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2. Introduction to Java Applications. 2.1 Introduction. Java application programming Display messages Obtain information from the user Arithmetic calculations Decision-making fundamentals. 2.2 First Program in Java: Printing a Line of Text. Application

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slide1
2
  • Introductionto Java Applications
2 1 introduction
2.1 Introduction
  • Java application programming
    • Display messages
    • Obtain information from the user
    • Arithmetic calculations
    • Decision-making fundamentals
2 2 first program in java printing a line of text
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
2 2 first program in java printing a line of text cont

1 // Fig. 2.1: Welcome1.java

2 // Text-printing program.

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
2 2 first program in java printing a line of text cont5

3

4 public class Welcome1

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
2 2 first program in java printing a line of text cont6

4 public class Welcome1

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
2 2 first program in java printing a line of text cont7

4 public class Welcome1

5 {

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)
2 2 first program in java printing a line of text cont8

7 public static void main( String args[] )

8 {

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)
2 2 first program in java printing a line of text cont9

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

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 ;
2 2 first program in java printing a line of text cont10

11 } // end method main

13 } // end class Welcome1

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
2 3 modifying our first java program

9 System.out.print( "Welcome to " );

10 System.out.println( "Java Programming!" );

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
2 3 modifying our first java program cont

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

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
2 4 displaying text with printf

9 System.out.printf( "%s\n%s\n",

10 "Welcome to", "Java Programming!" );

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
formatting output with printf
 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
formatting output with printf cont
  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
printing integers
 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
integer conversion characters
Integer conversion characters.
  • View demonstration program Fig. 28.3
    • Note output of positive, negative numbers
    • Note octal (base 8) and hexadecimal (base 16) options
printing floating point numbers
  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
floating point conversion characters
Floating-point conversion characters.
  • View program using printf for floating point numbers, Figure 28.4
printing strings and characters
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
printing with field widths and precisions
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
printing with field widths and precisions23
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
printing with field widths and precisions24
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 );

2 5 another java application adding integers
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
2 5 another java application adding integers cont

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

5 publicclass Addition

6 {

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
2 5 another java application adding integers cont27

10 // create Scanner to obtain input from command window

11Scanner input = new Scanner( System.in );

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
2 5 another java application adding integers cont28

13 int number1; // first number to add

14 int number2; // second number to add

15 int sum; // second number to add

int number1, // first number to add

number2, // second number to add

sum; // second number to add

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
2 5 another java application adding integers cont29

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

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

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()
2 5 another java application adding integers cont30

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

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

23 sum = number1 + number2;// add numbers

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 ofnumber1 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 number2are operands
2 5 another java application adding integers cont31

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

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

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
2 6 memory concepts
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
2 7 arithmetic
2.7 Arithmetic
  • Arithmetic calculations used in most programs
    • Usage
      • * for multiplication
      • / for division
      • % for remainder
      • +, -
    • Integer division truncates remainder

7 / 5 evaluates to 1

    • Remainder operator % returns the remainder

7 % 5 evaluates to 2

2 7 arithmetic cont
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 variablesa,b and c
      • Do not use: a + b + c / 3
      • Use: ( a + b + c ) / 3
slide37

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.