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Java Programming 1
Introduction to Basic GUI
Lesson 4
1
Outline

What is GUI?

Display Text in Dialog box
• JOptionPane
• Step-by-step explanation

Various Message Dialog Icons

Summary
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Graphic User Interface (GUI)

Graphic User Interface (GUI)
• Pronounced GOO-ee
• Program interface that takes advantage of the
computer’s graphics capabilities to make the
program easier to use

Most Java applications use windows or a dialog
box to display the output/input. So far we have
used command window via JCreator.
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
Swing GUI components
• Package javax.swing
• Components originate from AWT
(package java.awt)
• Contain look and feel

Appearance and how users interact
with program
• Lightweight components

Written completely in Java
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What is Packages in Java?

Packages :
• Set of predefined classes for us to use
• Groups of related classes called
packages
• Group of all packages known as Java
class library or Java Applications
Programming Interface (Java API)
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Displaying Text in a Dialog Box


JOptionPane is in the javax.swing
package
Class JOptionPane allows us to use dialog
boxes.
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Text in the dialog box
Displaying Text in a Dialog Box using
JOptionPane
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Displaying Text in a Dialog Box using
JOptionPane : Explanation

Line 7: import declarations
• Used by compiler to identify and locate classes used
in Java programs
• Tells the compiler to load class JOptionPane from
javax.swing package

Javax is the extension package from Java API
• Those that begin with java is the Java API core
package
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Displaying Text in a Dialog Box using
JOptionPane : Explanation

Line 14 : Call the showMessageDialog method from
JOptionPane class
•
This method requires two arguments/ parameters
•
Multiple arguments separated by commas (,)
•
For now, first argument always null
•
Second argument is String ( words/ phrases)

•
Must use double quote to enclose the words you
want to display on the dialog box.
As usual, all the statement must end with semicolon
(;)
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Displaying Text in a Dialog Box using
JOptionPane : Explanation

Line 17: System.exit(0); //exit the program

method exit of class System
• Terminates application
• Use in any application displaying GUI
• Identifiers start with capital letters usually class names

Argument of
0 (zero) means application ended successfully
• Non-zero usually means an error occurred

Class System part of package java.lang
• No import declaration is needed

java.lang is automatically import in every Java
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Another GUI Example

This example uses input dialogs to
input two values from user
• Use message dialog to display sum of
the two values

This is an interactive program
• Allow user to "provide" data to program
and process them.
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Another GUI Example:
Addition.java
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Another GUI Example (continue…)
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The Addition.java Program Output
1.
Input 10, press “OK”
2.
Input 20, press “OK”
3.
The result will show
in the message
dialog box
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Another GUI Example:
Addition.java (Explanation)

Line 16 and 17: are declarations statement
• strNum1 and strNum2 are variables.
• Variables are :


Location in memory that stores a value
All variables has to be declared with name
and type before use. ( week 2 )
• strNum1 and strNum2 are of type String
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Another GUI Example:
Addition.java (Explanation)

Line 19 – 21: are statements which declares
variables num1, num2, and sum of data type int
• You also can declare all the same data type in
a single line :

For Example:
int num1, num2, sum; // declaration
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Another GUI Example: Addition.java
(Explanation)


Operator
s
Associativit
y
Type
=
Right to left
assignment
Line 24: reads in the first input from the user,
representing the first number to be added.
Line 25: reads in the second input from the user,
representing the second number to be added.
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Another GUI Example:
Addition.java (Explanation)



Please take note that the input from the user to the
showInputDialog is always a String type.
We have to parse ( to change/ convert ) the String type to
the appropriate type.
Line 28 and 29: the method Integer.parseInt to
• Convert String argument into integer (with data type int)
• Integer returned by Integer.parseInt is assigned to
variable num1 Remember that num1 was declared as type
int (it means num1 can hold only integral value)
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Another GUI Example: Addition.java
(Explanation)

Line 32:
• Calculates the sum of num1 and num2 (from right to
left).
• Uses assignment operator, = , to assign result to
variable sum.
• Read as: sum gets the value of num1 + num2.
• sum has the type int.
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Another GUI Example: Addition.java
(Explanation)

Line 34 and 35: Use showMessageDialog to display results

"The sum is " +
sum
• Uses the operator + to "add" the string literal "The sum
is" and sum

+ operator means,
• Concatenation ( append) of a string and another type (int,
string etc ..) will results in a new string.
• For example:
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Another GUI Example: Addition.java (Explanation)
Different showMessageDialog

Compared to Welcome1 class example, here we have used different version of
showMessageDialog
• Previously, Welcome1class’ showMessageDialog only have two arguments
1
2
Here in Line 34-35, it requires four arguments (instead of two as before)

First argument: null for now

Second: string to display

Third: string in title bar

Fourth: type of message dialog with icon

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Line 35: JOptionPane.PLAIN_MESSAGE means no icon shown
Various Message Dialog Icons
Figure 2.12: JOptionPane constants for message dialogs
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Part 2: Java Assignment Statements and
Expressions
Operator
Meaning
+
Unary plus - no change/positive value
-
Unary minus - negation/negative value ( example : - 10 )
+
Binary plus - addition
-
Binary minus - subtraction ( example : 8 - 4 )
*
Multiplication
/
Division
%
Modulus - remainder from integer division
++
Unary increment - add one to variable
--
Unary decrement - subtract one from variable
( example : + 5 )
( example : 5 + 8 )
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Java Assignment Statements and
Expressions

Unary Operator : Represent the value, either
negative or positive
Example : - y ,



+5;
Binary Operator : Can perform arithmetic calculation.
Example : a – b , c / d ,
n % 5 , x + y;
Unary increment : an operation which will increase
by 1 to the variable. (post-increment and preincrement)
Example : a ++ , ++ a;
Unary decrement : an operation which will decrease
by 1 to the variable. (post-decrement and predecrement)
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Example : b -- , -- b ;
Unary increment/ Unary decrement

The following statement may look strange!
int count ;
// declaration
count = count + 1 ;


But, when we remember that the operator =
stands for assignment and not for equality, it
makes sense;
it is our instruction to the computer to increment
the current value of count by 1 and store to the
new result into count’s memory location
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Unary increment/ Unary decrement

The operators ++ and - -, (the unary increment
and unary decrement operators ) are used in Java
to increase or decrease the variable by one.
• Example : count = count + 1; can be written by
1.
count++;
// post-increment unary operator
OR
2.

++count; // pre-increment unary operator
Exercise : Write an expression count = count – 1
by using pre and post decrement unary operator.
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More Example on ++ and - 
What are the answers for the pre and
post-increment
Post-increment
int a = 10, b = 12, c= 0;
c = (a++) + b;
c = 10 + 12
c will have the value 22
a will increment by 1 after the
addition to 11 and kept in the
memory
Pre-increment
int a = 10, b= 12, c = 0;
c = (++a) + b;
c = 11 + 12
c will have the value 23
a will increment by 1 before the
addition to 11
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Summary


Basic Java GUI using JOptionPane
Two examples are given here
• Welcome1.java
• Addition.java

Various Message Dialog icons used in
JOptionPane
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