Most Sample Code from "Java How to Program, Fifth Edition" from Deitel and Deitel.

Packages

The following is taken from pages 228 and 229 (figure 6.6) of Deitel's book:

"As we have seen, Java contains many predefined classes that are grouped into categories of related classes called packages. Together, we refer to these packages as the Java Application Programming Interface (Java API), or the Java class library. ...import declarations specify the classes required to compile a Java program. For example, a program includes the declaration
import javax.swing.JApplet;
to specify that the program uses class JApplet from the javax.swing package."

Figure 6.6 from Java How to Program (5th Edition) has some of the packages in the java API:

• java.applet--The Java Applet Package contains the Applet class and several interfaces that enable applet/browser interaction and the playing of audio clips. In Java 2, class javax.swing.JApplet is used to define an applet that uses the Swing GUI components
• java.awt--The Java Abstract Window Toolkit Package contains the classes and interfaces required to create and manipulate GUIs in Java 1.0 and 1.1. In Java 2, the Swing GUI components of the javax.swing packges are often used instead
• java.awt.event--The Java Abstract Window Toolkit Event Package contains classes and interfaces that enable event handling for GUI components in both the java.awt and javax.swing packages
• java.io--The Java Input/Output Package contains classes that enable programs to input and output data...
• java.lang--The Java Language Package contains classes and interfaces... that are required by many Java programs. This package is imported by the compiler into all programs
• java.net--The Java Networking Package contains classes that enable programs to communicate via networks...
• java.text--The Java Text Package contains classes and interfaces that enable a Java program to manipulate numbers, dates, characters, and strings. The package provides many of Java's internationalization capabilities that enable a program to be customized to a specific locale (e.g., an applet may display strings in different languages, based on the user's country).
• java.util--The Java Utilities Package contains utility classes and interfaces, such as date and time manipulations, random-number processing capabilities with class Random, storing and processing large amounts of data and breaking strings into smaller pieces called tokens with class String Tokenizer...
• javax.swing--The Java Swing GUI Components Package contains classes and interfaces for Java's Swing GUI components that provide support for portable GUIs.
• javax.swing.event--The Java Swing Event Package contains classes and interfaces that enable event handling for GUI components in package javax.swing.

Creating our own packages

Use keyword "package" and then the location of the package

We would want to create our own package if we have a class that is useful and we want to share with others.

page 381 of Deitel states "Every package name should start with your Internet domain name in reverse order. For example, for the Internet domain name deitel.com, the package name should begin with com.deitel"

Code is:
Time1.java
TimeTest1.java

• Compile with:
  javac -d . Time1.java
• Notice that the directory structure has been created:
  com/deitel/jhtp5/ch08
• Inside the directory is Time1.class
• Notice that TimeTest1.java has the import statement:
  import com.deitel.jhtp5.ch08.Time1

Package Access

"If no access modifier (public, protected or private) is specified for a method or variable when it is declared in a class, the method or variable is considered to have package access." (pg 384, Deitel)

Code: PackageDataTest.java

Things to try:

• make "PackageData" a public class
  *note: cannot compile--only one public class allowed in a file
  *note: when you have multiple classes in a file, a separate .class file is generated for each class.
• make a separate file PackageDataTest2.java that has the code copied from the "PackageDataTest.java" file.
  *note: it is still able to find the PackageData class. That is because PackageData.class is in the same directory.

Inheritance and Abstract Classes

The following is from the On-line Course: "Implementing Object-Oriented Programming with Java Technology", Module 2, Topic 5, pages 1 and 2

"An abstract class is a class with at least one abstract operation (an operation for which there is no implementation or code specified). Assume you need to implement classes for a circle, rectangle, and triangle. These shapes must have operations to set the area and determine the shape's perimeter. Each shape implements these operations differently, so you decide to create an abstract class, Shape."

"You can declare an abstract class by marking it with the abstract keyword. Likewise, in the declaration of a method without implementation, the keyword abstract is used. Any attempt to instantiate an abstract class results in a compiler error."

