1 / 73

Chapter 3 Java Basics

Chapter 3 Java Basics. Introduction Primitive Types Constants Input Conversion between Types Ethics in Computing. Introduction: Skeleton Programs. Skeleton Program for turtle graphics program // Comment that describes the program import turtlegraphics.*; public class className {

irish
Download Presentation

Chapter 3 Java Basics

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Chapter 3Java Basics Introduction Primitive Types Constants Input Conversion between Types Ethics in Computing

  2. Introduction: Skeleton Programs • Skeleton Program for turtle graphics program • // Comment that describes the program • import turtlegraphics.*; • public class className • { • public static void main(String[] args) • throws TurtleException • { • // Put your Java statements here • } • } Programming and Problem Solving With Java

  3. Introduction: Skeleton Programs • Skeleton Program for non-turtle graphics program • // Comment that describes the program • public class className • { • public static void main(String[] args) • { • // Put your Java statements here • } • } Programming and Problem Solving With Java

  4. Introduction: Keywords • Keyword is reserved -- can’t use for identifier names • Keywords in Java Programming and Problem Solving With Java

  5. Introduction: Identifier Names • Programmer must make up names for new classes, methods, variables • Rules for forming identifier names • Must start with letter, underscore (_), or dollar sign ($) • Other characters must be letters, digits, underscores, or dollar signs • No spaces! • No keywords Programming and Problem Solving With Java

  6. Introduction: Identifier Names • Identifiers should also be meaningful to human readers • Part of good programming style Programming and Problem Solving With Java

  7. Introduction: Identifier Names • Many identifiers use more than one word • Examples: SmartTurtle, turnRight • Java conventions • After the first word, begin each word with a capital letter • Class names start with capital letter (SmartTurtle) • Other names start with lower-case letter (turnRight) Programming and Problem Solving With Java

  8. Introduction: The main() Method • The main() method is the one that starts with • public static void main(String[] args) … • Write executable instructions between braces • Executable instruction: makes computer do something • Examples of executable instructions • System.out.println("Hello!"); • myTurtle.turnRight(90); • Examples of non-executable instructions • public static void main(String[] args) • import turtlegraphics.*; • Computer starts executing the first statement in main() Programming and Problem Solving With Java

  9. Introduction: Flow of Control • Write executable statements like a list • Write first instruction you want the computer to do • Then write second, and so on • Sequential execution • Computer executes each instruction in turn, in order they appear in program • Computer stops after executing last instruction • "Control" • When computer executing instruction, control is at that instruction Programming and Problem Solving With Java

  10. Introduction: Semicolons • Semicolon required after each executable instruction • myTurtle.move(100); • myTurtle.turnRight(90); • Free-form input • Compiler ignores indentation, ends of lines (as long as words & other tokens not split) • Example of valid program • // (This program has poor formatting) • import turtlegraphics.*; public class DrawSimpleDesign { public static void main(String[] arguments) throws TurtleException { Turtle myTurtle = • new Turtle(); myTurtle.move(400); myTurtle.turnRight(90); myTurtle.move(200); } } Programming and Problem Solving With Java

  11. Introduction: Letter Case • Java is case-sensitive • Compiler treats upper and lower case letters differently • A different from a • B different from b • public static void different from PUBLIC STATIC VOID • Some languages (Pascal) are case-insensitive public static void main(String args[]) Programming and Problem Solving With Java

  12. Introduction: Comments • Comment starts with // and continues to end of line • // This program draws a square • myTurtle.move(100); // Position turtle for next figure • Compiler ignores comments • Programmer should include comments • Describe what the program does • Describe (in higher-level terms than the code) how the program works • Usually unnecessary to comment each line -- makes program too wordy • myTurtle.move(100); // Move 100 units • myTurtle.turnRight(90); // Turn 90 degrees Programming and Problem Solving With Java

  13. Introduction: Streams • Stream • A sequence of characters • Has a reader (consumer of information)at one end • Has a writer (producer of information) at the other • Program's input and output are streams • Output stream is the textual output of the program • Input stream is the textual input of the program Stream Writer Reader Programming and Problem Solving With Java

