Introduction to Primitives

Overview

• Today we will discuss:– The eight primitive types,

especially int and double– Declaring the types of variables– Operations on primitives– The assignment statement– How to print results

Primitives

• Primitives are the "basic" data values

• There are eight types of primitives:– boolean -- used for true and false values

– char -- used for single characters (letters, etc.)

– byte, short, int, long -- four different kinds of integer (whole number) values

– float, double -- two different kinds of decimal numbers (numbers with a decimal point)

int• The most important integer type is int

– An int is a "whole" number (no decimal point)

• Numbers occupy memory in the computer– Larger numbers require more memory– An int can be between about two billion (two

thousand million) and negative two billion

• If you just write a number, such as 25, Java assumes it is an int

• Use int in preference to other integer types

byte and short

• A byte can be between -128 and 127• A short can be -32768 to 32767• Why these numbers?

– I’ll answer that later in the course

• Use byte or short only when– You know the numbers are all small– There are millions of numbers to remember

• Extra syntax is needed (will be discussed later)

long

• long integers are for when two billion isn’t large enough for your needs– a long can be as long as about 19 digits– a long occupies twice as much space as an int– arithmetic on long values is slower– use only when you need really big numbers– Extra syntax is needed (will be discussed later)

• Even larger numbers are available in Java--but they are objects, not primitives

double

• A double represents a “real” number– Also sometimes called “floating point”– These are numbers with a decimal point

• A double has about 15 digits of accuracy

• If you just write a real number, such as 1.37, Java assumes it is a double

• Use double in preference to float

float

• float is the other kind of “real,” or “floating point” number

• float has about 8 digits of accuracy• Arithmetic with float is not faster• Use float only to save space when there are

millions of numbers involved• Extra syntax is needed (will be discussed later)

An aside: approximations

• Integers are precise, but real numbers are always approximate (inaccurate)

• Two numbers that look the same may actually be subtly different

• Never test floating point numbers for equality!– Only test for larger or smaller, or for “not

larger” or “not smaller”

Giving names to numbers

• Sometimes you know what a number is– You have 10 fingers– π is 3.1415926536– Numbers written like this are called literals– You can use literals anyplace in Java

• Sometimes you need to use names instead:– classSize, myBankBalance, price– Names like this are called variables– The value of a variable may change

Variables

• Before you use a variable, you must declare it (tell Java what type it is)

• There are two reasons for this:– Different types require different amounts of space– So Java can prevent you from doing something

meaningless (adding 5 to a frog)

• You must also tell Java what its value is– You might compute the value, or read it in

Declaring variables

• You declare variables like this: int classSize; double myBankBalance;

• When you declare a variable to be a primitive type, Java automatically finds space for it– The amount of space Java needs to find

depends on the type of the variable– Think of a variable as a specially shaped “box”

Giving values to variables

• A variable is just a name for some value– You have to supply the actual value somehow– Java tries to prevent you from using a variable

that you haven’t given a value

• You can assign values like this: classSize = 57; myBankBalance = 123.01; // no "$"!

Initializing variables

• You can give a variable an initial value when you declare it: int classSize = 30; double myBankBalance = 0.0;

• You can change the value of a variable many times: classSize = 57; myBankBalance = myBankBalance + 50.00;

Arithmetic

• Primitives have operations defined for them

• int and double have many defined operations, including+ for addition

- for subtraction

* for multiplication

(Old computers did not have thecharacter)

/ for division

Order of precedence

• Operations with higher precedence are done before operations with lower precedence

• Multiplication and division are done before addition and subtraction 2 + 3 * 4 is 14, not 20

• Operations of equal precedence are done left to right 10 - 5 - 1 is 4, not 6

Parentheses

• Operations inside parentheses are done first (2 + 3) * 4 is 20

• Parentheses are done from the inside out 24/(3*(10-6)) is 24/(3*4) is 24/12 is 2

• Parentheses can be used where not needed 2 + (3 * 4) is the same as 2 + 3 * 4

• [ ] and { } cannot be used as parentheses!

Assignment statements

• An assignment statement has the form: variable = expression ;

• Examples: price = 0.69; area = pi * radius * radius; classSize = classSize + 1;

• this means “add one to the value in classSize”

Printing out results, part 1

• In Java, “print” really means “display in a window on the screen”– Printing on actual paper is much harder!

• There are two commands for printing:– System.out.print(x);

• displays x

– System.out.println(x);• (pronounced “printline”)

displays x, then goes to the next line

Printing out results, part 2

• Examples: System.out.print("The sum of x and y is ");

System.out.println(x + y);

• If x and y are both 5, the result will be The sum of x and y is 10

• If you print from an application, an output window opens automatically

• If you print from a browser applet, you have to open the “Java Console” window to see your output

A BASIC program

• Here is a program, written in the BASIC language, to add two numbers and print out the result:

PRINT 2+2

A Java program

• Here is the same program, written in Java:

public class TwoPlusTwo { public static void main(String args[]) { System.out.println(2 + 2); }}

Why is Java so hard?

• BASIC is a beginner’s language, designed for small programs

• Java is a professional’s language, designed for large programs

• Just as a skyscraper needs a better foundation than a doghouse, Java programs need more structural support

• Java isn’t the best language for everything

New vocabulary

• primitive: one of the 8 basic kinds of values

• literal: an actual specified value, such as 42

• variable: the name of a “box” that can hold a value

• type: a kind of value that a literal has or that a variable can hold

• declare: to specify the type of a variable

More new vocabulary

• operation: a way of computing a new value from other values

• precedence: which operations to perform first (which operations precede which other operations)

• assignment statement: a statement that associates a value with a name

• initialize: to assign a “starting” value

The End