Transcript MF118_fhs_lnt_004_May09

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
2
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.
3

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
4
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)
5
Displaying Text in a Dialog Box


JOptionPane is in the javax.swing
package
Class JOptionPane allows us to use dialog
boxes.
6
Text in the dialog box
Displaying Text in a Dialog Box using
JOptionPane
7
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
8
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
(;)
9
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
10
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.
11
Another GUI Example:
Addition.java
12
Another GUI Example (continue…)
13
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
14
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
15
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
16
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.
17
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)
18
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.
19
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:
20
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

21
Line 35: JOptionPane.PLAIN_MESSAGE means no icon shown
Various Message Dialog Icons
Figure 2.12: JOptionPane constants for message dialogs
22
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 )
23
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)
24
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
25
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.
26
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
27
Summary


Basic Java GUI using JOptionPane
Two examples are given here
• Welcome1.java
• Addition.java

Various Message Dialog icons used in
JOptionPane
28