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2. Programming with Objects

2.1 Names

2.2 Objects and Classes

2.3 using Objects

2.4 Arithmetic Expressions

2.5 Parameters and Input

2.6 Getting Started with Applets and Events

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Objectives• Name objects and other Java entities• Build a class to define objects• Use methods to invoke other behavior• Use arithmetic expressions to compute value• Input in a dialog• Use an import statement• Get started with applet and events

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2.1 Names• Objects can communicate by calling each other’s

responsibilities by names• Names of objects, their data and responsibilities are

called identifiers<identifier> ::= <letter>{<letter> | digit}

<letter> ::= a..z | A..Z | _ | $

<digit> ::= 0..9

• Keywords are reserved identifiers for special uses

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• The (7-bit) ASCII character set contains 128 characters

• The (8-bit) Extended ASCII character set contains 256 characters

• In general, an n-bit character set contains 2 characters.• Java uses a 16-bit character set called Unicode that

may contain at most 65536 characters• Unicode contains ASCII code as a subset, and

characters of many countries

n

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Valid Identifiers savings, textLabel, rest_stop_12, x, I3, _test, $soup

Not valid Identifiers

4you // Starts with a numberx<y // Includes an illegal character, <top-gun // Includes an illegal character, -int // Reserved, see below

Figure 2.2 Valid and Invalid Java identifiers

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abstract do implements package throw boolean double import private throws break else inner protected transient byte extends instanceof public try case final int rest var cast finally interface return void catch float long short volatile char for native static while class future new super const generic null switch continue goto operator synchronized default if outer this

Figure 2.3 Java Keywords

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2.2 Objects and Classes• We can think of an object as a collection of

services that we can tell it to perform for us• A class defines a type of objects

public class Vending {

// variables, methods, and constructors

}

• Java’s style starts class names with an uppercase character

• Starts comment with // (through end of line), or enclose it with /* and */

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Java entity Use

class define a type of object instance variable define object state

instance method define object behavior constructor provide object identity

Figure 2.4 Java constructs for defining objects

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• Variables and their values define the state of an object

• A variable can hold either a primitive type or an object reference

• Primitive types include byte, short, int, long, float, double, char, and boolean

• Java uses 32-bit (signed) integers. The range is

-2,147,483,648 to 2,147,483,647

• Accessibility can be controlled by modifiers public, private or protected

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• Instance variables are those defined in a class without the static modifier

• Constructors have the same name as the class• A constructor allocates space for the instance

variables and may also initialize their values• A constructor may have formal parameters

• Default constructor is one without parameters.• A constructor doesn’t return any value

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• Methods specify the responsibilities, or behaviors, of the objects

• Instance methods are those defined without the static modifier

• A method has a header and a body• The header specifies the type, the name, a list of

parameters, and other modifiers• The body contains the details to accomplish the

designed responsibility or behavior• The body contains a series of declarations and

statements

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quarters numberOfQuarters

a. Before the assignment

quarters

b. After the assignment

Figure 2.5 The effect of an assignment statement

10 3

13

Consider the statement:

quarters = quarters + numberOfQuarters;

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Packages

• Example: package vend;• Java organizes classes into packages• A package name corresponds to a directory of the

host file system• Java style starts a package name with lowercase

letter• The package statement must be the first non-

comment line in the program• The import statement

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• Some of the packages in the standard class library are:

Package Purpose

java.lang General support

java.applet Creating applets for the web

java.awt Graphic and GUI

javax.swing Additional graphic capabilities

java.net Network communication

java.util Utilities

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• The assignment statement• The return statement• A string is a sequence of characters.• Constant strings are enclosed in double quotes,

e.g. “three blind mice”.• The String class• The void keyword• Unified Modeling Language (UML) object

diagram.

6/4/01 Figure 2.6 A Vending1 instance

___________:Vending1

quarters = 10

candyBars = 15enterMoney

selectMoney

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2.3 Using Objects• A variable whose type is a class is a reference

variable to an object

Vending1 byTheDoor = new Vending1(10,15);• Reference variable byTheDoor holds the address of

the newly created Vending1 object• The decalaration

Vending1 machineA;

Creates a reference variable referring to no object at this point (with value null)

6/4/01 Figure 2.7 A reference variable of type Vending1

___________:Vending1

quarters = 10

candyBars = 15enterMoney

selectMoney byTheDoor

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null

machineA

Figure 2.8 A null reference

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• An instance method must be invoked via an object reference

• Need testing (or driver) class to create objects and invoke their methods

• Special method in driver class:

public static void main(String[] args) {// code goes here

};• Class methods - declared with static• Java interpreter invokes the class method main to

start executing a Java program

6/4/01 Figure 2.9 After executing byTheDoor.enterMoney(3)

___________:Vending1

quarters = 13

candyBars = 15enterMoney

selectMoney byTheDoor

6/4/01 Figure 2.10 After executing byTheDoor.selectCandy()

___________:Vending1

quarters = 13

candyBars = 14enterMoney

selectMoney byTheDoor

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• Local variables: those declared inside a method• Output statement:

System.out.print(“Have a nice day ”);

System.out.println(12345);• System is a class defined in java.lang package,

and out is one of its class variable, a reference to a PrintStream object

• The PrintStream class has many methods including print and println

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2.4 Arithmetic Expressions• An expression is a combination of operators and

operands• Arithmetic expressions compute numeric results

and make use of the arithmetic operators:

Addition +

Subtraction -

Multiplication *

Division /

Remainder %

• If either or both operands are floating point, the result is floating point.

