Language Fundamentals - Source Files

A Java source code file or compilation unit has three basic parts, each of which is optional (JLS §7.3):

• A package declaration
• import declarations
• top-level class and interface declarations

Package declaration

• if used, it must be the first non-comment statement in the source code file
• you can not declare more than one
• syntax: package packageName;

Import declarations

• if used, must be the first non-comment statement directly following the package declaration.
• you can use as many import statements as you want
• if no package statement appears in the source code file, the import statement must be the first non-comment statement in the file

top-level class and interface declarations

• A top-level class or interface is defined as any class or interface whose declaration is not contained within the body of any other class or interface declaration. (JLS §8 and §9).
• you can declare multiple classes and interfaces within a file with the following caveats:
  
  
  • The Sun SDK allows one and only one public class or interface within a source code file.
  • The filename must exactly match the name of the public class or interface declared in the file and have the .java extension

Example Code

• TestPkgImport.java

Tips

• an empty source file will compile without error
• if a .java file does not contain a public class or interface it can have any name

Traps

• code with package or import declarations given in wrong order
• more than one package declaration
• file with more than one public class or interface declaration
• filename.java does not match public class name as declared within the file