How to extend built-in objects?

Minesha

I'm building a fancy XML parser/editor right now, and the syntax of my XML file gives every element a unique ID attribute. I wrote a function that returns a child element if it has an attribute whose value... You know what? Just take a look.

XML getChildAttr(XML Parent, String Attribute, String Value) {
  if (Parent.hasChildren()) {
    for (XML childN : Parent.getChildren()) {
      if (childN.getString(Attribute) != null) {
        if (childN.getString(Attribute).equals(Value)) {
          println(childN);
          return childN;
        }
      }
    }
  }
  return null;
}

I wish to be able to call it like this: Parent.getChildAttr(Attribute, Value);, but that would require it to be a method of the XML class. My question is this: Is it possible to add methods to built-in classes? If it is, how would I do so?

Chrisir

Look at extends?

Minesha

Do something like this, then? I was really hoping to be able to alter the XML class itself instead of making a new one.

class XMLN extends XML {
  XML getChildAttr(XML Parent, String Attribute, String Value) {
    if (Parent.hasChildren()) {
      for (XML childN : Parent.getChildren()) {
        if (childN.getString(Attribute) != null) {
          if (childN.getString(Attribute).equals(Value)) {
            println(childN);
            return childN;
          }
        }
      }
    }
  return null;
}

Wait... Do I need a constructor for this?

GoToLoop

• As you had noticed by now, Java can't append members to objects on-the-fly.
• At least not w/o some brutal, very complex secret techniques. :ar!
• Instead we need to use extends to established a derived class.
• However in Processing, class XML isn't supposed to be directly instantiated.
• Rather we need to use loadXML() or parseXML() functions for it.
• That's where the real problem lies: Both of them always return an XML, the original 1. :-<
• But what you need is your hackish derived class instance to be returned. #-o
• So you're gonna need to define a similar function for loadXML(), but returning a hacked XML.
• Below's some template attempt I did. You're still gonna need to customize it: I-)

1. https://GitHub.com/processing/processing/blob/master/core/src/processing/core/PApplet.java#L5787
2. https://GitHub.com/processing/processing/blob/master/core/src/processing/data/XML.java#L144

// forum.Processing.org/two/discussion/17028/how-to-extend-built-in-objects
// GoToLoop (2016-Jun-07)

import java.io.Reader;
import javax.xml.parsers.ParserConfigurationException;
import org.xml.sax.SAXException;

XML loadXMLExtended(String filename) {
  return loadXMLExtended(filename, null);
}

XML loadXMLExtended(String filename, String options) {
  try {
    return new XMLExtended(createReader(filename), options);
  }
  catch (IOException e) {
    throw new RuntimeException(e);
  }
  catch (ParserConfigurationException e) {
    throw new RuntimeException(e);
  }
  catch (SAXException e) {
    throw new RuntimeException(e);
  }
}

static class XMLExtended extends XML {
  XMLExtended (Reader input) throws
    IOException, ParserConfigurationException, SAXException {
    super(input, null);
  }

  XMLExtended (Reader input, String options) throws
    IOException, ParserConfigurationException, SAXException {
    super(input, options);
  }

  XML getChildAttr(XML parent, String attribute, String Value) {
    return parent;
  }
}

void setup() {
  exit();
}

Minesha

oh gosh. I'm gonna just stick with my boring little function, then. Code readability should never be worth using hackish cheats, which themselves are hard to decipher.

GoToLoop

• Making a derived class isn't a hack. Calling it such was just a tongue-in-cheek. :P
• And we can always place the whole solution aside in a different tab in the PDE, away from our main sketch's sight. ;))
• Then we just need to call our customized & "hacked" loadXMLExtended() in place of the Processing's original loadXML(). ;)