<p>I recently wrote a class for an assignment in which I had to store names in an ArrayList (in java). I initialized the ArrayList as an instance variable <code>private ArrayList&lt;String&gt; names</code>. Later when I checked my work against the solution, I noticed that they had initialized their ArrayList in the <code>run()</code> method instead.</p>

<p>I thought about this for a bit and I kind of feel it might be a matter of taste, but in general how does one choose in situations like this? Does one take up less memory or something?</p>

<p>PS I like the instance variables in Ruby that start with an @ symbol: they are lovelier.</p>

<p>(meta-question: What would be a better title for this question?)</p>

In the words of the great Knuth "Premature optimization is the root of all evil".
Just worry that your program functions correctly and that it does not have bugs. This is far more important than an obscure optimization that will be hard to debug later on.
But to answer your question - if you initialize in the class member, the memory will be allocated the first time a mention of your class is done in the code (i.e. when you call a method from it). If you initialize in a method, the memory allocation occurs later, when you call this specific method.
So it is only a question of initializing later... this is called lazy initialization in the industry.