Using JXPath to Query a List

Question

I have a simple class (for testing purposes) that I am trying to Query against using JXPath.

I create a list of various animal objects, and I want to get an Iterator for:

All Animals where type='CAT'
All Animals where numLegs = 4

Here is the simple class:

public class Animal {

    private UUID uuid;
    private int numLegs;
    private AnimalType type;

    public enum AnimalType {
        CHICKEN, FROG, DOG, CAT
    }

    public Animal(AnimalType type) {
        this.type = type;
        uuid = UUID.randomUUID();
        switch (type) {
        case CHICKEN:
            numLegs = 2;
            break;
        case FROG:
            numLegs = 2;
            break;
        case DOG:
            numLegs = 4;
            break;
        case CAT:
            numLegs = 4;
            break;
        }
    }

    public UUID getUuid() {
        return uuid;
    }

    public int getNumLegs() {
        return numLegs;
    }

    public AnimalType getType() {
        return type;
    }

    public String toString(){
        return "UUID: "+uuid+", Animal: "+type+ ", Legs: "+numLegs;
    }

}

Here is the method I am using to build a list of Animals for me to Query against:

private static List<Animal> getAnimals(int numAnimals) {
    ArrayList<Animal> animals = new ArrayList<Animal>();
    for(int i = 0; i<numAnimals; i++){
        if(i%4==0){
            animals.add(new Animal(AnimalType.CAT));
        }
        else if(i%3==0){
            animals.add(new Animal(AnimalType.DOG));
        }
        else if(i%2==0){
            animals.add(new Animal(AnimalType.FROG));
        }
        else{
            animals.add(new Animal(AnimalType.CHICKEN));
        }

    }

    return animals;
}

Here is how I am trying to perform the query:

public static void main(String[] args){
    List<Animal> animals = getAnimals(10000);

    JXPathContext animsContext = JXPathContext.newContext(animals);

    Iterator<BeanPointer> iter = 
        animsContext.iteratePointers("/*[type='CAT']");

    List<Animal> cats = new ArrayList<Animal>();


    while(iter.hasNext()){
        cats.add((Animal) iter.next().getParent().getValue());
    }
    System.out.println("Number of Cats is: "+cats.size());
}

This part:

    Iterator<BeanPointer> iter = 
        animsContext.iteratePointers("/*[type='CAT']");

is not working. What am I doing wrong? I cannot get it to work for /*[numLegs=4] either.

score 5 · Accepted Answer · answered May 29 '09 at 06:31

I've never used JXPath, but i would expect there to be a root node, in which case

/*[type='CAT']

would mean "give me the root node, only if it has an attribute of type equal to CAT"

I think what you are after is something more like

/*/*[type='CAT']

which would read like "give me nodes directly underneath the root node that have an attribute type which is equal to CAT"

Brian's suggestion

//*[type='CAT']

reads like "give me all nodes anywhere in the tree that have an attribute type that is equal to CAT"

Hope that helps a bit.

score 2 · Answer 2 · answered May 22 '09 at 17:10

I'm not sure what's not working in the above. However a couple of points.

I'm not sure enumerations work as you expect. Your JXPath query is querying for a type where getType() will return an enumeration and not a string. I'm not sure if JXPath is that clever wrt. enumerations.
[type='CAT'] by itself isn't an XPath expression. I would expect something like //*[type='CAT']

So I would try the following:

implement a getTypeName() method that returns the enumeration name e.g. CAT/DOG etc. and query using that
Try an XPath of //* (without any predicates) to confirm that JXpath will query an ArrayList as expected.

Using JXPath to Query a List

2 Answers2