I'm not an expert but I find NFC technology very interesting so, I asked GPT about it...

In theory, yes, a terminal could interact with a Java Card via NFC and instruct it to perform certain operations like selecting "1337 sats worth of ecash." However, the exact implementation would depend on several factors related to both the Java Card's applet (software) and the NFC terminal's capabilities. Here's how this interaction could conceptually work:

1. Java Card with a Custom Applet:

Java Cards run applets (Java Card applications) that handle specific tasks, such as managing digital currencies (e.g., ecash or Bitcoin sats). In your case, the Java Card would need to have a pre-programmed applet that knows how to store and manage ecash, including sats.

The applet would also need to have a coin selection algorithm that can calculate how to pick "1337 sats" from the available balance. This is similar to how Bitcoin wallets handle UTXO (Unspent Transaction Output) selection.

2. NFC Terminal Commands:

The NFC terminal (like a payment terminal or smartphone) would communicate with the Java Card over NFC using a set of APDU (Application Protocol Data Units) commands, which are the standard command/response format used to communicate with Java Cards.

The terminal would issue a SELECT command to the card, specifying the amount (e.g., 1337 sats). This command would be interpreted by the applet, which then runs the coin selection algorithm to choose a set of ecash coins that sum up to 1337 sats.

3. Coin Selection Algorithm:

The applet would implement a coin selection algorithm similar to those used in Bitcoin wallets, which would scan the stored coins (UTXOs) in the card’s memory. It would then select the best combination of UTXOs to form 1337 sats. This could involve selecting one or more UTXOs that total at least 1337 sats.

Any change (leftover sats from a larger UTXO) would need to be handled, potentially by generating a new UTXO to be stored back in the card or sent elsewhere.

4. Transaction Data and Sending:

Once the correct amount has been selected, the Java Card would prepare the transaction data (likely a signature or hash) and send it back to the terminal over NFC.

The terminal would then broadcast the transaction to the Bitcoin network or handle it in whatever way is required to finalize the transfer of ecash (e.g., uploading the transaction data to a blockchain or private ledger).

Technical Challenges:

Applet Development: The Java Card would need a specialized applet capable of storing Bitcoin UTXOs and implementing a coin selection algorithm.

Security: The transaction needs to be securely signed, ensuring that the Java Card's private keys are never exposed during the NFC exchange.

Communication Protocol: The terminal and Java Card need to agree on a specific protocol and set of APDU commands for selecting and transferring ecash.

In conclusion, while the concept is feasible, the implementation requires sophisticated programming on both the Java Card (for storing and managing ecash and coin selection) and the terminal (for securely requesting and processing the transaction).