A Java source file is a plain text file containing Java source code and having .java extension. The .java extension means that the file is the Java source file. Java source code file contains source code for a class, interface, enumeration, or annotation type. There are some rules associated to Java source file. We should adhere to following rules while writing Java source code.

• There can be only one public class per source code file.
• Comments can appear at the beginning or end of any line in the source code file; they are independent of any of the positioning rules discussed here. Java comment can be inserted anywhere in a program code where a white space can be
• If there is a public class in a file, the name of the file must match the name of the public class. For example, a class declared as public class Dog { } must be in a source code file named Dog.java.
• If the class is part of a package, the package statement must be the first line in the source code file, before any import statements that may be present.
• If there are import statements, they must go between the package statement (if there is one) and the class declaration. If there isn't a package statement, then the import statement(s) must be the first line(s) in the source code file. If there are no package or import statements , the class declaration must be the first line in the source code file.
• import and package statements apply to all classes within a source code file. In other words, there's no way to declare multiple classes in a file and have them in different packages, or use different imports.
• A file can have more than one non~public class.
• Files with non~public classes can have a name that does not match any of the classes in the file

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