Download 3 CREATING YOUR FIRST JAVA APPLICATION (USING WINDOWS)

GETTING STARTED: YOUR FIRST JAVA APPLICATION
3 CREATING YOUR FIRST JAVA
APPLICATION (USING WINDOWS)
GETTING STARTED: YOUR FIRST JAVA APPLICATION
Checklist:
The most recent version of Java SE Development Kit (JDK) – can be downloaded from
http://java.sun.com/javase/downloads/index.jsp
A text editor – such as Notepad or WordPad
Your first application, HelloWorld will simply display the greeting “Hello world!” using the command
prompt. The steps are:
1. Create a source file
To create your first Java source file (or called as source code):
i.
Start any text editor then create a new document, if necessary.
ii.
Type the class header public class HelloWorld. Press Enter once then type {.
iii.
Press Enter again and then type }. We will add main()method between these curly
braces.
iv.
As shown in Figure 3.1 below, add the main()method between the curly braces by
typing public static void main (String[] args). Then type a set of curly braces for
main().
public class HelloWorld
{
public static void main(String[] args)
{
}
}
Figure 3.1 The main()method for the HelloWorld class
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v.
Next, add the statement System.out.println(“Hello world!”) within the main()
method that can produce the output, Hello World!.
public class HelloWorld
{
public static void main(String[] args)
{
System.out.println("Hello world!"); // Display the string.
}
}
Figure 3.2 Complete HelloWorld class
vi.
Save the application as HelloWorld.java.
Important: Make sure the file extension is .java by appending the .java to the file name
when saving the file.
Now, you may exit the text editor.
2. Compile the source file
After saving the application, you must compile the source code into a .class file (or called as
bytecode). To compile your source code from the command line:
i.
Start the command prompt.
ii.
Change the default drive prompt to the drive and the directory where your application is
stored.
iii.
Type javac followed by the name of the file that contains the source code. Then press
Enter.
For example, to compile the HelloWorld.java, you type javac HelloWorld.java then
press Enter.
Figure 3.3 Changing the directory of the application
FIRST JAVA APPLICATION USING ECLIPSE IDE
Important: The name must match exactly, including the use of uppercase and lowercase
characters. The compiler (javac) and launcher (java) tools are case-sensitive.
Now that you have a .class file, you can run the application.
3. Run the application
When the compile is successful, in order to run the source code from the command line:
i.
Type java followed by the class name of the source code at the command line and
press Enter. For example, to run the HelloWorld.java, you type java HelloWorld.
ii.
The output should appear on the next line as shown in Figure 3.4 below.
Figure 3.4 Output of the HelloWorld application
Congratulations, your program works!
FIRST JAVA APPLICATION USING ECLIPSE IDE
Checklist:
The most recent version of Java SE Development Kit (JDK)
Eclipse IDE
The steps to display the greeting “Hello world!” using the Eclipse IDE are:
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1. Create a Java project
Before you can do anything else in Eclipse, such as creating a Java program, you need to create a
Java project.
To create a new Java project, follow these steps:
i.
Start Eclipse IDE and make sure it is in Java perspective. If you are not already in the Java
perspective, in the main menu select Window, point to Open Perspective and then
choose Java.
ii.
Click File in the main menu and then point to New, and choose Java Project.
iii.
Enter helloworld for the project name as shown in Figure 3.5, then click Finish.
iv.
The helloworld project will be shown as a folder on the Package Explorer.
2. Create a Java class
Once you have created a project for it to live in, you can create your first Java program.
Although doing so is not necessary, it's a good practice to organize your Java classes into
packages. Here, we'll use a package named helloworld. It is a good convention to use the
top package the same name as the project.
To create a new Java class, follow these steps:
i.
Right-click on the helloworld project and point to New, and then select Class to bring up
the New Java Project window.
ii.
Leave the Source Folder field as it is which is helloworld/src by default.
Figure 3.5 Creating new Java project named HelloWorld
FIRST JAVA APPLICATION USING ECLIPSE IDE
iii.
iv.
In the Package field enter helloworld and enter HelloWorld in the Name field.
Check the box for public static void main(String[] arg) in the ‘Which
Method Stubs Would You Like to Create?’ section as shown in Figure 3.6. Then, click
Finish.
Figure 3.6 Creating new Java class named HelloWorld
v.
vi.
The Java editor will automatically open showing your new class. The code that is
automatically generated includes a method stub for main().
Now that you have your HelloWorld class, in the main() method, add the following
statement:
System.out.println("Hello world!");
vii.
Then save your changes. The class will automatically compile upon saving.
3. Run the application
When the compile is successful, in order to run your application in Eclipse:
i.
Right-click on HelloWorld.java in the Package Explorer, point to Run As and select Java
Application.
ii.
The Console view should appear at the bottom and display the "Hello, world!" output as
shown in Figure 3.7.
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Figure 3.7 Output of the HelloWorld application
Congratulations! You have successfully created a Hello World application.
RUN JAVA APPLICATION OUTSIDE ECLIPSE IDE
In previous example, we run the application in the Eclipse IDE. To run a Java application outside the
Eclipse, we need to export the program as a Java ARchive (or JAR) file.
