Chapter 2 - Introduction to Java ApplicationsOutline 2.1 Introduction 2.2 A First Program in Java: Printing a Line of Text 2.3 Modifying Our First Java Program 2.4 Displaying Text in a Dialog Box 2.5 Another Java Application: Adding Integers 2.6 Memory Concepts 2.7 Arithmetic 2.8 Decision Making: Equality and Relational Operators
Chapter 2 - Introduction to Java Applications
Outline 2.1 Introduction 2.2 A First Program in Java: Printing a Line of Text 2.3 Modifying Our First Java Program 2.4 Displaying Text in a Dialog Box 2.5 Another Java Application: Adding Integers 2.6 Memory Concepts 2.7 Arithmetic 2.8 Decision Making: Equality and Relational Operators
2.1 Introduction
In this chapter
Introduce examples to illustrate features of Java
Two program styles - applications and applets
2.2 A First Program in Java: Printing a Line of Text
Application
Program that executes using the java interpreter
Sample program
Show program, then analyze each line
Welcome1.java Program Output 1 // Fig. 2.1: Welcome1.java
2 // Text-printing program.
3
4 public class Welcome1 {
5
6 // main method begins execution of Java application
7 public static void main( String args[] )
8 {
9 System.out.println( "Welcome to Java Programming!" );
10
11 } // end method main
12
13 } // end class Welcome1
Welcome to Java Programming!
2.2 A First Program in Java: Printing a Line of Text
1 // Fig. 2.1: Welcome1.java 2 // Text-printing program.
Comments start with: //
Comments ignored during program execution
Document and describe code
Provides code readability
Traditional comments: /* ... */
/* This is a traditional comment. It can be split over many lines */
Another line of comments
Note: line numbers not part of program, added for reference
2.2 A Simple Program: Printing a Line of Text
3 4 public class Welcome1 {
Blank line
Makes program more readable
Blank lines, spaces, and tabs are white-space characters
Ignored by compiler
Begins class declaration for class Welcome1
Every Java program has at least one user-defined class
Keyword: words reserved for use by Java
class keyword followed by class name
Naming classes: capitalize every word
SampleClassName
2.2 A Simple Program: Printing a Line of Text
4 public class Welcome1 {
Name of class called identifier
Series of characters consisting of letters, digits, underscores ( _ ) and dollar signs ( $ )
Does not begin with a digit, has no spaces
Examples: Welcome1, $value, _value, button7
7button is invalid
Java is case sensitive (capitalization matters)
a1 and A1 are different
For chapters 2 to 7, use public keyword
Certain details not important now
Mimic certain features, discussions later
2.2 A Simple Program: Printing a Line of Text
4 public class Welcome1 { 7 public static void main( String args[] )
Saving files
File name must be class name with .java extension
Welcome1.java
Left brace {
Begins body of every class
Right brace ends declarations (line 13)
Part of every Java application
Applications begin executing at main
Parenthesis indicate main is a method (ch. 6)
Java applications contain one or more methods
2.2 A Simple Program: Printing a Line of Text
7 public static void main( String args[] ) 8 {
Exactly one method must be called main
Methods can perform tasks and return information
void means main returns no information
For now, mimic main's first line
Left brace begins body of method declaration
Ended by right brace } (line 11)
2.2 A Simple Program: Printing a Line of Text
9 System.out.println( "Welcome to Java Programming!" );
Instructs computer to perform an action
Prints string of characters
String - series characters inside double quotes
White-spaces in strings are not ignored by compiler
System.out
Standard output object
Print to command window (i.e., MS-DOS prompt)
Method System.out.println
Displays line of text
Argument inside parenthesis
This line known as a statement
Statements must end with semicolon ;
2.2 A Simple Program: Printing a Line of Text
11 } // end method main 13 } // end class Welcome1
Ends method declaration
Ends class declaration
Can add comments to keep track of ending braces
2.2 A Simple Program: Printing a Line of Text
Compiling a program
Open a command prompt window, go to directory where program is stored
Type javac Welcome1.java
If no errors, Welcome1.class created
Has bytecodes that represent application
Bytecodes passed to Java interpreter
2.2 A Simple Program: Printing a Line of Text
Fig. 2.2 Executing Welcome1 in a Microsoft Windows 2000 Command Prompt. Executing a program
Type java Welcome1
Interpreter loads .class file for class Welcome1
.class extension omitted from command
Interpreter calls method main
2.3 Modifying Our First Java Program
Modify example in Fig. 2.1 to print same contents using different code
2.3 Modifying Our First Java Program
9 System.out.print( "Welcome to " );
10 System.out.println( "Java Programming!" ); Modifying programs
Welcome2.java (Fig. 2.3) produces same output as Welcome1.java (Fig. 2.1)
Using different code
Line 9 displays “Welcome to ” with cursor remaining on printed line
Line 10 displays “Java Programming! ” on same line with cursor on next line
Welcome2.java 1. Comments 2. Blank line 3. Begin class Welcome2 3.1 Method main 4. Method System.out.print 4.1 Method System.out.println 5. end main, Welcome2 Program Output 1 // Fig. 2.3: Welcome2.java
2 // Printing a line of text with multiple statements.
3
4 public class Welcome2 {
5
6 // main method begins execution of Java application
7 public static void main( String args[] )
8 {
9 System.out.print( "Welcome to " );
10 System.out.println( "Java Programming!" );
11
12 } // end method main
13
14 } // end class Welcome2
Welcome to Java Programming! System.out.print keeps the cursor on the same line, so System.out.println continues on the same line.
2.3 Modifying Our First Java Program
9 System.out.println( "Welcome\nto\nJava\nProgramming!" ); Newline characters (\n)
Interpreted as “special characters” by methods System.out.print and System.out.println
Indicates cursor should be on next line
Welcome3.java (Fig. 2.4)
Line breaks at \n
Usage
Can use in System.out.println or System.out.print to create new lines
System.out.println( "Welcome\nto\nJava\nProgramming!" );
Welcome3.java 1. main 2. System.out.println (uses \n for new line) Program Output
1 // Fig. 2.4: Welcome3.java
2 // Printing multiple lines of text with a single statement.
3
4 public class Welcome3 {
5
6 // main method begins execution of Java application
7 public static void main( String args[] )
8 {
9 System.out.println( "Welcome\nto\nJava\nProgramming!" );
10
11 } // end method main
12
13 } // end class Welcome3
Welcome
to
Java
Programming! Notice how a new line is output for each \n escape sequence.
2.3 Modifying Our First Java Program
Escape characters
Backslash ( \ )
Indicates special characters be output
2.4 Displaying Text in a Dialog Box
Display
Most Java applications use windows or a dialog box
We have used command window
Class JOptionPane allows us to use dialog boxes
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)
JOptionPane is in the javax.swing package
Package has classes for using Graphical User Interfaces (GUIs)
Comments