URGENT - Need help with setting up Transaction Input and Output objects

Transaction rawTransaction = new Transaction(params);
txnAmount = Coin.valueOf(50000000);
rawTransaction.addOutput(txnAmount, toAddress);

txFee = Coin.valueOf(12800);
amountToReturn = balance.subtract(txnAmount);
amountToReturn = amountToReturn.subtract(txFee);

rawTransaction.addOutput(amountToReturn, fromAddress);

I am able to setup transaction outputs but not able to figure out how to set transaction inputs for fee

Currently with above code the signed transaction is having 0 fee