— Scott McNealy, Sun Microsystems Chairman, President and ceo

java.util.ArrayList
, JButton is really javax.swing.JButton, and 
Socket is really java.net.Socket. Notice that two of those classes, 
ArrayList and Socket, both have java as their “fi rst name”. 
In other words, the fi rst part of their fully-qualifi ed names 
is “java”. Think of a hierarchy when you think of package 
structures, and organize your classes accordingly. 
java
net
util
text
awt
event
101101 10 
1001 101 10 
10 10010 
101 101 
101 101 101 
101101 10 
1001 101 10 
10 10010 
101 101 
101 101 101 
Socket
ActionEvent
101101 10 
1001 101 10 
10 10010 
101 101 
101 101 101 
FlowLayout
101101 10 
1001 101 10 
10 10010 
101 101 
101 101 101 
ArrayList
101101 10 
1001 101 10 
10 10010 
101 101 
101 101 101 
NumberFormat
Package structure of the Java API for:
What does this picture look like to 
you? Doesn’t it look a whole lot like 
a directory hierarchy?
java.text.NumberFormat
java.util.ArrayList
java.awt.FlowLayout
java.awt.event.ActionEvent
java.net.Socket

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588
 
chapter 17
Preventing package name conflicts
Putting your class in a package reduces the chances of naming 
conflicts with other classes, but what’s to stop two programmers 
from coming up with identical package names? In other words, 
what’s to stop two programmers, each with a class named Account, 
from putting the class in a package named shopping.customers? 
Both classes, in that case, would still have the same name: