Source code is a group of text listing commands, likely with comments, that can be compiled and executed by a machine but written and read by a human. Source code is what allows computer programmers to specify, read, and write the code that governs the actions of a Java Virtual Machine (JVM). As we discussed before, in order for the JVM to run a Java program, the Java compiler compiles the Java source code into Java bytecode, which is then interpreted by the Java Virtual Machine and converted into machine language.
The source code file has file extension ".java". This is the file that is converted into the Java bytecode file, also called the class file. Everything that you physically code is "source code". Thanks to Java source code, you will not have to deal with bytecode files or machine language files, as those files will be created automatically by Java from your source code when you compile and run your program.
Note: Every public class file that you write must be in a separate ".java" file, where the file name matches your class name.