Java – Reading a list of XML elements in Java

javaxml

I would like to iterate over an XML document that is essentially a list of identically structured XML elements. The elements will be serialized into Java objects.

<root>
    <element attribute="value" />
    <element attribute="value" />
    <element attribute="value" />
    ...
</root>

There are a lot of elements within the root element. I would prefer not to load them all into memory. I realize I could use a SAX handler for this, but using a SAX handler to deserialize everything into Java objects seems rather obtuse. I find JDOM very easy to use, but as far as I can tell JDOM always parses the entire tree. Is there a way I can use JDOM to parse the subelements one at a time?

Another reason for using JDOM is it makes writing serialization/deserialization code easy for the corresponding Java objects, which are meaningless if not entirely in memory. However, I don't want to load all of the Java objects into memory at the same time. Rather, I want to iterate over them once.

update: here is an example of how to do this in dom4j: http://docs.codehaus.org/display/GROOVY/Reading+XML+with+Groovy+and+DOM4J. Anyway to do this in jdom?

Best Answer

Why not use StAX (javax.xml.stream.*, an implementation is included in Java SE 6) to stream in the XML, and convert individual portions to objects?

import java.io.FileReader;
import javax.xml.bind.JAXBContext;
import javax.xml.bind.Unmarshaller;
import javax.xml.stream.XMLInputFactory;
import javax.xml.stream.XMLStreamReader;

public class Demo {

    public static void main(String[] args) throws Exception {
        JAXBContext jc = JAXBContext.newInstance(Element.class);
        Unmarshaller unmarshaller = jc.createUnmarshaller();

        XMLInputFactory xif = XMLInputFactory.newFactory();
        XMLStreamReader xsr = xif.createXMLStreamReader(new FileReader("input.xml"));
        xsr.nextTag();
        xsr.nextTag();
        while(xsr.hasNext()) {
            Element element = (Element) unmarshaller.unmarshal(xsr);
            System.out.println(element.getAttribute());
            if(xsr.nextTag() != XMLStreamReader.START_ELEMENT) {
                break;
            }
        }
    }

}

In the above example each individual "element" is unmarshalled into a POJO using JAXB (an implementation is included in Java SE 6), but you could process the fragment as you saw fit. JAXB model details below:

import javax.xml.bind.annotation.XmlAttribute;
import javax.xml.bind.annotation.XmlRootElement;

@XmlRootElement
public class Element {

    private String attribute;

    @XmlAttribute
    public String getAttribute() {
        return attribute;
    }

    public void setAttribute(String attribute) {
        this.attribute = attribute;
    }

}

Note:

StAX and JAXB are also compatible with Java SE 5, you just need to download the implementations separately.

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Java – What are the differences between a HashMap and a Hashtable in Java

There are several differences between HashMap and Hashtable in Java:

Hashtable is synchronized, whereas HashMap is not. This makes HashMap better for non-threaded applications, as unsynchronized Objects typically perform better than synchronized ones.
Hashtable does not allow null keys or values. HashMap allows one null key and any number of null values.
One of HashMap's subclasses is LinkedHashMap, so in the event that you'd want predictable iteration order (which is insertion order by default), you could easily swap out the HashMap for a LinkedHashMap. This wouldn't be as easy if you were using Hashtable.

Since synchronization is not an issue for you, I'd recommend HashMap. If synchronization becomes an issue, you may also look at ConcurrentHashMap.

Java – Is Java “pass-by-reference” or “pass-by-value”

Java is always pass-by-value. Unfortunately, when we deal with objects we are really dealing with object-handles called references which are passed-by-value as well. This terminology and semantics easily confuse many beginners.

It goes like this:

public static void main(String[] args) {
    Dog aDog = new Dog("Max");
    Dog oldDog = aDog;

    // we pass the object to foo
    foo(aDog);
    // aDog variable is still pointing to the "Max" dog when foo(...) returns
    aDog.getName().equals("Max"); // true
    aDog.getName().equals("Fifi"); // false
    aDog == oldDog; // true
}

public static void foo(Dog d) {
    d.getName().equals("Max"); // true
    // change d inside of foo() to point to a new Dog instance "Fifi"
    d = new Dog("Fifi");
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}

In the example above aDog.getName() will still return "Max". The value aDog within main is not changed in the function foo with the Dog "Fifi" as the object reference is passed by value. If it were passed by reference, then the aDog.getName() in main would return "Fifi" after the call to foo.

Likewise:

public static void main(String[] args) {
    Dog aDog = new Dog("Max");
    Dog oldDog = aDog;

    foo(aDog);
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    aDog.getName().equals("Fifi"); // true
    // but it is still the same dog:
    aDog == oldDog; // true
}

public static void foo(Dog d) {
    d.getName().equals("Max"); // true
    // this changes the name of d to be "Fifi"
    d.setName("Fifi");
}

In the above example, Fifi is the dog's name after call to foo(aDog) because the object's name was set inside of foo(...). Any operations that foo performs on d are such that, for all practical purposes, they are performed on aDog, but it is not possible to change the value of the variable aDog itself.

For more information on pass by reference and pass by value, consult the following SO answer: https://stackoverflow.com/a/430958/6005228. This explains more thoroughly the semantics and history behind the two and also explains why Java and many other modern languages appear to do both in certain cases.

Best Answer

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