Building Java Programs Chapter 2

Building Java ProgramsChapter 2 Variables and string concatenation

Variables

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: <type> <name>; // The name is an identifier. int x; double myGPA; x myGPA

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: <name> = <expression>; • intmyFirstVariable; • myFirstVariable = 307; • myFirstVariable = -3 - 4; myFirstVariable 307 myFirstVariable -7 myFirstVariable

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; // ??? 3 X 5 X

Expressions with variables • Variables can stand in for values in expressions. • double cookiesPerMinute = 3.5; • intcookiesInJar = 12; • double totalMinutes = cookiesInJar / cookiesPerMinute; • int minutes = (int) totalMinutes; • int seconds = ((int) (totalMinutes * 60)) % 60; 3.429 totalMinutes 3 minutes 25 seconds

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--;

Expanded Precedence Table This table is on page 86 of your book

Programming practice • Write a program that will calculate the percent you earned on your Monday quiz. • Declare variables for • percent, points earned, points possible

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 " + 3 + " years straight!"; • System.out.println(myMessage); Hello and welcome to my party! It's been going on for 3 years straight!

example • "2 + 2" + 3 + 4 • "2 + 23" + 4 • "2 + 234"

example • 3 + 4 + " 2 + 2" • 7 + " 2 + 2" • "7 2 + 2"

example • 2 + " times " + 3 + " = " + 2 * 3 • 2 + " times " + 3 + " = " + 6 • "2 times " + 3 + " = " + 6 • "2 times 3" + " = " + 6 • "2 times 3 = " + 6 • "2 times 3 = 6"

In your notebook… • 2 + " plus " + 3 + " = " + 2 + 3 • "2 plus" + 3 + " = " + 2 + 3 • … • "2 plus 3 = 23" • How do you fix it? • 2 + " plus " + 3 + " = " + (2 + 3) • … • "2 plus 3 = 5"

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!

Building Java Programs Chapter 2