The Structure of a Java Source Code File

We usually start with a brief description of the pupose of the java file (written as comments).
// Press.java	the Java file name
// remember that the file name must match the class name exactly // An applet for getting simple text input from a textField by pressing a button // lines starting with // are comments (ignored by the compiler) //
Next comes the import section, where we load the required, shared java libraries.
import		java.applet.*;	all the Applet Class definitions
import		java.awt.*;	all the GUI Class definitions
import		java.awt.event.*;	all the User-Action Class definitions
the import instruction		the Java Class Libraries	(the * means load All the Classes found in the specified library folder)

The first line of the class definition looks like this
public	class	Press	extends Applet	implements ActionListener
the visibility of the object public or private	the type of object a class	the name of the object the class name Press	creates an instance of an Applet to be viewed by a Browser	uses the ActionListener class which listens for button clicks
{	this opening brace, and the matching closing brace at the end of the file, enclose the objects encapsulated in the class
TextField	yourName	= new TextField	("Type your name in here", 25);
the predefined AWT class type of the data member	the variable name assigned to the data member	instantiate the new data member ie. allocate the memory storage	pass 2 parameters to the constructor method, the Prompt and the Field width
Button	pressMe	= new Button	("Press Me");
the predefined AWT class type of the data member	the variable name assigned to the data member	instantiate the new data member ie. allocate the memory storage	pass a parameter to the constructor method, the Text on the button
Label	theOutput	= new Label	("This is where the output will appear");
the predefined AWT class type of the data member	the variable name assigned to the data member	instantiate the new data member ie. allocate the memory storage	pass a parameter to the constructor method, the Text in the Label
public	void	init	( )
the visibility of the Method	the return value of the Method	the name assigned to the Method	no parameters are passed
{	this opening brace, and the matching closing brace at the end of the Method, enclose the functionality of the Method
add		(yourName);
the predefined Method to put a data member on the page		the data member to be added
add		(pressMe);
the predefined Method to put a data member on the page		the data member to be added
add		(theOutput);
the predefined Method to put a data member on the page		the data member to be added
pressMe	.addActionListener	(this);
the data member we are listening to	the predefined addActionListener Method to recieve action events	listen to this button
}	this closing brace, and the matching opening brace at the start of the Method, enclose the functionality of the Method
public	void	actionPerformed	(ActionEvent	thisEvent)
the visibility of the Method	the return value of the Method	the name assigned to the Method	the type of the parameter being passed	the variable name assigned to the parameter
{	this opening brace, and the matching closing brace at the end of the Method, enclose the functionality of the Method
if	(thisEvent	.getSource	== "pressMe")
the conditional keyword if	the passed parameter	the predefined Method to retreive the Button name that triggered the event	the name of the Button being compared
{	this opening brace, and the matching closing brace at the end of the conditional block, enclose the functionality of the condition
String	message	= yourName	.getText( )
the predefined LANG class type of the data member	the variable name assigned to the data member	the data member (textField) holding the text data	the predefined Method to retreive the text from the textField
theOutput	.setText	("Hello " + message + ", welcome.");
the data member (Label) recieving the text data	the predefined Method to insert text into the Label	the String message to insert into the Label Field
}	this closing brace, and the matching opening brace at the start of the conditional block, enclose the functionality of the condition
}	this closing brace, and the matching opening brace at the start of the Method, enclose the functionality of the Method
}	this closing brace, and the matching opening brace near the start of the file, enclose the objects encapsulated in the class

The Complete Java Code File looks like this

// Press.java // remember that the file name must match the class name exactly // Getting simple text input // lines starting with // are comments (ignored by the compiler) // import java.applet.*; import java.awt.*; import java.awt.event.*; // classes used by the applet public class Press extends Applet implements ActionListener { // Press class opening brace // a text field, a button and a label are initialised TextField yourName = new TextField("Type your name in here", 25); Button pressMe = new Button("Press Me"); Label theOutput = new Label("This is where the output will appear"); public void init() { // init Method opening brace // the objects are added to the page add( yourName ); add( pressMe ); add( theOutput ); // Make the button "listen" for an event (eg a press) pressMe.addActionListener(this); } // init Method closing brace public void actionPerformed(ActionEvent thisEvent) // the button is pressed { // actionPerformed Method opening brace if (thisEvent.getSource == "pressMe") // which button was pressed eg. the 'pressMe' button { // Get the name from the text box, then give the label object the new text String message = yourName.getText(); theOutput.setText("Hello " + message + ", welcome."); } } // actionPerformed Method closing brace } // Press class closing brace

The Finished Java Applet looks like this
The applet before any user interaction	After data has been entered and the button pressed

The original location of this page was http://www.uwcsea.edu.sg/comp/ib/javasource/SourceCode.html. It was mirrored from the web site of IB Computer Science, United World College of South East Asia, Singapore developed by IB Computer Science Teacher Richard Jones. This page was mirrored from that original site using the utility Webstripper. It was mirrored to ensure availability of it to local students as well as relieving hits on the original site. Aside from this paragraph and the immediately following footer, no other changes have been made.

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