Q: How to validate a XML file with a specific schema file?
A:
import java.io.IOException;
import java.util.LinkedList;

import nu.xom.Builder;
import nu.xom.Document;
import nu.xom.ParsingException;

import org.xml.sax.SAXException;
import org.xml.sax.XMLReader;
import org.xml.sax.helpers.XMLReaderFactory;
...		
try {
	/*
	 * Set specific SAX parser option, to validate the XML file with a the schema
 	 */
	XMLReader xerces = XMLReaderFactory.createXMLReader("org.apache.xerces.parsers.SAXParser");
	xerces.setFeature("http://apache.org/xml/features/validation/schema", true);
	xerces.setProperty("http://apache.org/xml/properties/schema/external-schemaLocation", SCHEMA_LOCATION);
			
	Builder parser = new Builder(xerces,true);
	Document doc = parser.build(xmlDocument);
	System.out.println(xmlDocument + " is schema valid.");
			
	} catch (ParsingException ex) {
		System.err.println("XML File is malformed.");
	} catch (IOException ex) {
		System.err.println("Problems reading the XML file.");
	} catch (SAXException e) {
		e.printStackTrace();
	}
}
...

Q: How to get all the nodes with a specific name?
A:
#XML FILE:

<?xml version="1.0" encoding="UTF-8"?>
<tns:Constraints xmlns:tns="http://www.example.org/ConstraintSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.example.org/ConstraintSchema ../../Structures/Constraints/ConstraintSchema.xsd ">
  
  <tns:Compatibility>
    <tns:Component>
      <tns:type>Model</tns:type>
      <tns:id>1</tns:id>
    </tns:Component>
    <tns:CompatibleComponents>
      <tns:Type>Tube</tns:Type>
      <tns:Component>1</tns:Component>
      <tns:Component>3</tns:Component>
    </tns:CompatibleComponents>
  </tns:Compatibility>
  
  <tns:Compatibility>
  	<tns:Component>
  		<tns:type>Model</tns:type>
  		<tns:id>2</tns:id>
  	</tns:Component>
  	<tns:CompatibleComponents>
  		<tns:Type>Tube</tns:Type>
  		<tns:Component>1</tns:Component>
  		<tns:Component>4</tns:Component>
  	</tns:CompatibleComponents>
  </tns:Compatibility>
  
</tns:Constraints>

#Example of java code to get all Compatibility nodes
1st - Get the root node
2nd - Get all the children of the root node

import nu.xom.Element;
import nu.xom.Elements;
...
Element rootNode = doc.getRootElement();
Elements constraints = rootNode.getChildElements();
Element constraint;
...