Organizing Related Classes into Packages

• Package: Set of related classes

• To put classes in a package, you must place a line

package packageName;

as the first instruction in the source file containing the classes

• Package name consists of one or more identifiers separated by periods

• For example, to put the Financial class introduced into a package named com.horstmann.bigjava, the Financial.java file must start as follows:

package com.horstmann.bigjava;

public class Financial
{
   . . .
}

• Default package has no name, no package statement

Importing Packages

• Can always use class without importing
  java.util.Scanner in = new java.util.Scanner(System.in);

• Tedious to use fully qualified name

• Import lets you use shorter class name
  import java.util.Scanner; . . .
  Scanner in = new Scanner(System.in)

• Can import all classes in a package
  import java.util.*;

• Never need to import java.lang

• You don't need to import other classes in the same package

Package Names and Locating Classes

• Use packages to avoid name clashes
  java.util.Timer vs. javax.swing.Timer

• Package names should be unambiguous

• Recommendation: start with reversed domain name
  com.horstmann.bigjava
  edu.sjsu.cs.walters: for Bertha Walters' classes (walters@cs.sjsu.edu)

• Path name should match package name
  com/horstmann/bigjava/Financial.java

• Path name starts with class path
  export CLASSPATH=/home/walters/lib:.
  set CLASSPATH=c:\home\walters\lib;

• Class path contains the base directories that may contain package directories

Which of the following are packages?

1. java

2. java.lang

3. java.util

4. java.lang.Math

   Answer:

1. No

2. Yes

3. Yes

4. No

Is a Java program without import statements limited to using the default and java.lang packages?

   Answer: No – you simply use fully qualified names for all other
   classes, such as java.util.Random and java.awt.Rectangle