Question

In Java, we can always use an array to store object reference. Then we have an ArrayList or HashTable which is automatically expandable to store objects. But does anyone know a native way to have an auto-expandable array of object references?

Edit: What I mean is I want to know if the Java API has some class with the ability to store references to objects (but not storing the actual object like XXXList or HashTable do) AND the ability of auto-expansion.

Answer 1

Java arrays are, by their definition, fixed size. If you need auto-growth, you use XXXList classes.

EDIT – question has been clarified a bit

When I was first starting to learn Java (coming from a C and C++ background), this was probably one of the first things that tripped me up. Hopefully I can shed some light.

Unlike C++, Object arrays in Java do not store objects. They store object references.

In C++, if you declared something similar to:

String myStrings[10]; 

You would get 10 String objects. At this point, it would be perfectly legal to do something like println(myStrings[5].length); – you’d get ‘0’ – the default constructor for String creates an empty string with length 0.

In Java, when you construct a new array, you get an empty container that can hold 10 String references. So the call:

String[] myStrings = new String[10]; println(myStringsp[5].length); 

would throw a null pointer exception, because you haven’t actually placed a String reference into the array yet.

If you are coming from a C++ background, think of new String[10] as being equivalent to new (String *)[10] from C++.

So, with that in mind, it should be fairly clear why ArrayList is the solution for an auto expanding array of objects (and in fact, ArrayList is implemented using simple arrays, with a growth algorithm built in that allocates new expanded arrays as needed and copies the content from the old to the new).

In practice, there are actually relatively few situations where we use arrays. If you are writing a container (something akin to ArrayList, or a BTree), then they are useful, or if you are doing a lot of low level byte manipulation – but at the level that most development occurs, using one of the Collections classes is by far the preferred technique.