Java Primitive Data Types and Variables, Study notes of Java Programming

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Primitive Data Types

15-110 Summer 2010

Margaret Reid-Miller

Data Types

• Data stored in memory is a string of bits (0 or 1).

• What does 1000010 mean?

'B'?

9.2E-44?

• How the computer interprets the string of bits

depends on the context.

• In Java, we must make the context explicit by

specifying the type of the data.

Common Primitive Types

Type Description Example of Literals int integers (whole numbers) 42 , 60634 , -8, 0 double real numbers 0.039, -10.2, 4.2E+ char single characters 'a', 'B', '&', '6' boolean logical values true, false

Numbers

Type Storage Range of Values byte 8 bits -128 to 127 short 16 bits -32,768 to 32, int 32 bits -2,147,483,648 to 2,147,483, long 64 bits -9x 18 to 9x 18 float 32 bits ± 10

• to ± 10 38 , 7 significant digits double 64 bits ± 10

to ± 10 308 , 15 significant digits

Variable Declaration

• Before you can use a variable, you must

declare its type and name.

• You can declare a variable only once in a

method.

• Examples:

int numDimes; double length; char courseSection; boolean done; String lastName;

Declaring Variables

• Declaring a variable instructs the compiler to set aside a portion of memory large enough to hold data of that type. int count; double length; count length
• No value has be put in memory yet. That is, the variable is undefined.

Re-Assigning Variables

• A variable must be declared exactly once.
• A variable can be assigned and re-assigned values many times after it is declared. Example: int x; x = 4; System.out.println(x); // prints 4 x = x + 1; System.out.println(x); // prints 5

Declaration/Initialization

• Variables can be declared and initialized in

one statement:

Examples:

int numDimes = 4; double length = 52.3; char courseSection = 'J'; boolean done = true; String lastName = "Reid-Miller"; int count = 3 + 2;

Division and Modulo

int a = 40; double x = 40.0; int b = 6; double y = 6.0; int c; double z; c = a / b; 6 c = a % b; 4 z = x / y; 6.66666667 c = b % a; 6 c = b / a; 0 c = 0 % a; 0 z = y / x; 0.15 c = b % 0; error c = 0 / a; 6 c = a / 0; error

• The operators *, /, % are evaluated before the operators +, - because *, /, % have higher precedence than +, -. Example: 2 + 4 * 5
• To change the order use parentheses: Example: (2 + 4) * 5 evaluates to ______

Operator Precedence

Other operators

• Assignment operators : =, +=, -=, *=, /=, %= Example:
  • Shortcut for x = x + 2; is x += 2; (“add 2 to x”)
  • Shortcut for y = y * 3; is y *= 3; (“multiply y by 3”)
• Increment / Decrement operators: ++, --
  • Shortcut for x = x + 1; is x++; (“increment x”)
  • Shortcut for y = y - 1; is y--; (“decrement y”)

Data Conversion

• Widening conversions convert data to another type that has the same or more bits of storage. E.g. , - short to int, long (safe) - int to long (safe) - int to float, double (magnitude the same but can lose precision)
• Narrowing conversions convert data to another type that has the fewer bits of storage and/or can lose information. E.g. , - double or float to any integer type - double to float

Mixing Types

• Conversions are done on one operator at a time in the order the operators are evaluated. 3 / 2 * 3.0 + 8 / 3 5. 2.0 * 4 / 5 + 6 / 4.0 3.

Mixing Types

• String concatenation has the same precedence as + - and is evaluated left to right. 1 + "x" + 4 "1x4" "2+3=" + 2 + 3 "2+3=23” 1 + 2 + " 3 " " 33 ” "23=" + 2 * 3 "23=6" 4 - 1 + "x" "3x" "x" + 4 - 1 error