question: public synchronized FilteredBlock applyAndUpdate ( Block block ) { List < Transaction > txns = block . getTransactions ( ) ; List < Sha256Hash > txHashes = new ArrayList < Sha256Hash > ( txns . size ( ) ) ; List < Transaction > matched = Lists . newArrayList ( ) ; byte [ ] bits = new byte [ ( int ) Math . ceil ( txns . size ( ) / 8.0 ) ] ; for ( int i = 0 ; i < txns . size ( ) ; i ++ ) { Transaction tx = txns . get ( i ) ; txHashes . add ( tx . getHash ( ) ) ; if ( applyAndUpdate ( tx ) ) { Utils . setBitLE ( bits , i ) ; matched . add ( tx ) ; } } PartialMerkleTree pmt = PartialMerkleTree . buildFromLeaves ( block . getParams ( ) , bits , txHashes ) ; FilteredBlock filteredBlock = new FilteredBlock ( block . getParams ( ) , block . cloneAsHeader ( ) , pmt ) ; for ( Transaction transaction : matched ) filteredBlock . provideTransaction ( transaction ) ; return filteredBlock ; }
target:  Creates a new FilteredBlock from the given Block, using this filter to select transactions. Matches can cause the filter to be updated with the matched element, this ensures that when a filter is applied to a block, spends of matched transactions are also matched. However it means this filter can be mutated by the operation. The returned filtered block already has the matched transactions associated with it.