Two Classes in Unnamed Packages - Hello.java and CallingHello.java

JDK Tutorials - Herong's Tutorial Examples

∟Calling and Importing Classes Defined in Unnamed Packages

∟Two Classes in Unnamed Packages - Hello.java and CallingHello.java

This section provides a tutorial example on how to place two Java classes in an unnamed package and how to call one class from another class in the unnamed package.

Testing with two classes is definitely more interesting. Let's enter another class called, CallingHello.java, in \herong\src directory, and run the following commands:

\herong\src> more CallingHello.java
/* CallingHello.java
 * Copyright (c) HerongYang.com. All Rights Reserved.
 */
public class CallingHello {
   public static void main(String[] a) {
      System.out.println("Calling Hello.main()...");
      Hello.main(a);
   }
}

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

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

\herong\src> java -classpath . CallingHello
Calling Hello.main()...
Hello world!

\herong\src> del *.class

\herong\src> cd ..\tmp

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

\herong\tmp> java -classpath ..\src CallingHello
Calling Hello.main()...
Hello world!

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

This test shows us that:

The compiler is smart. It automatically compiles the source code of the referred class, Hello, if Hello.class cannot be found in the class paths.

In the next test, let's separate the classes into two directories, and run the following commands:

\herong\tmp> copy ..\src\CallingHello.java

\herong\tmp> del ..\src\CallingHello.java

\herong\tmp> del *.class
Could Not Find \herong\tmp\*.class

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

\herong\tmp> javac -classpath . CallingHello.java
CallingHello.java:8: cannot resolve symbol
symbol  : variable Hello
location: class CallingHello
      Hello.main(a);
      ^
1 error

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

\herong\tmp> java -classpath . CallingHello
Calling Hello.main()...
Exception in thread "main" java.lang.NoClassDefFoundError: Hello
        at CallingHello.main(CallingHello.java:8)

\herong\tmp> java -classpath .;..\src CallingHello
Calling Hello.main()...
Hello world!

\herong\tmp> del *.class

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

This test confirms that:

The compiler is indeed smart. It will search for source code in class paths, even the referred class is not in the same directory as the class that is being compiled.