How extend LedgerJournalCheckPost class for method postJournal

I know extend class:

[ExtensionOf(ClassStr(LedgerJournalCheckPost))]

final class MyLedgerJournalCheckPost_Extension

{

}

Now I need add code in method postJournal:

[ExtensionOf(ClassStr(LedgerJournalCheckPost))]

final class MyLedgerJournalCheckPost_Extension

{

protected container postJournal()
{

}

}

but display error message:

Error Not all paths return a value in 'postJournal'.

and

Error Chain of command method must contain one next call. 

Please help!