Variables and Methods

Chapter 3 – Lecture Slides

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Variables vary...After all, change is the status quo.

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Variables Variable is data that is referenced by a named identifier

Variables need to be declared before you reference it

Variable declaration includes: data type variable name

Examples: JButton submit; JTextField tf_state;

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Variables - names

Descriptive of the data it represents Using the alphabet is not acceptable: a, b, c, d, e; nor is a1, a2, a3, a4 Lists of images in something like a slideshow can be named

img1, img2, img3, etc. No spaces allowed within a variable name Convention is to begin variable names with a lower case letter and separate

each word in the variable name by capitalizing subsequent words:

- firstName, lastName, midtermGrade, labGrade Abbreviations are good, but be consistent:

- tempFreq, hitFreq, qtyCount, qtyOfShirts Cannot use a Java keyword for a variable name

No spaces in names!

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Java Keywords

Reserved words Not to memorize, but take notice; which ones do you

already recognize?

abstract continue for new switch

assert  default goto  package synchronized

boolean do if private this

break double implements protected throw

byte else import public throws

case enum  instanceof return transient

catch extends int short try

char final interface static void

class finally long strictfp  volatile

const  float native super while

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Data Types There are 8

primitive data types

We will only be concerned with the most common ones:int, double, char and boolean

Note that these are the primitive data types; we also use a lot of objects as well.

Examples of objects are: JButton, String, JTextField, JPanel, Font, etc

data type description number of bits used

to represent the number

integers byte Byte-length integer 8-bit

short Short integer 16-bit

int Integer 32-bit

long Long integer 64-bit

reals float Single-precision floating point 32-bit

double Double-precision floating point 64-bit

char A single character 16-bit Unicode character

boolean holds either the value true or false 1-bit

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Characters

Data type: char

Enclose with single quotes

char initial = ‘E’;

char code = ‘!’;

Escape sequences

char singlequote = ‘\’’;

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Boolean

One of two values true false

boolean isOn = false;boolean available = true;

Variables Declaration

Graphics g; int numCourses; // integer values double salesTax; // floating pt number

Initialization – assignment for the first time Assignment

name = “Cookie Monster”;numCourses = 4;salesTax = 4.75;

Variable ReferenceString myName;myName = “Java Guru”;g.drawString( myName, 0, 12 );

Declaration includes the data type and a variable name.

Assignment changes the value of a variable..

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Variables

Instance variables Declared at the top of a class Data available to the entire class

Local variables Declared within a method Data only available within the method Includes method parameters

Scope: Instance Variable vs. Local VariableInstance Variable vs. Local Variable

import javax.swing.*;import java.awt.*;public class ColorEx extends JApplet { String favColor; String ski;

public void paint( Graphics g ) {

favColor = “Favorite color”; ski = “Love 2 ski”; g.setColor( Color.RED ); g.drawString( favColor, 30, 45 ); g.setColor( new

Color( 12,34,52) ); g.drawString( ski, 30, 53 ); }}

import javax.swing.*;import java.awt.*;public class ColorEx extends JApplet { public void paint( Graphics g ) {

String favColor = “Favorite color”; String ski = “Love 2 ski”;

g.setColor( Color.RED ); g.drawString( favColor, 30, 45 ); g.setColor( new

Color( 12,34,52) ); g.drawString( ski, 30, 53 ); }}

local: declared inside a method

instance: declared inside

the class

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Scope Scope

Check out the enclosing squigglys

You cannot declare two variables of the same name at the same scope level

Any variable declared strictly within the scope of another with the same name will shadow the outer named variable

Most variables are declared at the top of the class – called instance variables, so we can reference them throughout all the methods in the class

Methods Example public void paint( Graphics g )

Within the class squigglys { } Has it’s own scope squigglys { } Statements in method only run when the method is called/invoked Parameters (none or multiple) listed in parenthesis

Each must specify the data type and a variable name Separate multiple ones with a comma e.g. ( Graphics g )

return type of void designates the method doesn’t return anything g is a type of Graphics object,

which has methods such as drawString that we can call

paint is the name of the method

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Method call that returns a valueimport java.awt.*;import javax.swing.*;public class Calculate extends JApplet{

public void paint (Graphics g ) {

int addition = getAdd( 2, 7 );String added = "2 + 7 = " + addition;g.drawString ( added, 0, 12 );String subtracted = "2 - 7 = " + getSubtract( 2, 7 );g.drawString ( subtracted, 0, 24 );

} public int getAdd( int num1, int num2 ) {

return num1 + num2; } public int getSubtract( int n1, int n2 ) {

return n1 - n2; }

}

The # 2 gets copied into variable num1, the # 7 gets

copied into the variable num2

The value 9 gets returned to where the program called

this method

The # 2 gets copied into variable n1, the # 7 gets copied

into the variable n2

The value -5 gets returned to where the program called

this method

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3 Types of Method calling1. Within a class

add( panel );

2. In another class (type 1)Example: in the Graphics class

g.drawString(“Hi”, 20, 10 );

(where g is from the Graphics class, e.g., public void paint( Graphics g )

3. In another class (type 2)x = 22 + Math.abs( y );

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Why Have Methods?

