Introduction to Java Applications

1. 2 • Introductionto Java Applications

2. 2.1 Introduction • Java application programming • Display messages • Obtain information from the user • Arithmetic calculations • Decision-making fundamentals

3. 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

4. 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

5. 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

6. 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

7. 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)

8. 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)

9. 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 ;

10. 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

11. 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

12. 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

13. Fig. 2.5 | Some common escape sequences.

14. 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

15.  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

16.   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

17.  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

18. Integer conversion characters. • View demonstration program Fig. 28.3 • Note output of positive, negative numbers • Note octal (base 8) and hexadecimal (base 16) options

19.   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

20. Floating-point conversion characters. • View program using printf for floating point numbers, Figure 28.4

21. 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

22. 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

23. 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

24. 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 );

25. 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

26. 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

27. 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

28. 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

29. 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()

30. 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

31. 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

32. 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

33. 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

34. Fig. 2.11 | Arithmetic operators.

35. 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

36. Fig. 2.12 | Precedence of arithmetic operators.

37. 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.