  14. Introduction: System.out Stream • System.out is the standard Java output stream • System is the name of a standard Java class • out is the output stream object in the System class • Refer to this output stream as System.out • Allows displaying text output on the console • System.out.println("Hello!"); • println() is method of out stream • Syntax for method use • object.method(arguments); • Action of println() • Display message on consoleat cursor's position Hello! Programming and Problem Solving With Java

  15. Introduction: println() vs. print() • System.out.println() • Displays message, then moves cursor to beginning of next line • System.out.println("First message"); • System.out.println("Second message"); • First message • Second message • _ • System.out.print() • Just displays message (leaves cursor after) • System.out.print("First message"); • System.out.print("Second message"); • First messageSecond message_ Cursor Cursor Programming and Problem Solving With Java

  16. Introduction: Use of print() • Use System.out.print() to display several values on same line • // Displays the message, because there's a println() • // after the print(). • public class DisplayMessage • { • public static void main(String[] args) • { • System.out.print("This"); • System.out.print(" will"); • System.out.println(" display"); • } • } Programming and Problem Solving With Java

  17. Introduction: Use of flush() • Message from System.out.print doesn't display right away -- stored in buffer • Use System.out.flush() to force display of output from System.out.print() System.out.print("Hello!"); Hello! Hello System.out.flush(); Buffer Programming and Problem Solving With Java

  18. Here is a small test_ Here is _ Introduction: The Output Buffer • Output goes to the output buffer before the screen System.out.println("Here is"); System.out.print("a small"); System.out.print(" test"); System.out.flush(); Here is a small a small test Buffer Programming and Problem Solving With Java

  19. Displaying String Literals • Display string literals between quotes • System.out.println("This is a string literal"); • Three ways display a long string literal • Let the literal go past the edge of the editor screen • System.out.println("This is a very very very very very ver • Break the string into two strings, use print() on first, println() on second • System.out.print("This is a very very very very very "); • System.out.println("very very very long message"); • Use concatenation • System.out.println("This is a very very very very very " • + "very very very long message"); Programming and Problem Solving With Java

  20. Introduction: Escape Sequences • Can't display double quote " directly • This statement doesn't compile • System.out.println("She said, "Hi!""); • Compiler can't find end of the string • Use escape character \ before " in string • System.out.println("She said, \"Hi!\""); • Other escape sequences • \b Backspace • \\ Backslash • \a Bell • \n End of line • \t Tab Programming and Problem Solving With Java

  21. Primitive Types • Type is a kind of information • Must define the type of information in a program • Three common types of information • Textual • Numeric • Multimedia • Two kinds of numeric types • Integer: whole numbers (4, 99, -123) • Floating point (4.35, -33.4, 3.0) 2 43 18.6 Programming and Problem Solving With Java

  22. Primitive Type: Integers • Display integer • System.out.println(123); • Display result of integer arithmetic • System.out.println(123 + 456); • Display a message with an integer • System.out.println("The answer is " + 123); • Display a message with integer arithmetic (wrong) • System.out.println("The sum is " + 123 + 456); • Compiler treats + as concatenation! • Display a message with integer arithmetic (correct) • System.out.println("The sum is " + (123 + 456)); 123579The answer is 123The sum is 123456The sum is 579 Programming and Problem Solving With Java

  23. Primitive Types: Integer Operators Programming and Problem Solving With Java

  24. Primitive Types: Integer Operators • Operator precedence: order of execution of operators • Example • System.out.println(30 + 10 / 2); • Possible interpretations correct! Programming and Problem Solving With Java

  25. Primitive Types: Integer Operators • Evaluation of some sample expressions Programming and Problem Solving With Java

  26. Primitive Types: Integer Types Programming and Problem Solving With Java

  27. Primitive Types: Floating Point • Floating-point number has • Decimal point, or • Exponent, or both • Examples • 5.0, 12.34, 0.0, -45.8, 12. • Scientific notation • 5.6 x 1027 • = 5,600,000,000,000,000,000,000,000,000.0 • In Java • 5.6E27 Programming and Problem Solving With Java

