter has to decide whether to

 The difference between FilterResult.SKIP_CHILDREN and FilterResult.SKIP_ENTIRELY is that the first will invoke tail(Node, int) on the node, while the latter will not. Within tail(Node, int), both are equivalent to FilterResult.CONTINUE. 


/**
 * Node filter interface. Provide an implementing class to {@link NodeTraversor} to iterate through nodes.
 * <p>
 * This interface provides two methods, {@code head} and {@code tail}. The head method is called when the node is first
 * seen, and the tail method when all of the node's children have been visited. As an example, head can be used to
 * create a start tag for a node, and tail to create the end tag.
 * </p>
 * <p>
 * For every node, the filter has to decide whether to
 * <ul>
 * <li>continue ({@link FilterResult#CONTINUE}),</li>
 * <li>skip all children ({@link FilterResult#SKIP_CHILDREN}),</li>
 * <li>skip node entirely ({@link FilterResult#SKIP_ENTIRELY}),</li>
 * <li>remove the subtree ({@link FilterResult#REMOVE}),</li>
 * <li>interrupt the iteration and return ({@link FilterResult#STOP}).</li>
 * </ul>
 * The difference between {@link FilterResult#SKIP_CHILDREN} and {@link FilterResult#SKIP_ENTIRELY} is that the first
 * will invoke {@link NodeFilter#tail(Node, int)} on the node, while the latter will not.
 * Within {@link NodeFilter#tail(Node, int)}, both are equivalent to {@link FilterResult#CONTINUE}.
 * </p>
 */
public interface NodeFilter {
    
Filter decision.

/**
     * Filter decision.
     */
    enum FilterResult {
        
Continue processing the tree 
/** Continue processing the tree */
        CONTINUE,
        
Skip the child nodes, but do call NodeFilter.tail(Node, int) next. 
/** Skip the child nodes, but do call {@link NodeFilter#tail(Node, int)} next. */
        SKIP_CHILDREN,
        
Skip the subtree, and do not call NodeFilter.tail(Node, int). 
/** Skip the subtree, and do not call {@link NodeFilter#tail(Node, int)}. */
        SKIP_ENTIRELY,
        
Remove the node and its children 
/** Remove the node and its children */
        REMOVE,
        
Stop processing 
/** Stop processing */
        STOP
    }

    
Callback for when a node is first visited.

Params:
  • node – the node being visited.
  • depth – the depth of the node, relative to the root node. E.g., the root node has depth 0, and a child node of that will have depth 1.
Returns:Filter decision
/**
     * Callback for when a node is first visited.
     * @param node the node being visited.
     * @param depth the depth of the node, relative to the root node. E.g., the root node has depth 0, and a child node of that will have depth 1.
     * @return Filter decision
     */
    FilterResult head(Node node, int depth);

    
Callback for when a node is last visited, after all of its descendants have been visited.

Params:
  • node – the node being visited.
  • depth – the depth of the node, relative to the root node. E.g., the root node has depth 0, and a child node of that will have depth 1.
Returns:Filter decision
/**
     * Callback for when a node is last visited, after all of its descendants have been visited.
     * @param node the node being visited.
     * @param depth the depth of the node, relative to the root node. E.g., the root node has depth 0, and a child node of that will have depth 1.
     * @return Filter decision
     */
    FilterResult tail(Node node, int depth);
}