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Introductionto Java Applications

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2.1 Introduction

• Java application programming– Display messages

– Obtain information from the user

– Arithmetic calculations

– Decision-making fundamentals

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2.2 First Program in Java: Printing a Line of Text

• Application– Executes when you use the java command

to launch the Java Virtual Machine (JVM)

• Sample program– Displays a line of text

– Illustrates several important Java language features

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2.2 First Program in Java: Printing a Line of Text (Cont.)

– 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

1 // Fig. 2.1: Welcome1.java

2 // Text-printing program.

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2.2 First Program in Java: Printing a Line of Text (Cont.)

– 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

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4 public class Welcome1

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2.2 First Program in Java: Printing a Line of Text (Cont.)

– Java 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

– In chapters 2 to 7, use public class• Certain details not important now • Mimic certain features, discussions later

4 public class Welcome1

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2.2 First Program in Java: Printing a Line of Text (Cont.)

– 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)

4 public class Welcome1

5 {

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2.2 First Program in Java: Printing a Line of Text (Cont.)

– Part of every Java application• Applications begin executing at main

– Parentheses indicate main is a method (Ch. 3 and 6)

– Java applications contain one or more methods

• 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)

7 public static void main( String args[] )

8 {

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2.2 First Program in Java: Printing a Line of Text (Cont.)

– 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

– This line known as a statement• Statements must end with semicolon ;

9 System.out.println( "Welcome to Java Programming!" );

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2.2 First Program in Java: Printing a Line of Text (Cont.)

– Ends method declaration

– Ends class declaration

– Can add comments to keep track of ending braces

11 } // end method main

13 } // end class Welcome1

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2.3 Modifying Our First Java Program

• 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

9 System.out.print( "Welcome to " ); 10 System.out.println( "Java Programming!" );

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2.3 Modifying Our First Java Program (Cont.)

• Escape characters– Backslash ( \ )

– Indicates special characters be output

• Newline characters (\n)– Interpreted as “special characters” by methods System.out.print and System.out.println

– Indicates cursor should be at the beginning of the next line

– Welcome3.java (Fig. 2.4)

– Line breaks at \n

9 System.out.println( "Welcome\nto\nJava\nProgramming!" );

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Fig. 2.5 | Some common escape sequences.

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2.4 Displaying Text with printf

•System.out.printf– New feature of J2SE 5.0

– Displays formatted data

– Format string• Fixed text

• Format specifier – placeholder for a value

– Format specifier %s – placeholder for a string

– Other format specifiers

9 System.out.printf( "%s\n%s\n", 10 "Welcome to", "Java Programming!" );

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 Formatting Output with printf

•printf– Precise output formatting

• Conversion specifications: flags, field widths, precisions, etc.

– Can perform • rounding• aligning columns• right/left justification• inserting literal characters• exponential format• octal and hexadecimal format• fixed width and precision• date and time format

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  Formatting Output with printf (Cont.)

• Format String– Describe the output format

– Consist of fixed text and format specifier

• Format specifier– Placeholder for a value

– Specify the type of data to output

– Begins with a percent sign (%) and is followed by a conversion character

• E.g., %s, %d

– Optional formatting information• Argument index, flags, field width, precision

• Specified between % and conversion character

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 Printing Integers

• Integer– Whole number (no decimal point): 25, 0, -9– Positive, negative, or zero– Only minus sign prints by default (later we shall

change this)

• Format– printf( format-string, argument-list );– format-string

• Describe the output format

– argument-list• Contain the value corresponding to each format

specifier

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Integer conversion characters.

• View demonstration program Fig. 28.3– Note output of positive, negative numbers

– Note octal (base 8) and hexadecimal (base 16) options

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  Printing Floating-Point Numbers

• Floating Point Numbers– Have a decimal point (33.5)

– Computerized scientific notation (exponential notation)• 150.4582 is 1.504582 x 10² in scientific

• 150.4582 is 1.504582e+02 in exponential (e stands for exponent)

• use e or E

– f – print floating point with at least one digit to left of decimal

– g (or G) - prints in f or e (E) • Use exponential if the magnitude is less than 10-3, or

greater than or equal to 107

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Floating-point conversion characters.

• View program using printf for floating point numbers, Figure 28.4

Conversion character Description

e or E Display a floating-point value in exponential notation. When conversion character E is used, the output is displayed in uppercase letters.

F Display a floating-point value in decimal format.

g or G Display a floating-point value in either the floating-point format f or the exponential format e based on the magnitude of the value. If the magnitude is less than 10–3, or greater than or equal to 107, the floating-point value is printed with e (or E). Otherwise, the value is printed in format f. When conversion character G is used, the output is displayed in uppercase letters.

a or A Display a floating-point number in hexadecimal format. When conversion character A is used, the output is displayed in uppercase letters.

Conversion character Description

e or E Display a floating-point value in exponential notation. When conversion character E is used, the output is displayed in uppercase letters.

F Display a floating-point value in decimal format.

g or G Display a floating-point value in either the floating-point format f or the exponential format e based on the magnitude of the value. If the magnitude is less than 10–3, or greater than or equal to 107, the floating-point value is printed with e (or E). Otherwise, the value is printed in format f. When conversion character G is used, the output is displayed in uppercase letters.

a or A Display a floating-point number in hexadecimal format. When conversion character A is used, the output is displayed in uppercase letters.

