Compare new Integer Objects in ArrayList Question

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();

Answer 2

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?

The short answer is yes.

The long answer is ...

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 assume you mean "... will that be the same instance as ..."? The answer to that is no - calling new will always create a distinct instance separate from the previous instance, even if the constructor parameters are identical.

However, despite having separate identity, these two objects will have equivalent value, i.e. calling .equals() between them will return true.

Collection.contains()

It turns out that having separate instances of equivalent value (.equals() returns true) is okay. The .contains() method is in the Collection interface. The Javadoc description for .contains() says:

http://java.sun.com/javase/6/docs/api/java/util/Collection.html#contains(java.lang.Object)

boolean contains(Object o)

Returns true if this collection contains the specified element. More formally, returns true if and only if this collection contains at least one element e such that (o==null ? e==null : o.equals(e)).

Thus, it will do what you want.

Data Structure

You should also consider whether you have the right data structure.

Is the list solely about containment? is the order important? Do you care about duplicates? Since a list is order, using a list can imply that your code cares about ordering. Or that you need to maintain duplicates in the data structure.

However, if order is not important, if you don't want or won't have duplicates, and if you really only use this data structure to test whether contains a specific value, then you might want to consider whether you should be using a Set instead.

Answer 3

Short answer is yes, you should be able to do ArrayList.contains(new Integer(14)), for example, to see if 14 is in the list. The reason is that Integer overrides the equals method to compare itself correctly against other instances with the same value.