02장 Introduction to Java Applications

Java™ How to Program, 9/e
© Copyright 1992-2012 by Pearson Education, Inc. All Rights
Reserved.

 Java application programming
 Use tools from the JDK to compile and run programs.
 Videos at www.deitel.com/books/jhtp9/
 Help you get started with Eclipse and NetBeans integrated
development environments.

 Java application
 A computer program that executes when you use the java
command to launch the Java Virtual Machine (JVM).
 Sample program in Fig. 2.1 displays a line of text.

 Comments
// Fig. 2.1: Welcome1.java
 // indicates that the line is a comment.
 Used to document programs and improve their readability.
 Compiler ignores comments.
 A comment that begins with // is an end-of-line comment—it
terminates at the end of the line on which it appears.
 Traditional comment, can be spread over several lines as in
/* This is a traditional comment. It
can be split over multiple lines */
 This type of comment begins with /* and ends with */.
 All text between the delimiters is ignored by the compiler.

 Javadoc comments
 Delimited by /** and */.
 All text between the Javadoc comment delimiters is ignored by
the compiler.
 Enable you to embed program documentation directly in your
programs.
 The javadoc utility program (Appendix M) reads Javadoc
comments and uses them to prepare your program’s
documentation in HTML format.

 Blank lines and space characters
 Make programs easier to read.
 Blank lines, spaces and tabs are known as white space (or
whitespace).
 White space is ignored by the compiler.

 Class declaration
public class Welcome1
 Every Java program consists of at least one class that you
define.
 class keyword introduces a class declaration and is
immediately followed by the class name.
 Keywords (Appendix C) are reserved for use by Java and are
always spelled with all lowercase letters.

 Class names
 By convention, begin with a capital letter and capitalize the
first letter of each word they include (e.g.,
SampleClassName).
 A class name is an identifier—a series of characters consisting
of letters, digits, underscores (_) and dollar signs ($) that does
not begin with a digit and does not contain spaces.
 Java is case sensitive—uppercase and lowercase letters are
distinct—so a1 and A1 are different (but both valid) identifiers.

 Braces
 A left brace, {, begins the body of every class declaration.
 A corresponding right brace, }, must end each class
declaration.
 Code between braces should be indented.
 This indentation is one of the spacing conventions mentioned
earlier.

 Declaring the main Method
public static void main( String[] args )
 Starting point of every Java application.
 Parentheses after the identifier main indicate that it’s a
program building block called a method.
 Java class declarations normally contain one or more methods.
 main must be defined as shown; otherwise, the JVM will not
execute the application.
 Methods perform tasks and can return information when they
complete their tasks.
 Keyword void indicates that this method will not return any
information.

 Body of the method declaration
 Enclosed in left and right braces.
 Statement
System.out.println("Welcome to Java Programming!");
 Instructs the computer to perform an action
 Print the string of characters contained between the double
quotation marks.
 A string is sometimes called a character string or a string literal.
 White-space characters in strings are not ignored by the
compiler.
 Strings cannot span multiple lines of code.

 System.out object
 Standard output object.
 Allows Java applications to display strings in the command
window from which the Java application executes.
 System.out.println method
 Displays (or prints) a line of text in the command window.
 The string in the parentheses the argument to the method.
 Positions the output cursor at the beginning of the next line in
the command window.
 Most statements end with a semicolon.

 Compiling and Executing Your First Java Application
 Open a command window and change to the directory where the
program is stored.
 Many operating systems use the command cd to change directories.
 To compile the program, type
javac Welcome1.java
 If the program contains no syntax errors, preceding command creates
a.class file (known as the class file) containing the platform-
independent Java bytecodes that represent the application.
 When we use the java command to execute the application on a
given platform, these bytecodes will be translated by the JVM into
instructions that are understood by the underlying operating system.

 To execute the program, type java Welcome1.
 Launches the JVM, which loads the .class file for
class Welcome1.
 Note that the .class file-name extension is omitted
from the preceding command; otherwise, the JVM will
not execute the program.
 The JVM calls method main to execute the program.

 Class Welcome2, shown in Fig. 2.3, uses two
statements to produce the same output as that shown in
Fig. 2.1.
 New and key features in each code listing are
highlighted.
 System.out’s method print displays a string.
 Unlike println, print does not position the output
cursor at the beginning of the next line in the command
window.
 The next character the program displays will appear
immediately after the last character that print displays.

 Newline characters indicate to System.out’s print and
println methods when to position the output cursor at
the beginning of the next line in the command window.
 Newline characters are white-space characters.
 The backslash () is called an escape character.
 Indicates a “special character”
 Backslash is combined with the next character to form an
escape sequence.
 The escape sequence n represents the newline character.
 Complete list of escape sequences
java.sun.com/docs/books/jls/third_edition/html/
lexical.html#3.10.6.

 System.out.printf method
 f means “formatted”
 displays formatted data
 Multiple method arguments are placed in a comma-separated list.
 Java allows large statements to be split over many lines.
 Cannot split a statement in the middle of an identifier or string.
 Method printf’s first argument is a format string
 May consist of fixed text and format specifiers.
 Fixed text is output as it would be by print or println.
 Each format specifier is a placeholder for a value and specifies the type
of data to output.
 Format specifiers begin with a percent sign (%) and are followed
by a character that represents the data type.
 Format specifier %s is a placeholder for a string.

