One Class in an Unnamed Package

JDK Tutorials - Herong's Tutorial Examples

∟Calling and Importing Classes Defined in Unnamed Packages

∟One Class in an Unnamed Package - Hello.java

This section provides a tutorial example on how to place one Java class in an unnamed package and how to compile and run the class in an unnamed package.

I will begin with a simple case, where there is only one class that has no package declaration statement, so it can be stored in any unnamed package. Assuming that we have Hello.java entered in a directory called \herong\src, enter the following commands in a command window:

\herong\src> more Hello.java
class Hello {
   public static void main(String[] a) {
      System.out.println("Hello world!");
   }
}

\herong\src> del Hello.class
Could Not Find \herong\src\Hello.class

\herong\src> set CLASSPATH=

\herong\src> javac Hello.java

\herong\src> java Hello
Hello world!

\herong\src> del Hello.class
\herong\src> javac -classpath \herong Hello.java

\herong\src> java -classpath \herong Hello
Exception in thread "main" java.lang.NoClassDefFoundError: Hello

\herong\src> del Hello.class

\herong\src> javac -classpath . Hello.java

\herong\src> java -classpath . Hello
Hello world!

\herong\src> del Hello.class

This test tells us that:

If there is no -classpath command option specified, and no CLASSPATH environment variable defined, there is only one default class path: current directory.
Class paths are searched for the class being invoked in the execution process.

In the next test, let's move the current directory to a directory different than the Hello.java directory, and run the following commands:

\herong\src> mkdir ..\tmp

\herong\src> cd ..\tmp

\herong\tmp> del ..\src\Hello.class
Could Not Find \herong\src\Hello.class

\herong\tmp> javac -classpath . Hello.java
error: cannot read: Hello.java
1 error

\herong\tmp> javac -classpath . ..\src\Hello.java

\herong\tmp> java -classpath . Hello
Exception in thread "main" java.lang.NoClassDefFoundError: Hello

\herong\tmp> del ..\src\Hello.class

\herong\tmp> javac -classpath ..\src ..\src\Hello.java

\herong\tmp> java -classpath ..\src Hello
Hello world!

\herong\tmp> del ..\src\Hello.class