Bryce Canyon, Utah

CSE 114 – Computer Science IObjects and Reference

Classes

• Class—definition of a kind of object

• Like an outline or plan for constructing specific objects

• Class specifies what kind of data objects of that class have– Each object has the same data items but can have different

values

• Class specifies what methods each object will have– All objects of the same class have the same methods available to

them

Class as an Outline

Class Name: Automobile

Data:

amount of fuel ________

speed ________

license plate ________

Methods (actions):

increaseSpeed:

How: Press on gas pedal.

stop:

How: Press on brake pedal.

Class Definition

Objects that are instantiations of the class

First Instantiation:

Object name: patsCar

amount of fuel: 10 gallons

speed: 55 miles per hour

license plate: “135 XJK”

Second Instantiation:

Object name: suesCar

amount of fuel: 14 gallons

speed: 0 miles per hour

license plate: “SUES CAR”

Third Instantiation:

Object name: ronsCar

amount of fuel: 2 gallons

speed: 75 miles per hour

license plate: “351 WLF”

Objects

• Variables that are named instances of a class– the class is their type

• Have both data and methods– called members of the object

• Data items are also called fields or instance variables

• Invoking a method means to call the method, i.e. execute the method. Ex:– objectVariableName.method()– objectVariableName is the calling (invoking) object

Containment

• A class contains another class if it instantiates an object of that class– “HAS-A”

• PairOfDice HAS-A Die

PairOfDice dice

Die die1

int upValue

int numFaces

Die die2

int upValue

int numFaces

RollGames main method

Object Variables as Instance Variables for other Objects

Primitive Types vs. Objects (Class Types)

• Primitive variables:– assigned a memory location when declared– variable’s data stored there

• Object variables – assigned a memory location when it is declared– address of where the object data will be is stored there– address starts as null

• assigned when object variable is constructed

new

• Used when constructing an object

• Asks the JVM for a block of memory

• What for?– to store the instance variables of the object

• What does it return?– the address (#) of the memory block

Primitive vs. Class - Example

int sum = 0;

Die die1; MEMORY

die1 null

MEMORY

sum 0

die1.roll();

// CAN'T DO THIS! // die1 hasn’t been constructed

Before object variables are constructed, they store the address null (0)

Syntax or Runtime Error

Primitive vs. Class – Example (cont’d)

die1 = new Die(8,3);

die1.roll();

// Now it’s OK to roll

MEMORY

die1

upValue

numFaces

3

8

etc…

die1 stores an address, which is the address of the beginning of the memory block where the object’s data is

== and Objects

Die die1 = new Die(8,4);

Die die2 = new Die(8,4);

if (die1==die2)

FALSE!

if (die1.equals(die2))

SHOULD BE TRUE!

DEPENDING ON equals DEF.

MEMORY

4

8

4

8

etc…

die1

upValue

numFaces

die2

upValue

numFaces

etc…

.equals and Objects• equals should return true when the two objects have

equivalent state (instance variable) values

// INSIDE Die class

public boolean equals(Die otherDie)

{

return (

(upValue == otherDie.upValue)

&& (this.numFaces==otherDie.numFaces)

);

} this is optional, but what is it?

Call-by-value Revisited

• Note: – method arguments are copies of the original data

• Consequence?– methods cannot assign (‘=’) new values to arguments

and affect the original passed variables

• Why?– changing argument values changes the copy, not the

original

Java Primitives Examplepublic static void main(String[] args)

{

int a = 5;

int b = 5;

changeNums(a, b);

System.out.println(a);

System.out.println(b);

}

public static void changeNums(int x, int y)

{

x = 0;

y = 0;

}

Output?55

Java Objects (Strings) Examplepublic static void main(String[] args)

{

String a = "Hateful";

String b = "Career Opportunities";

changeStrings(a, b);

System.out.println(a);

System.out.println(b);

}

public static void changeString(String x, String y)

{

x = "The Magnificent Seven";

y = "The Magnificent Seven";

}• NOTE: When you pass an object to a method, you are passing a copy of the

object’s address

Output?HatefulCareer Opportunities

How can methods change local variables?

• By assigning returned values

• Ex, in the String class:– substring method returns a new String

String s = "Hello";

s.substring(0, 4);

System.out.println(s);

s = s.substring(0, 4);

System.out.println(s);

Output?HelloHell

Parameter Passing Revisited

• Primitive variables are passed using call-by-value.– a copy of the value is sent to the method

– any change to that value in the method does NOT affect the original primitive variable

• What about with objects?– a copy of the address is sent

– any changes made via mutator methods can change the original object

– Danger: Reconstructing an object parameter for a method does not reconstruct the originally passed variable

swap

a

die

x

y

3

6

1

3

upValue

numSides

• Example Method:

public void swap(int x, Die y){

int temp = x;x = y.getUpValue();y.setUpValue(temp);

}

• Example Method Call:int a = 3;Die die = new Die(6,1);swap(a,die);System.out.print("a is " + a);System.out.print(", die is " + die.getUpValue());

Parameter Passing Revisited (cont’d)

OUTPUT: a is 3, die is 3

x and y are temporary variables, and are stored in different places in memory than a and die

3

1

Changing passed parameterspublic class ParameterPassing {

public static void change(int idNum, String name, StringBuffer surname)

{ idNum = 15; name = "Oscar"; surname.replace(0, 4, "Jobs"); }

public static void main(String[] args) { int id = 0; String fName = "Steve"; StringBuffer lName = new StringBuffer("????"); change(id, fName, lName); System.out.println(fName + " " + lName + " is employee # " + id); }}

OUTPUT: Steve Jobs is employee # 0

What is the output?

These are only temporary variables

What’s happening in memory?

…

chars[0]"O"

chars[4]"r"

… …

"J"

"s"

15

…

change0idNum

surname

name

…

…

0id

lName

fName

…

…

…

MEMORY

main

chars[0]"S"

chars[4]"e"

… …

chars[0]"?"

chars[3]"?"

… …

…

…

…

MEMORY

…

…

= and Objects• = Assigns an ADDRESS TO

OBJECT VARIABLE!Die die1 = new Die(8,4);Die die2;die2 = die1;

• Doesn't make a new copy of object!

• Now die1 and die2 both refer to the same object!

die1.roll();• Causes same change indie1 AND die2

MEMORY

die2

upValue

numFaces

3

8

die1

etc…

6