  28. Primitive Types: Floating Point • Display floating point number • System.out.println(18.7856); • Display a message, too • System.out.println("F.P. # is " + 18.7856); • Display a large floating point number • System.out.println("F.P. # is " + 123456789.0); • Large number display rule • If more than 6 digits display in scientific notationElse display in conventional notation 18.7856F.P. # is 18.7856F.P. # is 1.23457e008 Programming and Problem Solving With Java

  29. Primitive Types: Floating Point Highlightedrowsrounded Programming and Problem Solving With Java

  30. Primitive Types: Floating Point • Floating Point Operators Programming and Problem Solving With Java

  31. Primitive Types: Floating Point • Floating point precedence Programming and Problem Solving With Java

  32. Primitive Types: Floating Point • Floating point types • Float point ranges Programming and Problem Solving With Java

  33. Use integers for counting Use floating-point numbers for measuring Primitive Types: Integer vs floating Programming and Problem Solving With Java

  34. Using Strings • String is a sequence of characters • Literal value: "This is a string" • Java strings • Not primitive (built-in) type • Standard class • String operations • Many operations: length, substring, search, etc. • Example • // Display the length of a string literal • public class FindLength • { • public static void main(String[] args) • { • System.out.println("This is a string literal".length()); • } • } Programming and Problem Solving With Java

  35. Variables • Variable: named location in memory • Can hold one value Programming and Problem Solving With Java

  36. Variables • Each variable like a calculator memory • Holds one value • Can retrieve value many times • Storing a new value erases old • Differences from calculator memory • Can have many variables • Variable can be one of many types • Each variable has a name Programming and Problem Solving With Java

  37. Variables Variable, schmariable • Kinds of variables • Local • Instance • Class (static) • Variable definitions • int count; • int sum, limit; • Example • public class IllustrateVariables • { • String anInstanceVariable; • static int aStaticVariable; • public static void main(String[] args) • { • int aLocalVariable; • } • } Programming and Problem Solving With Java

  38. Variables: Parameters • Parameters are like local variables • Difference: initial value of parameter passed in • class SmartTurtle • { • // drawSquare: Draws a square of the given size • public void drawSquare(int size) • { • for (int i = 1; i <= 4; i++) • { • this.move(size); • this.turnRight(90); • } • } • … • } • Counting variable of for statement is local variable • Scope restricted to for statement Parameter size Local variable i Programming and Problem Solving With Java

  39. Variables: Assignment • Assignment operator = • Stores value in a variable • Read as "becomes", not "equals" • Examples • int count; • count = 25; • count = sum; • count = sum + 15; • count = count + 1; • Syntax • Variable = Expression = Variable Expression Programming and Problem Solving With Java

  40. Variables: Initialization • Initialization symbol = • Optional • Gives variable its first value • Examples • int count = 0; • double weight = 10.2; • String firstName = "John", lastName = "Smith"; • Only one variable initialized per value • int first, second, third = 25; • Uninitialized variables don't have a value • int count; • System.out.println(count); // Wrong • Compiler output Test.java:7: Variable count may not have been initialized. System.out.println(count); // Wrong Programming and Problem Solving With Java

  41. Variables: Assign vs Initialize • Assignment & initialization use same symbol • Different operations • // Demonstrates assignment and initialization • public class StringDemonstration • { • public static void main(String[] args) • { • String firstName = "Linda"; // Initialize firstName • String lastName; // No initial value • String name; // No initial value • lastName = "Smith"; // Assign to lastName • name = firstName; // Assign to name • name = name + " " + lastName; // Assign to name again • System.out.println("Name is " + name); • } • } Name is Linda Smith Programming and Problem Solving With Java

  42. Variables: Assign & Initialize • Assignment and initialization are operators • Not statements or commands • Part of expression • Very low precedence • = inside expressions • x = y = 0; • Same as • x = (y = 0); • Both x and y get 0 • Associativity • Two of same operators in expression • Tells which the computer executes first Programming and Problem Solving With Java

