Basic Java knowledge required in

Currently, most people have learned from the beginning of the Java program. When learning from the Java program, they always start learning from the basics of Java. Currently, all the books on the basics of Java are based on J2SE, this creates some unnecessary troubles for learning. The following describes the basic knowledge of Java used in j2s and the basic knowledge of Java that is not needed:
Basic Java knowledge used in j2s:
1. Java syntax basics: including basic data types, keywords, operators, etc.
2. Object-oriented Thinking: Concepts of classes and objects, inheritance and polymorphism, etc.
3. Exception Handling
4. Multithreading
Basic Java knowledge not used in j2s:
1. Use of javac and java commands in JDK
2. Many classes in the Java foundation do not exist in j2s, or the methods in the classes are greatly simplified. Therefore, it is recommended that you familiarize yourself with the class library in j2s.
3. Knowledge of Applet, AWT, and Swing cannot be used in j2s.
To put it simply, I hope that my friends who want to learn j2s can take less detours. I hope you can actively correct and supplement the shortcomings.