CS 99

Summer 2001 07.03

Java Basics I:

Variables

• Named storage location in memory with an associated type

Declaring Variables

• Syntax:

<type> name [= init] [, name = init]…;

• Examples:

int y;

int x = y;

double pi = 3.14;

String hi = “Hi!”;

int a = 1, b, c = 2;

Types

• Every value in a program has a type

• Every variable, since it holds values, also has a type

• Java has some built-in types called intrinsic types or primitive types

• Programmers can also create their own types using classes

• Integers

– Numbers that are whole valued and signed

– e.g., 5, -1000, 42, 0

– Java types byte, short, int, long

• Floating point numbers

– Numbers that have a decimal component

– e.g., 3.14, 1.78, .9944, -1.69, 1.0, 0.0

– Java types float, double

• We’ll usually use int and double

• Characters

– The symbols in a character set, such as letters, numerals, punctuation, etc.

– e.g., ‘a’, ‘b’, ‘c’, ‘X’, ‘Y’, ‘Z’, ‘1’, ‘%’

– Java type char

• Booleans

– Values that are either true or false

– Java type boolean

• The String type

• String is technically not a primitive type, but an object type

• Strings are sequences of characters

• e.g., “Hello, world!”, “1 + 1 = 2”

Naming Variables

• Follow Style Guide

• First character in name must be a letter

• Remaining characters can be letters, numbers, $, or the underscore “_” (e.g., cs99_2000su)

• Can be (practically) as long a name as you want

Literals

• Variables are placeholders for values in a program

• Literals are actual values written directly in a program:

int x = 5;

double y = x + 2;

String s = “5 + 2 = “ + y;

• Literals above: 5, 2, “5 + 2 = ”

Assignment Statement

• Syntax:

variable = value

• Examples:

x = 5;

y = x;

z = x + y;

d = Math.round(b) – 1;

Expressions

• Values combined by operators

• Have a value, and therefore a type; but not all expressions return values.

Operators

• Operators allow values to be combined

• Categories of operators

– Arithmetic

– Relational

– Logical

Arithmetic Operators

Modulus %

Increment ++

Decrement --

Addition assignment += (also -=, *=, /=, %=)

Division /

Multiplication *

Subtraction -

Addition +

Relational Operators

<= (less than or equal to), >= (greater or equal to), < (less than), > (greater than), != (not equal to).

== (equal to)

• For equality operators:

– Operands must be of the same type

• For ordering operators:

– Operands must be of numeric type

• For both:

– Resulting value is a boolean

• Common error:

– Using the assignment operator instead of the equality operator

Logical Operators

! (NOT), || (OR), && (AND)Operands and resulting values are boolean