Memory profiling of ABCL shows that the classes with largest allocation count are LispStackFrame and LispStackFrame.UnavailableArgument . It seems to me that LispStatckFrame.UNAVAILABLE_ARG can be a singleton object, can't it ? =================================================================== --- LispStackFrame.java (revision 14571) +++ LispStackFrame.java (working copy) @@ -44,7 +44,7 @@ private final LispObject third; private final LispObject[] args; - private final class UnavailableArgument extends LispObject + private static final class UnavailableArgument extends LispObject { public UnavailableArgument () { } @Override @@ -53,7 +53,7 @@ } } - private final LispObject UNAVAILABLE_ARG = new UnavailableArgument(); + private static final LispObject UNAVAILABLE_ARG = new UnavailableArgument(); public LispStackFrame(LispObject operator) { ====================================================================