How Multibit choose input ( and output ) address?

[El Emperador]

I've  used Multibit for a while and know that there isn't the possibility to choose the input and output addresses in a transaction, because Multibit itself choose the addresses on its own.

But I've not understood yet why Multibit choose one address instead of another... what's the concept involved? Or it chooses addresses randomly?

[jim618]

In the Bitcoin transactions MultiBit constructs:

1) It chooses transactionInputs by age (oldest first) and keeps adding inputs until it has enough to cover the amount you want to spend.
2) Your send address is one transactionOutput and the change address is the other.
3) Change is either the first receiving address in a wallet, or the second if there is more than one. (This latter rule helps when a user imports a paper wallet address as the change goes to the address they just imported).

The change address algorithm isn't great for privacy but that is fixed with MultiBit HD.