BUILDING JAVA PROGRAMS CHAPTER 2 VARIABLES AND STRING CONCATENATION.

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Presentation transcript:

BUILDING JAVA PROGRAMS CHAPTER 2 VARIABLES AND STRING CONCATENATION

22 VARIABLES

33 VARIABLE DECLARATION Declaring a variable tells Java to set aside a place to store a value. Variables must be declared before they can be used. Syntax: ;// The name is an identifier. int x; double myGPA; x myGPA

44 VARIABLE ASSIGNMENT Assigning a value to a variable updates its contents, dumping out whatever used to be there. The value can be an expression: the variable stores the result. Syntax: = ; int myFirstVariable; myFirstVariable = 307; myFirstVariable = ; myFirstVariable 307 myFirstVariable -7 myFirstVariable

55 VARIABLE ASSIGNMENT Assignment uses = It is not an algebraic equation. = means: "store the value at right in variable at left“ The right side expression is evaluated first, and then its result is stored in the variable at left. What happens here? int x = 3; x = x + 2; // ??? 3X3X 5X5X

66 EXPRESSIONS WITH VARIABLES Variables can stand in for values in expressions. double cookiesPerMinute = 3.5; int cookiesInJar = 12; double totalMinutes = cookiesInJar / cookiesPerMinute; int minutes = (int) totalMinutes; int seconds = ((int) (totalMinutes * 60)) % 60; totalMinutes 3 minutes 25 seconds

77 SHORTHAND ARITHMETIC OPERATORS Some operations are so commonly done that shorthand operators are included in Java. total = total + 33;=>total += 33; price = price * taxRate;=>price *= taxRate; numAwards = numAwards + 1;=> ++numAwards; or numAwards++; cookiesInJar = cookiesInJar – 1;=> --cookiesInJar; or cookiesInJar--;

88 EXPANDED PRECEDENCE TABLE DescriptionOperators unary operators +, -, ++, -- multiplicative operators *, /, % additive operators +, - assignment operators=, +=, -=, *=, /=, %= This table is on page 86 of your book

99 PROGRAMMING PRACTICE Write a program that will calculate the percent you earned on your Monday quiz. Declare variables for percent, points earned, points possible

10 STRING CONCATENATION Like “adding” strings together, it uses the + or += operators. String myMessage = "Hello" + " and welcome"; myMessage += " to my party!"; myMessage += "\nIt's been going on for " " years straight!"; System.out.println(myMessage); Hello and welcome to my party! It's been going on for 3 years straight!

11 EXAMPLE "2 + 2" "2 + 23" + 4 " "

12 EXAMPLE " 2 + 2" 7 + " 2 + 2" " "

13 EXAMPLE 2 + " times " " = " + 2 * " times " " = " + 6 "2 times " " = " + 6 "2 times 3" + " = " + 6 "2 times 3 = " + 6 "2 times 3 = 6"

14 IN YOUR NOTEBOOK… 2 + " plus " " = " "2 plus " " = " … "2 plus 3 = 23" How do you fix it? 2 + " plus " " = " + (2 + 3) … "2 plus 3 = 5"

15 PROGRAMMING PRACTICE 2 Create a Java class called Area that has the following methods: Square Rectangle Triangle Circle Sphere Each method should declare 1-2 variables (one for base, height, length, etc.), initialize the variables to 1, and print the area or surface area of the shape. Have the main method call each shape method to test that your program works!