Why does Idea IDE run code but javac throws errors?

Question

Java

I wrote a small calculator in Java and Swing, the calculator is semi-working, the code in the Idea IDE starts up and the calculator window appears on the screen. But the same code refuses to compile to javac and throws 3 errors.
Why is this happening? Are there different compiler versions? If so, how to understand which compiler to compile the code with?
Here is my mini-project, if it's important: https://github.com/roman-deniskin/swing-calculator... Attached
errors on the screen. I ask you not to throw slippers at me for stupid questions, I just started studying Java, but I couldn’t google it.

Answer 1

1. Do not upload compiled .class files to git. Upload only source code. (google .gitignore)
2. Your project consists of several classes, in several files. So compile them all together:
javac App.java Expression.java ProcessCalc.java