Importing Classes from Unnamed to Named Packages

JDK Tutorials - Herong's Tutorial Examples

∟Calling and Importing Classes Defined in Unnamed Packages

∟Importing Classes from Unnamed to Named Packages

This section provides a tutorial example on how to import classes defined in an unnamed package to a named package. The imported class must be defined as 'public'.

Recently, I received an email from a reader asking me how to refer to a class defined in an unnamed package if the calling class is in a named class with JDK 1.4.1 or higher. To answer this question, let's enter a new class called "ImportingHelloCom" in a package called "com", and run the following commands:

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

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

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

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

\herong\tmp> \jdk1.3.1_05\bin\javac com\ImportingHelloCom.java
com\ImportingHelloCom.java:5: cannot resolve symbol
symbol: class Hello
import Hello;
^
com\ImportingHelloCom.java:9: cannot resolve symbol
symbol  : variable Hello
location: class ImportingHelloCom
      Hello.main(a);
      ^
2 errors

\herong\tmp> \jdk1.3.1_05\bin\javac -classpath .
   com\ImportingHelloCom.java
com\ImportingHelloCom.java:6: Hello is not public in empty package;
cannot be accessed from outside package
import Hello;
^
com\ImportingHelloCom.java:10: Hello is not public in empty package;
cannot be accessed from outside package
      Hello.main(a);
      ^
com\ImportingHelloCom.java:10: main(java.lang.String[]) in Hello is
not defined in a public class or interface; cannot be accessed from
outside package
      Hello.main(a);
           ^
3 errors

This test shows us that:

Only public classes in an unnamed package can be imported into named packages.
Comparing with the testing results from the previous sections, there is really only one unnamed package. All classes defined without a package name are considered to be in this single package. All classes can be imported into each other without the "public" attribute.

To fix the last compilation error, just declare the Hello class as "public". Now let's modify the classes and run the following commands:

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

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

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

\herong\tmp> \jdk1.3.1_05\bin\javac -classpath .
   com\ImportingHelloCom.java

\herong\tmp> \jdk1.3.1_05\bin\java -classpath . com.ImportingHelloCom
Calling the imported Hello.main()...
Hello world!