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Printing Strings and Characters

• Conversion character c and C– Require char

– C displays the output in uppercase letters

• Conversion character s and S– String

– Object• Implicitly use object’s toString method

– S displays the output in uppercase letters

• View program demonstrating character conversion with printf, Fig. 28.5

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Printing with Field Widths and Precisions

• Field width– Size of field in which data is printed

– If width larger than data, default right justified• If field width too small, increases to fit data

• Minus sign uses one character position in field

– Integer width inserted between % and conversion specifier

• E.g., %4d – field width of 4

– Can be used with all format specifiers except the line separator (%n)

• View program demonstrating field width, Fig. 28.12

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Printing with Field Widths and Precisions

• Precision– Meaning varies depending on data type– Floating point

• Number of digits to appear after decimal (e or E and f)• Maximum number of significant digits (g or G)

– Strings• Maximum number of characters to be written from

string

– Format• Use a dot (.) then precision number after %

e.g., %.3f

• View program demonstrating precision, Fig. 28.13

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Printing with Field Widths and Precisions

• Field width and precision– Can both be specified

• %width.precision

%5.3f

– Negative field width – left justified

– Positive field width – right justified

– Precision must be positive• Example:

printf( "%9.3f", 123.456789 );

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2.5 Another Java Application: Adding Integers

• Program Fig. 2.7 to do keyboard input– Use Scanner to read two integers from user

– Use printf to display sum of the two values

– Use packages

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2.5 Another Java Application: Adding Integers (Cont.)

– import declarations • Used by compiler to identify and locate classes used in

Java programs

• Tells compiler to load class Scanner from java.util package

– Begins public class Addition• Recall that file name must be Addition.java

– Lines 8-9: begins main

3 import java.util.Scanner; // program uses class Scanner

5 public class Addition 6 {

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2.5 Another Java Application: Adding Integers (Cont.)

– Variable Declaration Statement– Variables

• Location in memory that stores a value– Declare with name and type before use

• Input is of type Scanner – Enables a program to read data for use

• Variable name: any valid identifier

– Declarations end with semicolons ;– Initialize variable in its declaration

• Equal sign• Standard input object

– System.in

10 // create Scanner to obtain input from command window11 Scanner input = new Scanner( System.in );

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2.5 Another Java Application: Adding Integers (Cont.)

– Declare variable number1, number2 and sum of type int• int holds integer values (whole numbers): i.e., 0, -4, 97

• Types float and double can hold decimal numbers

• Type char can hold a single character: i.e., x, $, \n, 7

• int, float, double and char are primitive types

– Can add comments to describe purpose of variables

– Can declare multiple variables of the same type in one declaration

– Use comma-separated list

13 int number1; // first number to add14 int number2; // second number to add15 int sum; // second number to add

int number1, // first number to add number2, // second number to add sum; // second number to add

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2.5 Another Java Application: Adding Integers (Cont.)

– Message called a prompt - directs user to perform an action

– Package java.lang

– Result of call to nextInt given to number1 using assignment operator =

• Assignment statement• = binary operator - takes two operands

– Expression on right evaluated and assigned to variable on left

• Read as: number1 gets the value of input.nextInt()

17 System.out.print( "Enter first integer: " ); // prompt

18 number1 = input.nextInt(); // read first number from user

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2.5 Another Java Application: Adding Integers (Cont.)

– Similar to previous statement• Prompts the user to input the second integer

– Similar to previous statement• Assign variable number2 to second integer input

– Assignment statement• Calculates sum of number1 and number2 (right hand side)• Uses assignment operator = to assign result to variable sum• Read as: sum gets the value of number1 + number2• number1 and number2 are operands

20 System.out.print( "Enter second integer: " ); // prompt

21 number2 = input.nextInt(); // read second number from user

23 sum = number1 + number2; // add numbers

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2.5 Another Java Application: Adding Integers (Cont.)

– Use System.out.printf to display results

– Format specifier %d• Placeholder for an int value

– Calculations can also be performed inside printf

– Parentheses around the expression number1 + number2 are not required

25 System.out.printf( "Sum is %d\n: " , sum ); // display sum

System.out.printf( "Sum is %d\n: " , ( number1 + number2 ) );

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2.6 Memory Concepts

• Variables

– Every variable has a name, a type, a size and a value

• Name corresponds to location in memory

– When new value is placed into a variable, replaces (and destroys) previous value

– Reading variables from memory does not change them

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2.7 Arithmetic

• Arithmetic calculations used in most programs

– Usage • * for multiplication

• / for division

• % for remainder

• +, -

– Integer division truncates remainder7 / 5 evaluates to 1

– Remainder operator % returns the remainder 7 % 5 evaluates to 2

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Fig. 2.11 | Arithmetic operators.

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2.7 Arithmetic (Cont.)

• Operator precedence – Some arithmetic operators act before others

(i.e., multiplication before addition)• Use parenthesis when needed

– Example: Find the average of three variables a, b and c

• Do not use: a + b + c / 3

• Use: ( a + b + c ) / 3

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Fig. 2.12 | Precedence of arithmetic operators.

Operator(s) Operation(s) Order of evaluation (precedence)

*

/

%

Multiplication

Division

Remainder

Evaluated first. If there are several operators of this type, they are evaluated from left to right.

+

-

Addition

Subtraction

Evaluated next. If there are several operators of this type, they are evaluated from left to right.

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Operator Precedence Chart

Hierarchy of Java operators with the highest precedence shown first.• Expressions inside parentheses are evaluated first; nested parentheses are evaluated from the innermost parentheses to the outer. • Operators in the same row in the chart have equal precedence.

Operator Type Order of Evaluation

(   )   [   ]    .

ParenthesesArray subscriptMember access

left to right

++   -- Prefix increment, decrement

right to left

++   -- - Postfix increment, decrementUnary minus

right to left

*   /   % Multiplicative left to right

+   - Additive left to right

<   >   <=   >= Relational left to right

==   != Equality left to right

&& And left to right

|| Or left to right

?   : Conditional right to left

=   +=   -=   *=   /=   %=

Assignment right to left