Question

I am storing Integer objects representing an index of objects I want to track. Later in my code I want to check to see if a particular object’s index corresponds to one of those Integers I stored earlier. I am doing this by creating an ArrayList and creating a new Integer from the index of a for loop:

ArrayList<Integer> courseselectItems = new ArrayList();

//Find the course elements that are within a courseselect element and add their indicies to the ArrayList
for(int i=0; i<numberElementsInNodeList; i++) {
    if (nodeList.item(i).getParentNode().getNodeName().equals("courseselect")) {
        courseselectItems.add(new Integer(i));
    }
}

I then want to check later if the ArrayList contains a particular index:

//Cycle through the namedNodeMap array to find each of the course codes
for(int i=0; i<numberElementsInNodeList; i++) {
    if(!courseselectItems.contains(new Integer(i))) {
        //Do Stuff
    }
}

My question is, when I create a new Integer by using new Integer(i) will I be able to compare integers using ArrayList.contains()? That is to say, when I create a new object using new Integer(i), will that be the same as the previously created Integer object if the int value used to create them are the same?

I hope I didn’t make this too unclear. Thanks for the help!

Answer 1

Yes, you can use List.contains() as that uses equals() and an Integer supports that when comparing to other Integers.

Also, because of auto-boxing you can simply write:

List<Integer> list = new ArrayList<Integer>();
...
if (list.contains(37)) { // auto-boxed to Integer
  ...
}

It’s worth mentioning that:

List list = new ArrayList();
list.add(new Integer(37));
if (list.contains(new Long(37)) {
  ...
}

will always return false because an Integer is not a Long. This trips up most people at some point.

Lastly, try and make your variables that are Java Collections of the interface type not the concrete type so:

List<Integer> courseselectItems = new ArrayList();

not

ArrayList<Integer> courseselectItems = new ArrayList();