[Soot-list] Singleton ArrayLists

[Ohad Shacham]

Hi,

We recently built a tool called Chameleon (
http://www.research.ibm.com/qvm/chameleon.html) for automatically managing
collections in Java. Running Chameleon on Soot, it reported, among other
things, that in the following places a Singleton (non-modifiable) list
should be used:

soot.jimple.internal.JIfStmt - targetBoxes
soot.baf.internal.BLoadInst - useBoxes
soot.jimple.internal.JIdentityStmt - defBoxes
soot.baf.internal.BStoreInst - defBoxes
soot.baf.internal.BIdentityInst - defBoxes

Checking the code, it seems that indeed there's no reason to use a
full-fledged ArrayList in these places, as the collections are singletons
and are immutable.

For example: in soot.jimple.internal.JIfStmt

  protected JIfStmt(ValueBox conditionBox, UnitBox targetBox)
    {
        this.conditionBox = conditionBox;
        this.targetBox = targetBox;

        targetBoxes = new ArrayList();
        targetBoxes.add(this. targetBox);
        targetBoxes = Collections.unmodifiableList(targetBoxes);
    }

This ArrayList allocates a default size of 10, and will always use only 1
element in the underlying array. This might lead to significant waste of
space.

We noticed that in some other places in Soot, the constructors are already
using a SingletonList (e.g., JAssignStmt), and wondered what is the reason
that the aforementioned locations remained as using ArrayLists?
Thanks,
Ohad
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