Transcript Class 1 ~ Chapter 1

CSE1340
Class 4
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Objectives

Write a simple computer program in Java

Use Swing components to build the GUI

Use proper naming conventions for
classes and files
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Adding Interface Components
to a java application
JLabel
– An area where uneditable text can be
displayed.
 JButton
– An area that triggers an event when
clicked.
 JTextField
– An area in which the user inputs data
from the keyboard. The area can also
display information.
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
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Simple steps for creating a user interface:
1. Create a window where gui components are
displayed
2. Set the layout of the window
3. Create the gui components
4. Add them to the window
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Step 1: Create a window on which to place the gui
components
Container myWindow = getContentPane();
Part of the Java API
Method call to perform a
task
Programmer defined name
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Step 2: Set the layout of the window
myWindow.setLayout (null);
(Java doesn’t know where to place the components
so you will have to set the bounds (specify where)
each component should go)
// or
myWindow.setLayout (new FlowLayout());
// Java knows where to place the components
// (left to right, top to bottom)
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3. Creating the gui components
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Kind of object defined in Java api
JLabels

Programmer defined name
of object
JLabel label = new JLabel();
– Constructs an empty Label-text is not
displayed
 JLabel label1 = new JLabel( “Label with text” );
– Constructs a Label that displays the text with
default left-justified alignment.
// where do you want it to go in the window and
// what do you want the size to be
label1.setLocation(100,20);
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label1.setSize(200,100);
JLabels
// decide on the font or use the default font
label1.setFont(new Font(“Arial”, Font.PLAIN,40));
Any valid font name
Font.BOLD
Font.ITALIC
Size of font in
pixels
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JLabels
// where within the allocated space should the
// label appear ?
label1.setHorizontalAlignment( JLabel.CENTER);
// other alignment choices: LEFT
RIGHT
// add the label to the window
myWindow.add(label1);
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JLabel
JLabel label2= new JLabel( );
– Constructs a label that is empty
label2.setIcon (new ImageIcon(“java.jpg”));
The label contains a picture instead of text.
// set the bounds
label2.setBounds(300,100);
// set the alignment
label2. setHorizontalAlignment(
JLabel.CENTER);
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JLabel
// add label2 to the window
myWindow.add(label2);
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Creating a complete Java
application
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Coding/Implementing the Solution
Integrated development environments (IDE) such as
NetBeans
can be used to make coding simpler. NetBeans is a
free download from Sun and is installed on the
computers in the Junkins lab and the SIC open lab.
Instructions for using NetBeans are available via a link
at the top of your outline. You should print them out
before next week’s lab. Lab assistants should show
you how to use NetBeans next week.
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Coding the Program Comments as Documentation

Purpose of comments
– Provides clear description when reviewing
code
– Helps programmer think clearly when coding
 Placement of comments
– Use a comment header to identify a file and
its purpose
– Place a comment at the beginning of code for
each event and method
– Place comments near portions of code that
need clarification
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Coding the Program Comments as Documentation
General form:
/* block comments */
// line comments
Example: /* Programmer:
Judy
Date:
Sept. 3, 2007
Filename:
MyProgram.java
Purpose: This program displays the name
and webaddress of a company */
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Import Packages

Use the import statement to access
classes in the SDK
– The java.lang package is
automatically imported
– Place the import statement before
the class header
– Use an asterisk (*) after the
package name and period delimiter
to import all necessary classes in
the package
import javax.swing.*;
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import javax.swing.JOptionPane;
The import statement includes particular
classes from a particular package (folder)
in the java library of classes
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import javax.swing.JApplet;
The import statement includes particular
classes from a particular package (folder) in
the java library of classes
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import javax.swing.*;
public class Welcome4
All code in java must be inside a class
definition; the above line begins the
definition of a class called Welcome4.
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Coding the Program The Class Header

Identify how the code can be accessed with an
access modifier
– public indicates that the code can be accessed
by any and all entities
 Specify a unique name for the class
– The class name at the beginning of the
program must match the file name exactly
– Java is case-sensitive
– Must begin with an underscore, dollar sign or
letter and can then contain underscores, $,
letters, or digits (no special characters)
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Coding the Program The Class Header
– Cannot be reserved words
– By convention, uppercase letters are used
for class names and to distinguish words
in class names
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Sample Class Header

Use braces { } after the class header to
enclose the class body
public class Person {
// body of the class
}
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import javax.swing.*;
public class Welcome extends JFrame
Keyword extends gives us inheritance in Java
We want our class to have the characteristics
of a JFrame
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Coding the Program The Method Header


The method header contains modifiers, return
value, method name, and parameters along with
their data type
Every stand-alone Java application must contain
a main() method, which is the starting point
during execution
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Coding the Program The Method Header

Modifiers set properties for a method
– public allows other programs to invoke
this method
– static means this method is unique and
can be invoked without creating a
subclass or instance
 Return value is the data type of the data
returned by the method
– If no data is returned, the keyword void
is used
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Coding the Program The Method Header

Parameters are pieces of data received by
the method to help the method perform its
operation
– Identifiers are used to name the variable
sent to the method
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Special Characters/
keywords
Character
Use
// Double slash
Marks the beginning
of a comment
import
Tells the compiler
where to search for
the packages that
contain predefined
code
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Special Characters
Character
Use
{ } Open / close
braces
Encloses a group of
statements, such as
the contents of a
method
( ) Open / close
parentheses
Used in naming a
method such as in
public void paint (….)
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import javax.swing.*;
// import statements required to use part of the Java API
import java.awt.*;
public class LabelFrame extends JFrame {
private JLabel label1, label2;
public LabelFrame( )
{
Container myWindow= getContentPane();
myWindow.setLayout(null);
myWindow.setBackground(Color.CYAN);
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label1 = new JLabel(“Label with text”);
label1.setLocation(300,20);
label1.setSize(200,100);
myWindow.add( label1 );
label2= new JLabel( );
label2.setIcon (new ImageIcon(“java.jpg”));
label2.setBounds(300,100);
label2. setHorizontalAlignment(JLabel.CENTER);
myWindow.add(label2);
setTitle(“Testing JLabel”);
setSize(300,200);
setVisible(true);
} // end of LabelFrame()
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// Every Java application must have a method main
public static void main(String a[])
{ LabelFrame l = new LabelFrame();
l.setDefaultCloseOperation(
JFrame.EXIT_ON_CLOSE);
}// end of main
}// end of class
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Label with text
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Testing the Solution

Compile the source code
 If the compiler detects errors, fix the
errors and compile again
 If the compilation was successful, a new
bytecode file for each class is created with
a .class extension
 run the program (test it logically)
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Debugging the Solution

System Errors
– System command is not set properly
– Software is installed incorrectly
– Location of stored files is not accessible
 Syntax Errors
– One or more violations of the syntax rules of
Java
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Debugging the Solution

Logic and Run-Time Errors
– Unexpected conditions during execution of a
program
– Wront results
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Running the Application

After compilation is successful, run the
program to test for logic and run-time
errors
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Editing the Source Code - cont.
Recompile and run the application
– The bytecode should be updated
after any changes to the source
code
 Print a hard copy of the source code
– The final step of the program
development cycle is to document
the solution

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