The two best introductory tutorials I have found are:
- "Lecture 14 - Javacard" https://edux.pjwstk.edu.pl/mat/268/lec/lect14/lecture14.html
and this one at JavaWorld,
- How to write a Java Card applet: A developer's guide
For example code there is a section titled Learning in this Github repo,
javacard-curated-list of applets
To give an idea of the first topics that are typically covered, the headings from the second link mentioned are:
Architecting the applet
Specifying the functions of the applet
Specifying AIDs
Defining the class structure and method functions of the applet
Defining the interface between an applet and its terminal application
An APDU primer
Defining APDU commands
The Javacard APDU Class
The applet processes an APDU command by invoking methods on the APDU object. In general it performs these steps:
Step 1. Retrieve the APDU buffer
Step 2. Receive data
Step 3. Return data
Step 4. Return status word
Constructing the applet code
Implementing error checking
This article is by Zhiqun Chen who also wrote the book Java Card Technology for Smart Cards