Objectives
- Discuss what variables are
- Determine validity of variable names in Java
- Discuss and enumerate the primitive data types in Java
- Create variables and store data in variables
- Cast variables to different data types
- Declare constants
Variables
- memory locations where values are stored
- naming rules
- must start with letter, '_', or '$'
- can only contain letters, digits, '_', and '$'
- case-sensitive
- may not be one of Java's reserved words
- declaring (e.g. "int num;")
- initializing (e.g. "int num = 7;")
Data types
- char: single characters (e.g. 'a', '%', '*', '2')
- byte: -128 to 127
- short: -32768 to 32767
- int: -2147483648 to 2147483647
- long: -922337203684547758808 to 922337203684547758807
- float: numbers including fractional parts
- double: larger, higher precision float
- boolean: true or false
You can make a variable or value temporarily look like a different data type by "casting" it. You "cast" a variable by putting another data type name within parentheses in front of the value or variable. For example, the following code examples will display 5 instead of 5.9:
double x = 5.9; System.out.println((int) x); System.out.println((int) 5.9);
Notice that casting a floating point number to an integer truncates instead of rounding.
Constants
- names of constants are usually uppercase by convention (but not required by compiler)
- values of constants may not be changed after they are initialized
- follow normal identifier naming rules
- the final keyword is what makes it a constant
- final double TAX_RATE = 7.5;
- TestFinal.java: constants