  43. Variables: Increment & Decrement • Can use assignment to increment • count = count + 1; • Or use increment operator • count++; // Postfix version • ++count; // Prefix version • Difference between post- and pre- • Postfix: increment after evaluating expression • int x = 0, y = 1; • x = y++; // y is 2, x is 1 • Prefix: increment before evaluating expression • int x = 0, y = 1; • x = ++y; // y is 2, x is 2 • Also post- and prefix decrement operators -- • count--; • --count; ++ Programming and Problem Solving With Java

  44. Variables: Displaying Values • // Displays the average of four floating • // point numbers • public class DisplayAverage • { • public static void main(String[] args) • { • double firstNum = 10.0; • double secondNum = 12.3; • double thirdNum = 15.4; • double fourthNum = 18.9; • double average; • average = (firstNum + secondNum + thirdNum + fourthNum) • / 4; • System.out.println("The average is " + average); • } • } Programming and Problem Solving With Java

  45. Constants • Constant is like a variable • Has name • Has value • Constant is unlike a variable • Value can't change • Defining • Must define as a class (static) variable • Defined in the class, outside of any method • static final double TARGET_SALES = 350000.0; • Makes program more readable • System.out.println("Widget have sold " • + (sales / TARGET_SALES * 100) • + " percent of target sales"); Programming and Problem Solving With Java

  46. Constants: Uses • Give meaning to meaningless literal value • static final double TARGET_SALES = 350000.0; • Makes program easier to read • Convention: ALL_CAPITAL_LETTERS for constants • Values that occur several times in a program • Names of companies, departments, etc. • static final String BANK_NAME = "First National Bank"; • static final String BRANCH_NAME = "Springfield Branch"; • Makes it easier to update the program • How about constants for 0 and 1? • static final int ONE = 1; • … • count = count + ONE; • No more readable than using literal value 1 0 1 Programming and Problem Solving With Java

  47. Constants: Numeric Limits • Predefined constants for largest and smallest numbers • System.out.println("Range of int: " + Integer.MIN_VALUE • + " to " + Integer.MAX_VALUE); • System.out.println("Range of long: " + Long.MIN_VALUE • + " to " + Long.MAX_VALUE); • System.out.println("Range of float: " + Float.MIN_VALUE • + " to " + Float.MAX_VALUE); • System.out.println("Range of double: " + Double.MIN_VALUE • + " to " + Double.MAX_VALUE); • Output • Range of int: -2147483648 to 2147483647 • Range of long: -9223372036854775808 to 9223372036854775807 • Range of float: 1.4013e-045 to 3.40282e+038 • Range of double: 2.22507e-308 to 1.79769e+308 Programming and Problem Solving With Java

  48. Input • Many programs require input from user • Input devices • Keyboard • Mouse • Stylus • Scanner • Keyboard input is complex in Java • Will use Keyboard class for now • Will learn other techniques later Programming and Problem Solving With Java

  49. Input: Keyboard Class • import Keyboard; • public class DemonstrateKeyboardInput • { • public static void main(String[] args) • throws java.io.IOException • { • int height, width; • System.out.print("Enter height of rectangle: "); • System.out.flush(); • height = Keyboard.readInt(); • System.out.print("Enter width of rectangle: "); • System.out.flush(); • width = Keyboard.readInt(); • System.out.println("The area of the rectangle is " • + (width * height)); • } • } Enter height of rectangle: 4 Enter height of rectangle: 4 Enter width of rectangle: Enter height of rectangle: 4 Enter width of rectangle: 3 The area of the rectangle is 12 Enter height of rectangle: Enter height of rectangle: 4 Enter width of rectangle: 3 Note use of exception Programming and Problem Solving With Java

  50. Input: Keyboard Class • Methods in Keyboard class • readInt() • readByte() • readShort() • readLong() • readFloat() • readDouble() • readString() • When control reaches Keyboard method • Computer waits for user to enter value • Method returns value user typed Programming and Problem Solving With Java

More Related