java.util and anonymous class (original) (raw)

Maurizio Cimadamore maurizio.cimadamore at oracle.com
Mon Dec 5 10:37:03 UTC 2011

On 05/12/11 10:20, Rémi Forax wrote:

While removing generics warnings in java.util, I've seen that some anonymous classes are coded in a way that make javac to generate too much fields.

This code: final List list = Arrays.asList(args); Iterator iterator = new Iterator() { private Iterator it = list.iterator(); ... } }; generates an anonymous class with two fields: final class Foo$1 implements java.util.Iterator<java.lang.String> { java.util.Iterator<java.lang.String> it; final java.util.List val$list; FinalCaptureTooBig$1(java.util.List); ... while this code: List list = Arrays.asList(args); final Iterator it = list.iterator(); Iterator iterator = new Iterator() { ... generates an anonymous class with only one field: final class FinalCaptureTooBig$1 implements java.util.Iterator<java.lang.String> { final java.util.Iterator val$it; FinalCaptureTooBig$1(java.util.Iterator); ... If javac was smart, it should generate only one field because the variable 'list' is only use in the initializer of a field, so 'list' has to be passed as parameter of the constructor but don't need to be stored as a field. Note that this change in javac may break the serialization compatibility, so may be it's not a good idea to change javac. Or we can say that we don't care because there is an item in Effective Java that says that you should not use an anonynous class. Should we change the code of java.util or javac ? My 0.02$ is that it is potentially a risky change (as you point out) to do in the compiler, and not such a straightforward one as well (javac currently does not distinguish as to whether a variable is accessed from a variable initializer w.r.t. form inside a method body). Of course, it could be done - but I think it'd be wiser to invest time on making lambda better so that people will be less and less forced to use inner classes ;-)

Maurizio

cheers, Rémi

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