code: public boolean isPresent(int accessor) {
  
      if (TraceComponent.isAnyTracingEnabled() && tc.isEntryEnabled()) JmfTr.entry(this, tc, "isPresent", new Object[]{Integer.valueOf(accessor)});
      boolean result;
  
      if (accessor < cacheSize) {
        result = super.isPresent(accessor);
      }
      else if (accessor < firstBoxed) {
        result = getCase(accessor - cacheSize) > -1;
      }
      else if (accessor < accessorLimit) {
        // Conservative answer: a boxed value is present if its containing box is present;
        // this is enough to support creation of the JSBoxedImpl for the value, which can
        // then be interrogated element by element.
        synchronized (getMessageLockArtefact()) {
          result = super.isPresent(boxManager.getBoxAccessor(accessor - firstBoxed));
        }
      }
      else {
        if (TraceComponent.isAnyTracingEnabled() && tc.isEntryEnabled()) JmfTr.exit(this, tc, "isPresent", "IndexOutOfBoundsException");
        throw new IndexOutOfBoundsException();
      }
  
      if (TraceComponent.isAnyTracingEnabled() && tc.isEntryEnabled()) JmfTr.exit(this, tc, "isPresent", Boolean.valueOf(result));
      return result;
  
    }
docs: The BoxManager stuff is a black art so we'll lock round it to be safe.