To create a JAR file for the helloworld project:
1. Right-click on the helloworld project and select Export.
2. In the Export window, select Java and then select JAR file. Click Next.
3. In the JAR Export window as shown in Figure 3.8, select helloworld project as the resource to be
export. Select an export destination and a name for the JAR file. For example, choose your
Desktop as the destination and helloworld.jar as the JAR file name.
4. Click Finish. A JAR file will be created in your selected destination.
To run the JAR file:
1. Start the command prompt.
2. Change the default drive prompt to the drive and the directory where your JAR file is stored.
3. Type the following statement and press Enter.
java -classpath helloworld.jar helloworld.HelloWorld
4. The output should appear on the next line as shown in Figure 3.9.
CREATING A DIALOG BOX
Figure 3.8 JAR Export window
Figure 3.9 Output when running the helloworld.jar
CREATING A DIALOG BOX
To create a Java application that produces output in a dialog box:
1. Open the Eclipse IDE, if necessary.
2. To create a new class, right-click on the helloworld package, point to New and then click Class.
3. Enter HelloDialog in the Name field then click Finish.
4. The Java editor will automatically open showing your new class. The code that is automatically
generated includes a class header and an opening and closing brace for the class named
HelloDialog.
5. Position the insertion point at the end of the first line that contains the package declaration
then press Enter to insert a new line.
6. Type the import statement that allows you to use the JOptionPane class:
import javax.swing.JOptionPane;
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7. Position the insertion point after the opening curly brace, press Enter, and then type the
following:
public static void main(String[] args)
{
JOptionPane.showMessageDialog(null, “Hello world!”);
}
8. Save the application. The class will automatically compile upon saving.
9. Run the application. The output should appear as shown in Figure 3.10. Click OK to close the
dialog box.
Figure 3.10 Output of the HelloDialog application
ADDING COMMENTS TO A CLASS
You will add comments to your HelloWorld.java application and save it as a new class named
HelloWorld2.
To add comments to your application and save it as a new class:
1. Open the Eclipse IDE, if necessary.
2. Double-click the HelloWorld.java in the Package Explorer to open the file in the Java Editor.
3. Position the insertion point at the top of the file that contains the package declaration then
press Enter to insert a new line.
4. Press the Up arrow to go to that line, and then type the following comments at the top of the
file. Press Enter after typing each line. Insert your name and today’s date where indicated.
ADDING COMMENTS TO A CLASS
// Filename HelloWorld2.java
// Written by <your name>
// Written on <today’s date>
5. Change the HelloWorld class name to HelloWorld2, press Enter and then type the following
block comment:
/* This class demonstrates the use of the print() and println()
method to print the message Hello, world! and I am a new student.I'm
ready for Java programming. */
6. The HelloWorld2 class name will be underlined with a problem highlight line. Ignore this
problem.
7. Add the following statements below the statement that prints “Hello, world!” as shown in
Figure 3.11:
System.out.print(“I am a new student.”);
System.out.println(“I’m ready for Java programming!”);
8. Click File on the main menu and click Save As.
9. In the Save As window, change the File name to HelloWorld2.java. Click OK. You will see a new
class named HelloWorld2.java created in the Package Explorer. The problem highlight name is
also being fixed.
10. Run the application. The output should appear as shown in Figure 3.12.
Figure 3.11 The HelloWorld2 class
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Figure 3.12 Output of the HelloWorld2 application
Note: As you work through this book, add comments as the first three lines of every exercise
that you do. The comments should contain the number of question and the class name,
purpose of the source code, your name and the date created.
Do it yourself
3.1
Write, compile and test a class that prints your name on the screen. Save the class as
Name.java.
3.2
Write, compile and test a class that prints your full name, matric number and your address in
three separate lines on the screen. Save the class as Student.java.
3.3
Write, compile and test a class that produces a series of three dialog boxes so that each
displays one line of the output from Exercise 3.2 in turn. Save the class as StudentDialog.java.
3.4
Identify and fix the errors in the following code:
Public class Welcome {
public void Main(string[] args) {
System.out.println(‘Welcome to Java’)
}
Do it yourself
3.5
Identify the output of the following code:
// Ex 3.5: Hello.java
// Printing a line of text with multiple statements
public class Hello
{
public static void main(String[] args) //begin execution of program
{
System.out.print(“Welcome to ”);
System.out.println(“Java Programming!”);
}// end method main
}// end class Hello
3.6
Write a Java program that will prints multiple lines of text as follows:
Welcome
to
Java
Programming!
CASE PROJECT
Making Money Bank intends to provide a new automated teller machine (ATM) for the
customers to perform basic financial transactions. Each user has only one account at the
bank. ATM users should be able to view their account balance, withdraw cash and
deposit funds. The company has asked you to write a Java application for the ATM.
Write, compile and test a Java class that displays “Making Money Bank ATM Service”
and “Welcome!” in an attractive layout on the console screen. Be certain to use
appropriate comments in your class. Save the class as MakingMoneyATM.java.
Example of the layout you can create:
********************************************************************
*
*
*
Making Money Bank ATM Service
*
*
*
********************************************************************
Welcome!
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NOTES