Variables and TypesJava Part 3

Class Fields Aka member variable, field variable,

instance variable, class variable. These are the descriptors of the object. The type associated with these

variables can be built in or user-defined.

8 Intrinsic (Primitive) Types int integer (4 bytes) double number with a decimal (8 bytes) boolean true / false (2 bytes) char single character (2 bytes)

byte (1 byte) short (2 bytes) long (8 bytes) float (4 bytes)

Note: these all begin with a lowercase letter

Abstract Types String This is a class defined in the

java library and is used to hold strings of characters.

For ex, name=“Joe Smith”

Font used to change the font style of your output

Note: These begin with an uppercase letters – denotes a class name.

User-Defined Types Another type of abstract data You can declare member variables to be

of user-defined types, such as any class that you have created.

For ex, if you create a Widget class, and save it, you can declare things of type Widget in another project, or this same project.

Declaring Variablesint length;int length, width;int length = 10;

In the last statement, the variable is declared and initialized in one statement.

Monster m = new Monster();

This is how you declare a variable of a user-defined type.

Referencing VariablesIntrinsic variables store the value of the variable.

Object variables (such as type String, or anything you create) store the reference (address) to the data, not the data itself.

This will become important when you are changing the values of an object later on.

What is a reference?Monster m = new Monster();m

Monster Object

0xF50xF5

m stores the address of a Monster

What is a primitive variable?int numDays = 365;

numDays

365

numDays stores an integer value

Real Numbers

Reals – predict the outputdouble one = 99.57;double two = 3217;float three = 23.32f;

System.out.println(one);System.out.println(two);System.out.println(three);

OUTPUT99.573217.023.32

double one = 120.7; System.out.println(one);one = 125;System.out.println(one);System.out.println((int)one);

OUTPUT120.7125.0125

Characters Character variables are used to store a

single letter. Character variables are actually

integers.char let = 'A';char fun = 65;

char test = 'a';char go = 97;

char what = 48;

Ascii ValuesASCII values you MUST know!!!

‘A’ 65‘a’ 97‘0’ 48

char letter = 97; out.println(letter); //outs aletter = 'A';out.println(letter); //outs A

Characterschar alpha = 'A';char ascii = 65;char sum = 'B' + 1;

System.out.println(alpha);System.out.println(ascii);System.out.println(sum);System.out.println('B'+1);

OUTPUTAAC67

Booleans

Booleans can only store true or false

boolean go = true;System.out.println(go);boolean stop = false;System.out.println(stop);

OUTPUTtruefalse

Strings String is a class that stores a group of

characters

String dude = "hello world";String buddy = "whoot - \\\\\\\\\\\\";

System.out.println(dude);System.out.println("buddy = " + buddy);

OUTPUThello worldbuddy = whoot - \\\\\\

Variables AssignmentThe statement

receiver = 57; assigns the value 57 to the variable receiver.

It is assumed that the variable receiver has already been declared before this point.

int num = 82; declares the variable and assigns it a value in

one statement.

Assignment Practiceint number = 75, bigNum=99;double hTownTax = 8.25;char bigA = 'A', littleA = 'a';boolean isPrime = false;String s = "abc";

System.out.println(number);System.out.println(bigNum);System.out.printf("%.2f\n",hTownTax);System.out.println(bigA);System.out.println(littleA);System.out.println(isPrime);System.out.println(s);

OUTPUT75998.25Aafalseabc

How Big and Small can my variables go?The MIN_VALUE and MAX_VALUE fields store the minimum and maximum values that can be stored in a particular type.

System.out.println(Byte.MIN_VALUE);System.out.println(Byte.MAX_VALUE);

System.out.println(Short.MIN_VALUE);System.out.println(Short.MAX_VALUE);

OUTPUT-128127-3276832767

Mixing Types Java is a stong typed language. You must pay attention to a variable’s type

when assigning a value. Integers cannot store decimals unless a

cast is used.

int one=90;char letter= ‘A’;char let= 97;

one=letter; letter=let; one=let;

When assigning values from one variable to another, data types and data type sizes play an important part.

Large size types can be assigned same size type and smaller type values.

Smaller size types cannot be assigned larger type values.

16 bit = 16 bit and smaller //legal 8 bit = 16 bit and smaller //illegal

without a cast()

int one = 90;double dec = 234;char letter = 'A';

System.out.println( one );one = letter; //char to intSystem.out.println( one );

one = 'A'; //char to intSystem.out.println( one );

System.out.println( dec );dec = one; //int to doubleSystem.out.println( dec );

OUTPUT906565234.065.0

Data type sizesoften determine if assignment is legal.32 bit == 32 bit

AutoBoxing and AutoUnboxingIn JAVA, you have 8 primitive data types.

All other variables in Java are referencevariables. References refer to objects.

Monster m = new Monster();

For each of the primitive types, there is a corresponding wrapper class.

primitive objectbyte Byte short Short

int Integer long Long float Float

double Double char Character

boolean Boolean

Box/UnboxInteger numOne = 99;Integer numTwo = new Integer(99);

=99;=new Integer(99);These two lines are equivalent.

Box/UnboxDouble numOne = 99.1;Double numTwo = new Double(99.1);

=99.1;=new Double(99.1);These two lines are equivalent.

With the introduction of Java 5, the new Double() Object instantiation code happens in the background. Java takes care of these details, but does not show the work.

GUI Help//gui example

import javax.swing.JOptionPane;

public class GuiHelp { public static void main(String args[]) { { int one, two, total; one = Integer.parseInt(JOptionPane.showInputDialog("Enter an integer :: ")); two = Integer.parseInt(JOptionPane.showInputDialog("Enter an integer :: "));

total = one + two; JOptionPane.showMessageDialog(null,"Total ::" + total); System.out.println(total); }}

Constants In java, we refer to constants as finals. A declaration would look like:

final double PI = 3.14; The name of the final would be in

UPPERCASE letters. Use finals whenever you have values

that don’t change while the program is running.

Assignment Operators total = total + number; total += number;

numPlayers = numPlayers -3; numPlayers -= 3;

Assignment operators include:= , += , -= , *=, /= , %=

Increment and Decrement The increment operator (++) means

add by 1 The decrement operator (--) means

decrease by 1 These can be prefix or postfix.

Prefix: ++x; increment, then fetchPostfix: x++; fetch, then increment

There is a difference…

Concatention The + sign is used to concatenate

strings and/or variable values.

Relational Operators!= not equal to== equal to

format() This can be used to format numeric values. A specifier takes the form:

%[alignment][width][.decimal]f

where [.decimal] indicates the number of decimal places and f indicates a floating point number.For an integer:

%[alignment][width]d

% start of specifier[alignment]skip for right alignment, - for left align[width]num of characters useds indicates String

NumberFormat Class import java.text.NumberFormat;

money=NumberFormat.getCurrencyInstance();number=NumberFormat.getIntegerInstance();decimal = NumberFormat.getNumberInstance();percent=NumberFormat.getPercentInstance();

Getting Input from User An input stream is a sequence of

characters received from an input device, such as a keyboard.

import java.util.Scanner to get input from the keyboard.

Use System.in to read in the input

Scanner Methods next() returns a string (no spaces)

use after reading in numeric data as nextLine() will read in the EOL character, not the next actual line

nextLine() returns string up to EOL nextInt() returns the int from the

stream nextDouble() nextBoolean() close() closes the input stream

Assignments Glencoe book, p 130, #5 – use wordpad Worksheets

Variables Wk 0-2 Assignments from weebly