Possible heap pollution via varargs parameter

java eclipse generics variadic-functions
hertzsprung picture hertzsprung · Sep 17, 2012 · Viewed 110.9k times · Source

I understand this occurs with Java 7 when using varargs with a generic type;

But my question is..

What exactly does Eclipse mean when it says "its use could potentially pollute the heap?"

And

How does the new @SafeVarargs annotation prevent this?

Answer

Ben Schulz picture Ben Schulz · Sep 17, 2012

Heap pollution is a technical term. It refers to references which have a type that is not a supertype of the object they point to.

List<A> listOfAs = new ArrayList<>();
List<B> listOfBs = (List<B>)(Object)listOfAs; // points to a list of As

This can lead to "unexplainable" ClassCastExceptions.

// if the heap never gets polluted, this should never throw a CCE
B b = listOfBs.get(0);

@SafeVarargs does not prevent this at all. However, there are methods which provably will not pollute the heap, the compiler just can't prove it. Previously, callers of such APIs would get annoying warnings that were completely pointless but had to be suppressed at every call site. Now the API author can suppress it once at the declaration site.

However, if the method actually is not safe, users will no longer be warned.

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