Variables
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What are variables
Think of variables as a box you can put values into
Variables are place holders for data. Data comes in the form on whole numbers, decimals, text, and objects.
Data types in Java
String = Text data which is enclosed by double quotes " "
"Hello"
"Goodbye1"
"12345"
int = Integers are numbers wiithout decimals
123
34
55
double = Doubles are numbers with decimals
50.99
33.4433
123.2
How to declare a variable format
To declare a variable, follow this format.
(data type) (variable name) = (value - optional)
Setting the value of a variable is optional
Example
String name = "James"; //creates a string
int num1 = 9; //creates an integer
int num2; //creates an integer variable with no value
num2=23; // sets value to 23
double num3= 9.78; //creates a double
Student joeObj = new Student(“Joe Schmo” , 16, 23432); //creates a new instance of a student class
Concat variables - Appending Variables
You can concat a variable to String by using the plus + operator. If you concat a number type (double or int) to a String, it makes the data a String.
String fname = "Joe"; // name is a String
String lname = "Schmo"; // name is a String
int id=44523; //id is an integer
String fullIdName = fname + "--" id + "--" + lname;
System.out.println(fullIdName);
output
Joe--44532--Schmo
System.out.println("The id value is " + id);
output
The id value is 44532
Variable Expressions - Making Calculations
You normally use numeric variables to make calculations.
The + Operator
Use the + operator to add two ints together
int a;
int b;
a = 5 + 6; // a contains 11
b = a + 3; // b contains 14
a + b; // not allowed, does nothing
a = a + 1; // a contains 12, and b?
double a;
double b;
a = 3.14159; // a contains 3.14159
b = a + 2.1; // b contains 5.24159
a += 1.6; // shortcut for a = a + 1.6;
a++; // shortcut for a = a + 1.0;
The - Operator
Exactly like + except performs subtraction
int a;
int b;
a = 5 - 6; // a contains -1
b = 3 - a; // b contains 4
a -= 10; // shortcut for a = a – 10;
a--; // shortcut for a = a – 1;
double a;
double b;
a = 3.14159;// a contains 3.14159
b = a - 2.1; // b contains 1.04159
The * Operator
The * operator performs multiplication
int a;
int b;
a = 5 * 6; // a contains 30
b = a * 3; // b contains 90
a *= 2; // shortcut for a = a * 2;
double a;
double b;
a = 3.14159;// a contains 3.14159
a = b * 0.5; // a contains 0.520795
The / Operator
The / operator performs integer division
Not the same as regular division
int a;
int b;
a = 3; // a contains 3
b = a / 2; // b contains 1
a /= 2; // shortcut for a = a / 2;
The % Operator for int
The % operator is the mod operator
It finds the remainder after division
int a;
int b;
a = 8; // a contains 8
b = a % 5; // b contains 3
a %= 2; // shortcut for a = a % 2;
Shortcuts
Some expressions are used so often, Java gives us a short cut
x = x + y; can be written x += y;
x = x + 1; can be written x++;
int x;
x = 6; // x contains 6
x += 4; // x contains 10
x++; // x contains 11
Casting
You cannot directly store a double value into an int variable
int a = 2.6; // fails!
However, you can cast the double value to convert it into an int
int a = (int)2.6; // succeeds! (a = 2)
Casting tells the compiler you want the loss of precision to happen
You can always store an int into a double
Rounding
In Java, the conversion of a double into an int does not use rounding
As in the case of integer division, the value is always rounded down
Data Types
Credit: A Slides are modified with permission from Barry Wittman at Elizabethtown College
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