Code: Shape.java (note abstract class)
Point.java (Point extends Shape)
Circle.java(Circle extends Point)
Cylinder.java(Cylinder extends Circle)
AbstractInheritanceTest.java(Implementation file)

********Notice: In "Cylinder.java" code like the following:

            public double getVolume()
            {
                return super.getArea() * getHeight();
            }
          

Things to try:

• Try to instantiate a "Shape" object: Shape shape= new shape();
• Turn the Abstract class into an "interface" (see below)

Interface

"An interface declaration begins with the keyword "interface" and contains a set of public abstract methods. Interfaces may also contain public static final data. To use an interface, a class must specify that "implements" the interface and must declare each method in the interface with the signature specifiied in the interface declaration. If the class does not implement any method of the interface, the class is an abstract class and must be declared abstract. Implementing an interface is like signing a contract with the compiler that states, "I will declare all the methods specified by the interface"
...An interface is typically used in place of an abstract class when there is no default implementation to inherit .ie. no instance variables and no default method implementations." (page 467, Deitel)

You can turn the abstract Shape class into an interface by making the following changes:

1. modify Shape.java to declare the class as:

                    public interface Shape {
                       public double getArea();
                       public double getVolume();
                       public String getName();
                    }
                   

   Notice: the functions are just prototypes; they don't do anything. Anything that implements Shape will have to specify the details of what these functions do

2. change Point.java so that the class declaration reads:

                    public class Point extends Object implements Shape
                    

3. also in Point.java, add a declaration for the abstract methods "getArea" and "getVolume"--Otherwise: compiler error.

                    public double getArea()
                    {
                        return 0.0;
                    }
                    public double getVolume()
                    {
                        return 0.0;
                    }
                    

4. the changes yield the following two files that can be used with the other three files from above:
   
   • Shape.java
   • Point.java

Review of Applets

Code:
SquareIntegers.java
SquareIntegers.html

*applet needs init, start, or paint function (no main)

*SquareIntegers extends JApplet

"The on-screen display area for an object of class JApplet has a content pane--an object of class Container from the java.awt package--to which the GUI components must be attached so that they can be displayed at execution time.
"...when the applet executes, all GUI components attached to the applet's content pane are displayed" (page 222, Deitel)

Notice the "getContentPane()" and the syntax to "add" GUI components to the applet's content pane.

See below to add more GUI components:

Adding GUI (Graphical User Interface) Components and Making them Interactive

Follow these steps to add a button and a textfield:

1. You can change SquareIntegers.html so that the width="350" and the height="300" (so that we have more room to add things)
2. Add the following right before the "public void init()"

                JTextField myField;
                JButton myButton;
                

3. add the following line after instantiating container

                container.setLayout (new FlowLayout());
                

   This lays things out from left to right

4. Add the following to add a textfield and a button:

                myField = new JTextField (10);
                myField.setEditable(true);
                container.add(myField);
                myField.setText("Testing");
    
                myButton=new JButton("My Button");
                container.add(myButton);
                
            

1. add the following import statement so that you can handle events (such as clicking a button or hitting return):

                 import java.awt.event.*;
                 

2. change the declaration of SquareIntegers so that it "implements ActionListener". If a class implements ActionListener interface, you must define the actionPerformed function.

                 public class SquareIntegers extends JApplet implements ActionListener
                 

3. before the closing curly bracket for the class add the actionPerformed function:

                 public void actionPerformed (ActionEvent actionEvent)
                 {
                     //If the button is clicked
                     if (actionEvent.getSource()==myButton)
                     {
                          showStatus ("You hit the button");
                     }
   
                     //If you press Enter in the textfield
                     else if (actionEvent.getSource()==myField)
                     {
                          showStatus ("You changed the field");
                     }
                 }
                

4. You need to specify that clicking the button or pressing Enter on the textfield will trigger an event (to be handled by the actionPerformed function). We use the addActionListener function. Add this code after you have created a myField and myButton instance.

                     myField.addActionListener(this);
                     ...
                     myButton.addActionListener(this); 
                   

The code:
SquareIntegers2.java
SquareIntegers2.html