Packages

Figure 240. Why packages ?

Grouping of related classes (e.g. subsystems).
Structuring big systems.
Provide access restrictions using:

public, private and protected modifier
Resolving class name clashes. Example:

java.lang.String vs. my.personal.String

Figure 241. Rules and conventions

Package names below java. are reserved.
Package names should not start with javax. either.
Package names must not contain operators:

mi.hdm-stuttgart.de → de.hdm_stuttgart.mi.
Packages should start with reversed DNS avoiding clashes.

Figure 242. Fully qualified class name vs. import

Fully qualified class name:

java.util.Scanner ❶ scanner =          // Clumsy and
   new java.util.Scanner ❷(System.in); // redundant

Using import :

import java.util.Scanner; ❶

public class Q {

  static void main() {
     Scanner ❷ scanner = new Scanner ❷(System.in);
       ...
  }
}

Fully qualified class name Using import

❶	Using the fully qualified class i.e. including its package name name for defining a variable.
❷	Creating an instance by using the fully qualified class name again.

❶	Importing the `Scanner` class once.
❷	Unqualified class use due to `import`.

Figure 243. Don't be too lazy!

Bad	Good
import java.util.*; public class Q { static void main() { Scanner s = new Scanner(System.in); Date today = new Date(); } }	import java.util.Scanner; import java.util.Date; public class Q { static void main() { Scanner s = new Scanner(System.in); Date today = new Date(); } }

Bad

Good

import java.util.*;

public class Q {
  static void
        main() {
    Scanner s = 
      new Scanner(System.in);
    Date today = new Date();
  }
}

import java.util.Scanner;
import java.util.Date;

public class Q {
  static void
         main() {
    Scanner s =
       new Scanner(System.in);
    Date today = new Date();
  }
}

Figure 244. Special: Classes in package java.lang

import java.lang.String; ❶  // Optional
import java.util.Scanner;❷  // Required
public class Q {

  static void main() {
    String message = "Hello!";
    Scanner ❸ s = new Scanner(System.in);
  }
}

❶

Classes belonging to the java.lang package are being imported automatically.

❷

The Scanner class belongs to the java.util package and must thus be imported.

❸

Without the import java.util.Scanner statement we need the fully qualified class name:

java.util.Scanner s = new java.util.Scanner(System.in);

Figure 245. Class, package and file system

❶	A class Print defined in package my.first.javapackage.
❷	Generated byte code in corresponding directory hierarchy.

Figure 246. Source hierarchy view

package my.first.javapackage;
        ❷ ❸     ❹
public class Print { 
              ❺
  ...


}

❶	Our project's start folder containing Java™ classes.
❷	First package name component.
❸	Second package name component.
❹	Third package name component.
❺	Class `Print` being contained within our package.

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