BUILDING JAVA
PROGRAMS
CHAPTER 2
VARIABLES AND STRING CONCATENATION
VARIABLES
2
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;
x
double myGPA;
myGPA
3
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>;
int myFirstVariable;
myFirstVariable = 307;
307
-7
myFirstVariable
myFirstVariable = -3 - 4;
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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
5
X
5
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;
3.429
3
25
totalMinutes
minutes
seconds
6
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;
or
cookiesInJar--;
cookiesInJar = cookiesInJar – 1;
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EXPANDED PRECEDENCE TABLE
Description
Operators
unary operators
+, -, ++, --
multiplicative operators
*, /, %
additive operators
+, -
assignment operators
=, +=, -=, *=, /=, %=
This table is on page 86 of your book
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PROGRAMMING PRACTICE
Write a program that will calculate the percent you earned on your Monday
quiz.
Declare variables for
percent, points earned, points possible
9
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!
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EXAMPLE
"2 + 2" + 3 + 4
"2 + 23" + 4
"2 + 234"
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EXAMPLE
3 + 4 + " 2 + 2"
7
+ " 2 + 2"
"7 2 + 2"
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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"
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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"
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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!
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