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Operation Math notation Java (constants) Java (variables)

Addition a + b 3 + 4 score1 + score2 Subtraction a - b 3 - 4 bats - gloves Multiplication ab 12 * 17 twelve * dozens Division a/b 7 / 3 total / quantity Remainder r in a=qb+r 43 % 5 cookies % people Negation -a -6 -amount

Figure 2.13 Java arithmetic operations

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Operator Precedence• Operators can be combined into complex

expressions

result = total + count / max - offset;• Operators have a well-defined precedence which

determines the order of evaluation• Multiplication, division, and remainder are

evaluated prior to addition, subtraction and string concatenation

• Arithmetic operators with same precedence are evaluated from left to right

• Parentheses can be use to change the order

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3 + 4 * 5

Figure 2.14 Multiplication gets its operands first

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* 5(3 + 4)

Figure 2.15 Compute within parentheses first

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Increment & Decrement Operators

• The increment and decrement operators are unary• The increment operator (++) adds one to its

operand• The decrement operator (--) subtracts one from its

operand• The statement

count++;

is essentially equivalent to

count = count + 1;

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• The increment and decrement operators can be applied in prefix form (before the variable) or postfix form (after the variable)

Expression Operation Value of Expression

count++ add 1 old value

++count add 1 new value

count-- subtract 1 old value

--count subtract 1 new value• When used alone in a statement, prefix and postfix

forms make no difference• Have different effect in larger expressions

6/4/01 Figure 2.16 Expression a) and equivalent expression b)

then

3 + x

x++

a. b.

3 + x++

6/4/01 Figure 2.17 Expression a) and equivalent expression b)

then

++x

3 + x

a. b.

3 + ++x

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Assignment Operators• Often we perform an operation on a variable, then

store the result back into that variable• Java provides assignment operators to simplifies the

notations

Operator Example Equivalent To

+= x += y x = x + y -= x -= y x = x - y *= x *= y x = x * y /= x /= y x = x / y %= x %= y x = x % y

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Parameters• A method may have a list of (formal) parameters• Another method calls it by providing a matching list

of arguments (or actual parameters)• Argument-parameter matching is the primary

mechanism for the calling method to pass data to the called method

• Java always passed arguments by value, meaning that the called method receives the values of the arguments

• Changing value of a parameter has no effect on the correspond argument

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v.enterMoney

Figure 2.19 The enterMoney “machine”

Arguments

Return Value

2, 2, 1

75

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v :Vending2

quarters = 20

dimes = 20

nickels = 20

candy = 10

enterMoney

selectCandy

test

before

v :Vending2

quarters = 22

dimes = 22

nickels = 21

candy = 10

enterMoney

selectCandy

test

after

Figure 2.20 The state change from enterMoney

6/4/01 Figure 2.21 Memory usage

cube aNumber

12x

value

0

After the call

12

17280

12 then 17

During the call

12

1728

main

result

Before the call to cube(x)

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Message and Input Dialogs• Output can be displayed in a popup message dialog

rather than a console window

JOptionPane.showMessageDialog(null, “hi there”);

• The JOptionPane class is in javax.swing

import javax.swing.JOptionPane;

• The showInputDialog method of JOptionPane pops up a window that accepts input from the user

String s = JOptionPane.showInputDialog

(“How many quarters deposited? “);

int q = Integer.parseInt(s);

6/4/01 Figure 2.23 Package diagram

default

TestVending2

//other classes in //the same //directory

Javax.swing

JOptionPane

<<imports>>

vend

Vending1

<<imports>>

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Colors and Coordinate Systems• A picture to be displayed is broken down into

pixels, and each pixel is stored separately• Each pixel can be identified using a two-

dimensional coordinate system• In Java, each pixel can be represented by three

numbers between 0 and 225, the RGB value, that specify the intensity of the three primary colors

• When referring to a pixel in a Java program, we use a coordinate system with the origin in the upper left corner

6/4/01 Figure 2.26 Coordinates, in pixels, for 400 by 300 window

x

(0,0)

(0,299)

(399,0)

(399,299)

y

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• Applets runs on a browser or applet viewer• Any applet always extends the Applet class• The Applet class provides a standard interface

between the applets and their environment• We override the paint method of the Applet class

to handle the paint events• A Graphics object can be used to write strings or

draw pictures• An applet is part of a web page

<applet code=“HelloApplet” width=400 height=300>

</applet>