Lecture 21 Objects and References COMP1681 / SE15 Introduction to Programming

Lecture 21

Objects and References

COMP1681 / SE15

Introductionto Programming

SE15: Objects and References 21–2

This week’s Learning Objectives

To review the notion of a reference. To be aware of the implications of using references Revisit Arrays


Lecture Outline

Memory allocation of different types What does new do? Implications of using references Defining equals methods

Savitch section 4.3


Variables of Primitive Types

Every variable, whether a class type or a primitive type, is implemented as a memory location.

For primitive types, the value of the variable is stored in the memory location assigned to the variable.

int score = 10;

char symbol = ‘a’;

10

a

score

symbolTwo memory Locations for thetwo variables(known size)


Class variables

For class variables, then the object named by the variable is stored in some other location in memory, and the memory address of where the object is located is stored in the variable that names the object.

The memory address of where the object is located is called a reference to the object

Book myBook1;Book myBook2;

?

?

myBook1

myBook2Two memory Locations for thetwo variables

:


What’s new?

Book myBook1;

At this point your program has a place to store a memory address, but no where to store the data in the instance variables.

new assigns a memory location for an object.

myBook1 = new Book();

assigns a memory location for a Book object and places the memory address of that location in the variable myBook1.

Informally new creates the instance variables of the object


References to objects

Book myBook1 = new Book();

Book myBook2 = new Book();

3000

1052

myBook1

myBook2

??

??

1052

3000

Two memory Locations for thetwo objects(could be any value)

:


References to objects

myBook1.setTitle(‘Macbeth’);

myBook1.setAuthor(‘Shakespeare’);

myBook2.setTitle(‘Programming’);

myBook2.setAuthor(‘Pickle’);

3000

1052

myBook1

myBook2

ProgrammingPickle

MacbethShakespeare

1052

3000

:


Assignment (=) with primitive types

int score1 = 10;

int score2 = 34;

score2 = score1;

10

34

score1

score2

:

10

10

score1

score2

:


Assignment (=) with Class variables

myBook2 = myBook1;

3000

3000

myBook1

myBook2

ProgrammingPickle

MacbethShakespeare

1052

3000

:


myBook2.setTitle(‘Java’);

myBook2.setAuthor(‘Savitch’);

myBook1 and myBook2 are now

two names for the same object!

3000

3000

myBook1

myBook2

ProgrammingPickle

JavaSavtich

1052

3000

:


Equality (==) with variables of a Primitive type

int a = 10;

int b = 10;

if (a == b)

System.out.println(“They are EQUAL”);

else

System.out.println(“They are NOT equal”);

This will produce the output

They are EQUAL


Equality (==) with variables of a Class type

Book myBook1 = new Book();Book myBook2 = new Book();myBook1.setTitle(‘Java’);myBook2.setTitle(‘Java’);myBook1.setAuthor(‘Savitch’);myBook2.setAuthor(‘Savitch’);

if (myBook1 == myBook2)System.out.println(“They are EQUAL”);

elseSystem.out.println(“They are NOT equal”);

This will produce the outputThey are NOT equal


Why are they NOT equal?





Because 1052 is not equal to 3000

(The == operator only checks if the

memory addresses are the same)

3000

1052

myBook1

myBook2

JavaSavitch

JavaSavitch

1052

3000

:


Defining an equals method

• When you compare two objects using the == operator you are checking to see if they have the same memory address.

• You are NOT testing for what you would intuitively call “being equal”.

• To do this you should define a method equals


An equals method for a Date class

public class Date{

private int year;private int month;private int day;

public boolean equals(Date otherDate){

if((year == otherDate.year)&& (month == otherDate.month)&& (day == otherDate.day))return true;

elsereturn false;

}}


References and Arrays

For Primitive types

To create a collection of seven variables of type double

double[] temperature = new double[7];

For Class types To create a collections of seven objects of type Book

Book[] libraryBook = new Book[7];

for( i=0; i < 7; i++)

libraryBook[i] = new Book();


Arrays and references

2678

2426

libraryBook[0]

JavaSavitch

MacbethShakespeare

2380

2426

:

libraryBook[1]

2380libraryBook[2]

ProgrammingPickle

2678

?

?

libraryBook[0]

:

libraryBook[1]

?libraryBook[2]

?

?

?

libraryBook[3]

libraryBook[4]

libraryBook[5]

libraryBook[6] ?


NullPointerException

No doubt you have all seen this error!

It indicates your code tried to access some member of a class variable, but the class variable names no object

(it does not contain a reference to any object)

Book[] libraryBook = new Book[7];

libraryBook[0].setAuthor(‘Andy’);


Objects as properties of Objects

References are used to

refer to one object from another

Board

a1 =

a2 =

White Queen:Piece

Colour = white

name = q

inPlay = yes

Position = a1

White King:Piece

Colour = white

name = k

inPlay = yes

Position = a2


Objects as properties of Objects

Person

name = Ann Bennet

age = 22

mother =

Person

name = Liz Bennet

age = 22

mother =

Person

name = Mrs Bennet

age = 22

mother =


Summary

Looked at how Java uses references to identify objects Identified some potential pitfalls of this model Looked at defining equals methods Seen how arrays make use of references.


Follow-up Work

Savitch chapter 4 Read up on using Primitive and Class types as

parameters for methods What is the biggest difference between a parameter of a

primitive type and a parameter of a class type?

Documents

Lecture 21 Objects and References COMP1681 / SE15 Introduction to Programming