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:

1. All Animals where type='CAT'

2. 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.

Answer 1

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

1. 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.
2. [type='CAT'] by itself isn't an XPath expression. I would expect something like //*[type='CAT']

So I would try the following:

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

Answer 2

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.