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Creating Packages

The package statement must be the first statement in your class.

  • Only comments, not code, can precede the package statement.

Just as the name of the public class in a .java file must match the file's name, the name in the package declaration must match the directory the .java file is in.

  • In the package name we replace the directory path separator (\ on Windows, / everywhere else) with ..

File utilities/parsing/Parser.java: 

package utilities.parsing;

public class Parser {
  public void parse() { /* ... */ }
  public static void main(String[] args) { /* ... */ }
}

  • This allows us to organize our project's code hierarchically.

Note that we don't need to use the absolute pathname in our package name.

  • If our code is in /Users/dobbs/Development/Java/utilities/parsing/, we just use the part of the path that's relevant:
package utilities.parsing;

To run a class in a package Java needs to know its fully qualified name - that is, both its package and its class names.

cd /Users/dobbs/Development/Java/
java utilities.parsing.Parser
  • java looks for a directory path matching the package name.

Classpath

Normally you would make sure your current working directory is at the top of the package directory, so java can see the package path.

You can also tell java other places to look for packages and classes by specifying a classpath.

classpath

A list of directory locations where Java commands will look for packages and classes. A classpath can list multiple directories separated by colon, :, on Unix/Linux systems or semicolon, ;, on Windows.

You can set the classpath with the -classpath or -cp option on the command line, or via the CLASSPATH environment variable.

cd ~   # Go to my home directory
java -classpath /Users/dobbs/Development/Java utilities.parsing.Parser

export CLASSPATH=/Users/dobbs/Development/Java
java utilities.parsing.Parser


Practice Exercise

One of the many tasks an IDE like Eclipse handles for us is automatically managing the classpath for each project.

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