Code Easier to read:

Repeated code:

For access by other objects:

Events: (we’ll discuss later)

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Why Have Methods?import java.awt.*;import javax.swing.*;public class House extends JApplet{ public void paint (Graphics g ) { g.setColor( Color.pink ); g.fillRect ( 100,100,200,200 ); g.setColor( Color.black ); Polygon poly = new Polygon( ); poly.addPoint(100,100); poly.addPoint(200,50); poly.addPoint(300,100); g.fillPolygon(poly);

g.setColor( Color.blue ); g.fillRect ( 200,230,40,70); g.fillRect ( 120,150,20,30); g.fillRect ( 150,150,20,30); g.fillRect ( 200,150,20,30); g.fillRect ( 230,150,20,30);

g.setColor( Color.black ); g.fillRect ( 400,130,30,170 ); g.setColor( Color.green ); g.fillOval( 370,80,100,100 ); g.fillRect ( 0,295,500,5 ); }}

// Code Easier to read: with methodsimport java.awt.*;import javax.swing.*;public class HouseMethods extends JApplet{ int WINDOW_WIDTH = 20; int WINDOW_HEIGHT = 30; public void paint (Graphics g ) {

paintHouse( g );paintLandscape( g );

} public void paintHouse( Graphics grph ) { grph.setColor( Color.pink );

grph.fillRect ( 100,100,200,200 ); grph.setColor( Color.black );

Polygon poly = new Polygon();poly.addPoint(100,100);poly.addPoint(200,50);poly.addPoint(300,100);grph.fillPolygon(poly);

grph.setColor( Color.blue );grph.fillRect ( 200,230,40,70);paintWindow( grph, 120, 150 );paintWindow( grph, 150, 150 );paintWindow( grph, 200, 150 );paintWindow( grph, 230, 150 );

} public void paintWindow( Graphics gp, int x, int y ) {

gp.setColor( Color.blue );gp.fillRect ( x, y, WINDOW_WIDTH, WINDOW_HEIGHT );

} public void paintLandscape( Graphics g ) {

g.setColor( Color.black ); // treeg.fillRect ( 400,130,30,170 );g.setColor( Color.green );g.fillOval( 370,80,100,100 );g.fillRect ( 0,295,500,5 ); // grass

}}

Which code is easier to figure out

how to add a new

window?

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Why Have Methods?import javax.swing.*;import java.awt.*;

public class NeedMethods

extends JApplet{ public void paint( Graphics g ) { // row 1 g.fillRect( 20,20, 10,10 ); g.fillRect( 40,20, 10,10 ); g.fillRect( 60,20, 10,10 ); g.fillRect( 80,20, 10,10 ); g.fillRect( 100,20, 10,10 ); // row 2 g.fillRect( 30,30, 10,10 ); g.fillRect( 50,30, 10,10 ); g.fillRect( 70,30, 10,10 ); g.fillRect( 90,30, 10,10 ); g.fillRect( 110,30, 10,10 );

}}

// Repeated code using a method:

import javax.swing.*;import java.awt.*;

public class NeedMethods2 extends JApplet{ public void paint( Graphics g ) { drawRows( g, 20, 20 ); drawRows( g, 30, 30 ); } public void drawRows( Graphics graphics, int x, int y ) { graphics.fillRect( x,y, 10,10 ); graphics.fillRect( x+20, y, 10, 10 );

graphics.fillRect( x+40, y, 10, 10 ); graphics.fillRect( x+60, y, 10, 10 ); graphics.fillRect( x+80, y, 10, 10 );

}}

Which program would be easier to use to create a full-size checkerboard?

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Why Have Methods?

For access by other objects: Graphics gr

gr.drawString( gr.fillRect(gr.setColor(

If these weren’t methods, we wouldn’t be able to draw anything!

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Why Have Methods?

Events Events in Java are triggered when:

User selects a button/checkbox/item in list/etc. User moves/drags the mouse User types a key Timer expires More…

Each event automatically calls a particular method to handle the type of event that occurred.

We’ll discuss events in more detail later

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Summary

Variables Scope: instance variables vs. local variables Method Structure Purpose of methods

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