Lecture 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
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
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!
1 // Fig. 2.1: Welcome1.java
–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 */
2 // Text-printing program.
– Another line of comments
– Note: line numbers not part of program, added for reference
3
– Blank line
• Makes program more readable
• Blank lines, spaces, and tabs are white-space characters
– Ignored by compiler
4 public class
Welcome1
{
–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
– 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
– 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)
7 public
static void
main( String args[] )
– Part of every Java application
•
Applications begin executing at main
–
Parenthesis indicate main is a method (see ch. 6)
–
Java applications contain one or more methods
7 public
static void main( String args[] )
• Exactly one method must be called
main
–
Methods can perform tasks and return information
•
void means
main returns no information
8 {
– Left brace begins body of method declaration
•
Ended by right brace } (line 11)
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 ;
11
} //
end method main
– Ends method declaration
13
}
//
end class Welcome1
– Ends class declaration
–
Can add comments to keep track of ending braces
–
Remember: compiler ignores comments
–
Comments can start on same line after code
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
• Executing a program
–
Type java Welcome1
•
Interpreter loads .class file for class Welcome1
•
.class extension omitted from command
–
Interpreter calls method main
Executing Welcome1 in a Microsoft Windows 2000 Command Prompt.
Modifying Our First Java Program
• Modify example in Fig. 2.1 to print same contents using different code
• Modifying programs
– Welcome2.java (Fig. 2.3) produces same output as
– Welcome1.java (Fig. 2.1)
– Using different code
9
System.out.print(
"Welcome
to " );
10
System.out.println(
"Java
Programming!" );
– Line 9 displays “Welcome to ” with cursor remaining on printed
line
–
Line 10 displays “Java Programming! ” on same line with cursor
on next line
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.)
• 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)
9
System.out.println( "Welcome\nto\nJava\nProgramming!" );
– 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!" );
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.)
Escape characters
– Backslash ( \ )
– Indicates special characters be output