 Integers
 Whole numbers, like –22, 7, 0 and 1024)
 Programs remember numbers and other data in the
computer’s memory and access that data through
program elements called variables.
 The program of Fig. 2.7 demonstrates these concepts.

 import declaration
 Helps the compiler locate a class that is used in this program.
 Rich set of predefined classes that you can reuse rather than
“reinventing the wheel.”
 Classes are grouped into packages—named groups of related
classes—and are collectively referred to as the Java class
library, or the Java Application Programming Interface (Java
API).
 You use import declarations to identify the predefined
classes used in a Java program.

 Variable declaration statement
Scanner input = new Scanner( System.in );
 Specifies the name (input) and type (Scanner) of a variable that
is used in this program.
 Variable
 A location in the computer’s memory where a value can be stored for
use later in a program.
 Must be declared with a name and a type before they can be used.
 A variable’s name enables the program to access the value of the
variable in memory.
 The name can be any valid identifier.
 A variable’s type specifies what kind of information is stored at that
location in memory.

 Scanner
 Enables a program to read data for use in a program.
 Data can come from many sources, such as the user at the keyboard or a
file on disk.
 Before using a Scanner, you must create it and specify the source of
the data.
 The equals sign (=) in a declaration indicates that the variable
should be initialized (i.e., prepared for use in the program) with
the result of the expression to the right of the equals sign.
 The new keyword creates an object.
 Standard input object, System.in, enables applications to read
bytes of information typed by the user.
 Scanner object translates these bytes into types that can be
used in a program.

 Variable declaration statements
int number1; // first number to add
int number2; // second number to add
int sum; // sum of number1 and number2
declare that variables number1, number2 and sum hold
data of type int
 They can hold integer.
 Range of values for an int is –2,147,483,648 to +2,147,483,647.
 Actual int values may not contain commas.
 Several variables of the same type may be declared in one
declaration with the variable names separated by commas.

 Prompt
 Output statement that directs the user to take a specific action.
 System is a class.
 Part of package java.lang.
 Class System is not imported with an import declaration at
the beginning of the program.

 Scanner method nextInt
number1 = input.nextInt(); // read first number from
user
 Obtains an integer from the user at the keyboard.
 Program waits for the user to type the number and press the
Enter key to submit the number to the program.
 The result of the call to method nextInt is placed in
variable number1 by using the assignment operator, =.
 “number1 gets the value of input.nextInt().”
 Operator = is called a binary operator—it has two operands.
 Everything to the right of the assignment operator, =, is always
evaluated before the assignment is performed.

 Arithmetic
sum = number1 + number2; // add numbers
 Assignment statement that calculates the sum of the variables
number1 and number2 then assigns the result to variable sum
by using the assignment operator, =.
 “sum gets the value of number1 + number2.”
 In general, calculations are performed in assignment statements.
 Portions of statements that contain calculations are called
expressions.
 An expression is any portion of a statement that has a value
associated with it.

 Integer formatted output
System.out.printf( "Sum is %dn", sum );
 Format specifier %d is a placeholder for an int value
 The letter d stands for “decimal integer.”

 Variables
 Every variable has a name, a type, a size (in bytes) and a value.
 When a new value is placed into a variable, the new value
replaces the previous value (if any)
 The previous value is lost.

 Arithmetic operators are summarized in Fig. 2.11.
 The asterisk (*) indicates multiplication
 The percent sign (%) is the remainder operator
 The arithmetic operators are binary operators because
they each operate on two operands.
 Integer division yields an integer quotient.
 Any fractional part in integer division is simply discarded (i.e.,
truncated)—no rounding occurs.
 The remainder operator, %, yields the remainder after
division.

 Arithmetic expressions in Java must be written in
straight-line form to facilitate entering programs into
the computer.
 Expressions such as “a divided by b” must be written
as a / b, so that all constants, variables and operators
appear in a straight line.
 Parentheses are used to group terms in expressions in
the same manner as in algebraic expressions.
 If an expression contains nested parentheses, the
expression in the innermost set of parentheses is
evaluated first.

 Rules of operator precedence
 Multiplication, division and remainder operations are applied first.
 If an expression contains several such operations, they are applied from
left to right.
 Multiplication, division and remainder operators have the same level of
precedence.
 Addition and subtraction operations are applied next.
 If an expression contains several such operations, the operators are
applied from left to right.
 Addition and subtraction operators have the same level of precedence.
 When we say that operators are applied from left to right, we are
referring to their associativity.
 Some operators associate from right to left.
 Complete precedence chart is included in Appendix A.

 As in algebra, it’s acceptable to place redundant
parentheses (unnecessary parentheses) in an ex-
pression to make the expression clearer.

 Condition
 An expression that can be true or false.
 if selection statement
 Allows a program to make a decision based on a condition’s value.
 Equality operators (== and !=)
 Relational operators (>, <, >= and <=)
 Both equality operators have the same level of precedence,
which is lower than that of the relational operators.
 The equality operators associate from left to right.
 The relational operators all have the same level of
precedence and also associate from left to right.

 An if statement always begins with keyword if,
followed by a condition in parentheses.
 Expects one statement in its body, but may contain multiple
statements if they are enclosed in a set of braces ({}).
 The indentation of the body statement is not required, but it
improves the program’s readability by emphasizing that
statements are part of the body.
 Note that there is no semicolon (;) at the end of the
first line of each if statement.
 Such a semicolon would result in a logic error at execution
time.
 Treated as the empty statement—semicolon by itself.

02장 Introduction to Java Applications

More Related Content

What's hot (15)

Similar to 02장 Introduction to Java Applications (20)

More from 유석 남 (14)

Recently uploaded (20)

02장 Introduction to Java Applications