9.1 Java Packages

9.1.1 A collection of classes

• Allows classes to be grouped arbitrarily

• Hierarchical structure independent of inheritance

• Classes can be grouped by function without needing to be directly related

The PATH environment variable

• The PATH environment variable is very similar to a package

• Files are stored on your local hard disk in an hierarchical structure

• The executable files could be distributed throughout this structure

• The PATH variable defines a package of folders that have executable files in them

9.1.2 Class loading and how it works

• Classes are declared as belonging to a package using the package keyword

• If there is no such declaration, the class belongs to the default package which corresponds to the current directory

• To access any class, you must specify the package

• The only exception to this is the java.lang package

9.1.3 Locating explicit package declarations

• When you specify a class in your program the JVM must find the.class file to load it

• The JVM searches as follows:1. The standard directories (in the JDK

directory structure)2. The default package (current directory)3. Directories specified by the CLASSPATH

environment variable

9.1.3 Locating explicit package declarations

• To include a class that is not in java.lang or in the default package:

java.util.Date myDate;myDate = new java.util.Date();

Orimport java.util.Date;import java.util.*;<…>Date myDate;myDate = new Date();

9.2.2 Packaging classes

• Declare the package that the class is to be a member of

• Create the directory structure exactly matching the package declaration

• Use javac to compile, and specify the –d option, whose argument is the root of the package tree

9.2.2 Packaging classes

• For your program to run correctly, the package directory structure must exist on the target computer

• This can be inconvenient if the program is loading over the Internet

• Your entire package directory structure can be compressed in to one ‘jar’ file

• The jar file can be easily downloaded, and the JVM can search it for the package classes

9.3.1 Accessing packages

• Declaring a class to be a member of a package must be the first non-comment in the file

• Importing other packages must occur before the class definition

9.5.1 GUI and terminology

• Components are the core building blocks of a GUI

• Pre-defined elements, such as Buttons, Windows, TextFields and Panels

• Components that can hold other components are containers

• The Java Foundation Classes support five GUI APIs

• This course focuses on AWT only

9.5.1 Understanding the model view controller pattern

• Users today expect a Graphical User Interface (GUI) when using applications

• Java provides the building blocks to create GUIs

• Model View Controller is a design pattern for class design

• MVC requires separate classes for data modelling, interfacing and event handling

9.5.2 Applying GUI concepts

• Creating a GUI requires a thorough understanding of AWT

• Navigating the AWT API documentation is criticaljava.awt.Frame

java.awt.Button

9.5.3 AWT categories • Draw graphics or manipulate images

Graphics, Image, Insets, Point, Polygon, Rectangle

• Position visual elementsBorderLayout, CardLayout, CheckboxGroup, java.lang.Object,

FlowLayout, GridBagLayout, GridLayout

• Change the properties of the visual elementsToolkit, Color, Font, FontMetrics, CheckboxGroup

• Create graphical componentsMenuComponent, Component, MenuBar, MenuItem, , Menu,

PopupMenu, CheckboxMenuItem, Button, Canvas, Checkbox, Choice, Label, List, Scrollbar, TextComponent, TextArea, TextField,

• Components that hold other componentsContainer, Panel, Window, ScrollPane, Dialog, Frame, Applet,

(java.applet, package), FileDialog