How to sort XML elements by value of attributes in Java?

Question

I have a XML string which I need to sort it by the attributes. Here is the input XML string:

<graph>
  <root root_id="1"/>
  <type name="A">
    <extends name="Sphere"/>
  </type>
  <node id="3" name="" type="A">
    <property name="radius" value="0.1"/>
  </node>
  <edge id="2" src_id="5" dest_id="3" type="successor"/>
  <node id="4" name="" type="L">
    <property name="length" value="3.0"/>
  </node>
  <edge id="3" src_id="1" dest_id="4" type="successor"/>
  <node id="5" name="" type="P">
    <property name="color" value="14"/>
  </node>
  <edge id="4" src_id="4" dest_id="5" type="successor"/>
</graph>

Here is the expecting result:

<graph>
  <root root_id="1"/>
  <type name="A">
    <extends name="Sphere"/>
  </type>
  <edge id="3" src_id="1" dest_id="4" type="successor"/>
  <node id="4" name="" type="L">
    <property name="length" value="3.0"/>
  </node>
  <edge id="4" src_id="4" dest_id="5" type="successor"/>
  <node id="5" name="" type="P">
    <property name="color" value="14"/>
  </node>
  <edge id="2" src_id="5" dest_id="3" type="successor"/>
  <node id="3" name="" type="A">
    <property name="radius" value="0.1"/>
  </node>
</graph>

Basically, the nodes have to been linked with the edges one after another, with the help of edge's attribute src_id and dest_id. Any idea will be very appreciated! Thank you very much!

How are you reading the XML input? As just a string or as JAXB object? — Craig, Aug 31 '15 at 09:09
You want to just sort it in the code, or just for clearness? Because if you just want to see xml more clearly, you can just use XML editor like XmlSpy — Thamiar, Aug 31 '15 at 09:11

score 4 · Answer 1 · edited May 23 '17 at 10:29

This Class sorts NodeList by attribute's value given the type of data (String, Integer, Double etc).

Here is the solution in full:

package mynewpackage;

// for the method
import java.lang.reflect.Constructor;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Comparator;
import java.util.List;
import org.w3c.dom.Element;
import org.w3c.dom.Node;
import org.w3c.dom.NodeList;

// for the test example
import org.xml.sax.InputSource;
import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import java.io.StringReader;
import org.w3c.dom.Document;
import java.math.BigDecimal;

public class NodeTools {
    /**
     * Method sorts any NodeList by provided attribute.
     * @param nl NodeList to sort
     * @param attributeName attribute name to use
     * @param asc true - ascending, false - descending
     * @param B class must implement Comparable and have Constructor(String) - e.g. Integer.class , BigDecimal.class etc
     * @return Array of Nodes in required order
     */
    public static Node[] sortNodes(NodeList nl, String attributeName, boolean asc, Class<? extends Comparable> B)
    {        
        class NodeComparator<T> implements Comparator<T>
        {
            @Override
            public int compare(T a, T b)
            {
                int ret;
                Comparable bda = null, bdb = null;
                try{
                    Constructor bc = B.getDeclaredConstructor(String.class);
                    bda = (Comparable)bc.newInstance(((Element)a).getAttribute(attributeName));
                    bdb = (Comparable)bc.newInstance(((Element)b).getAttribute(attributeName));
                }
                catch(Exception e)
                {
                    return 0; // yes, ugly, i know :)
                }
                ret = bda.compareTo(bdb);
                return asc ? ret : -ret; 
            }
        }

        List<Node> x = new ArrayList<>();
        for(int i = 0; i < nl.getLength(); i++)
        {
            x.add(nl.item(i));
        }
        Node[] ret = new Node[x.size()];
        ret = x.toArray(ret);
        Arrays.sort(ret, new NodeComparator<Node>());
        return ret;
    }   

// Test with simple example 
    public static void main(String... args)
    {
        DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();  
        DocumentBuilder builder;
        String s = "<xml><item id=\"1\" price=\"100.00\" /><item id=\"3\" price=\"29.99\" /><item id=\"2\" price=\"5.10\" /></xml>";
        Document doc = null;
        try 
        {  
            builder = factory.newDocumentBuilder();  
            doc = builder.parse(new InputSource(new StringReader(s)));
        }
        catch(Exception e) { System.out.println("Alarm "+e); return; }

        System.out.println("*** Sort by id ***");
        Node[] ret = NodeTools.sortNodes(doc.getElementsByTagName("item"), "id", true, Integer.class);

        for(Node n: ret)
        {
            System.out.println(((Element)n).getAttribute("id")+" : "+((Element)n).getAttribute("price"));
        }

        System.out.println("*** Sort by price ***");
        ret = NodeTools.sortNodes(doc.getElementsByTagName("item"), "price", true, BigDecimal.class);
        for(Node n: ret)
        {
            System.out.println(((Element)n).getAttribute("id")+" : "+((Element)n).getAttribute("price"));
        }
    }
}

In my simple test it prints:

*** Sort by id ***
1 : 100.00
2 : 5.10
3 : 29.99
*** Sort by price ***
2 : 5.10
3 : 29.99
1 : 100.00

all this for sorting?have you found any new methods – Feb 25 '18 at 10:15 — , Feb 25 '18 at 10:15

How to sort XML elements by value of attributes in Java?

1 Answers1

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