Question

Did some searching before asking, some not-so-reliable sources suggest that there’s an underlying Object[] array.

Is it as simple as that? i.e. it handles resizing when necessary, maybe does a few tricks like doubling the size to get better amortized runtimes, and keeps track of where the first empty slot in the array is.

Or, are there optimizations done for membership testing and sparse arrays?

Answer 1

It is an array of Object. From the source:

http://hg.openjdk.java.net/jdk7/jdk7/jdk/file/tip/src/share/classes/java/util/ArrayList.java

private transient Object[] elementData;