What is the difference between JRE and JDK, why do we use JDK?

1.1.2. What is the difference between JRE and JDK? Why do we use JDK?

1.1.3. How to configure and modify environment variables, what is the use of environment variables?

1.1.4. What is the difference between system environment variables and user environment variables?

1.1.5. What is the use of the PATH variable in the system environment variable, for example, what is his value looks like, what is meaningful?

1.1.6. What order is used to run the Java target program? What parameters have this command, what is the use of these parameters? 1.1.7. What is the role in the MAIN method? What is the use of each parameter? What meanings represent?