Java – the purpose of defining a package in a Java file?

javapackage

I am a newbie and just learned that if I define say

package my.first.group.here;
...

then the Java files that are in this package will be placed under my/first/group/here directory.

What is the main purpose of putting some Java files in a package? Also, if I choose to adopt this, how should I group them?

Thank you

EDIT: For anyone who might have the same question again, I just found this tutorial on packages from Sun.

Best Solution

Let's start with the definition of a "Java package", as described in the Wikipedia article:

A Java package is a mechanism for organizing Java classes into namespaces similar to the modules of Modula. Java packages can be stored in compressed files called JAR files, allowing classes to download faster as a group rather than one at a time. Programmers also typically use packages to organize classes belonging to the same category or providing similar functionality.

So based on that, packages in Java are simply a mechanism used to organize classes and prevent class name collisions. You can name them anything you wish, but Sun has published some naming conventions that you should use when naming packages:

Packages

The prefix of a unique package name is always written in all-lowercase ASCII letters and should be one of the top-level domain names, currently com, edu, gov, mil, net, org, or one of the English two-letter codes identifying countries as specified in ISO Standard 3166, 1981.

Subsequent components of the package name vary according to an organization's own internal naming conventions. Such conventions might specify that certain directory name components be division, department, project, machine, or login names.

Examples:

com.sun.eng

com.apple.quicktime.v2

edu.cmu.cs.bovik.cheese

Related Solutions

Java – What are the differences between a HashMap and a Hashtable in Java

There are several differences between HashMap and Hashtable in Java:

Hashtable is synchronized, whereas HashMap is not. This makes HashMap better for non-threaded applications, as unsynchronized Objects typically perform better than synchronized ones.
Hashtable does not allow null keys or values. HashMap allows one null key and any number of null values.
One of HashMap's subclasses is LinkedHashMap, so in the event that you'd want predictable iteration order (which is insertion order by default), you could easily swap out the HashMap for a LinkedHashMap. This wouldn't be as easy if you were using Hashtable.

Since synchronization is not an issue for you, I'd recommend HashMap. If synchronization becomes an issue, you may also look at ConcurrentHashMap.

Java – How does the Java ‘for each’ loop work

for (Iterator<String> i = someIterable.iterator(); i.hasNext();) {
    String item = i.next();
    System.out.println(item);
}

Note that if you need to use i.remove(); in your loop, or access the actual iterator in some way, you cannot use the for ( : ) idiom, since the actual iterator is merely inferred.

As was noted by Denis Bueno, this code works for any object that implements the Iterable interface.

Also, if the right-hand side of the for (:) idiom is an array rather than an Iterable object, the internal code uses an int index counter and checks against array.length instead. See the Java Language Specification.

Best Solution

Related Solutions

Java – What are the differences between a HashMap and a Hashtable in Java

Java – How does the Java ‘for each’ loop work

Related Question