Java 5: Autoboxing

This will not be covered on the AP exam, but your students will discover it on their own, accidentally...

When necessary, Java automatically converts ints into Integers and vice versa. You can write:

ArrayList<Integer> numbers = new ArrayList<Integer>();
numbers.add(3);
numbers.add(5);
int total = 0;
for(int i : numbers) total += i;

Without autoboxing, this would have to be written:

ArrayList<Integer> numbers = new ArrayList<Integer>();
numbers.add(new Integer(3));
numbers.add(new Integer(5));
int total = 0;
for(Integer i : numbers) total += i.intValue();

Autoboxing is rather problematic. Consider the following example.

if(numbers.get(0) == numbers.get(1)) { // ...

Even if the two underlying ints are equal, the if's body may not execute, because this compares Integers, not ints. (However, it may, because there is some internal caching...)

Another possible unexpected behavior is this:

students.remove(numbers.get(0));

This uses the remove method for removing an Object, not the remove method for removing the object at an int index.

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