Collection of Objects 15-121 Fall 2020 Margaret Reid-Miller Today - - PowerPoint PPT Presentation

Collection of Objects

15-121 Fall 2020 Margaret Reid-Miller

Today

• Quiz will be due Sunday 11:55pm
• Homework3: start early
• due Monday September 21th at 11:55pm
• Review of how objects are stored
• Arrays of objects
• ContactList implementation

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Primitive type variables hold values

(e.g., int, double, char)

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• Variables of primitive types name a storage location

in memory in which we can store a value.

Primitive Types

balance1 1000.0 1000.0

double balance1 = 1000.0;

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• Simply declaring a local variable does not provide a

value for the storage location. You cannot use the variable until it is assigned a value.

Primitive Types

balance2 balance1 1000.0 1000.0

double balance1 = 1000.0; double balance2;

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• Assigning the value of the one variable to another

copies the value:

Primitive Types

balance2 1000.0 balance1 1000.0

double balance1 = 1000.0; double balance2; balance2 = balance1;

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• You can assign a new value to a variable. The

previous value is lost.

Primitive Types

balance2 1000.0 balance1 1000.0 500.0

double balance1 = 1000.0; double balance2; balance2 = balance1; balance1 = 500;

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Object type variables hold references to objects.

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• Alice gets a $100 gift card from Target.
• Object type variables also name a memory location.

But the memory is too small to hold an object. It can

• nly hold a reference (pointer) to the object.

GiftCard alice = new GiftCard("Macy's", 100.0); ; (60.0); ;

100.0 name balance GiftCard alice String "Macy's"

Object Types

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GiftCard alice = new GiftCard("Macy's", 100.0); GiftCard bob = alice; (60.0); ;

100.0 bob name balance GiftCard alice String "Macy's"

• Bob takes Alice’s gift card.

Object References

Assigning alice to bob copies the reference from alice to bob. We say bob is an alias for alice.

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GiftCard alice = new GiftCard("Macy's", 100.0); GiftCard bob = alice; bob.buyGoods(60.0); ;

40.0 bob name balance GiftCard alice String "Macy's"

• Bob spends $60. Alice can see that her card now

has only $40.

Object References

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GiftCard alice = new GiftCard("Macy's", 100.0); GiftCard bob = alice; bob.buyGoods(60.0); alice = new GiftCard("Target", 75.0);

40.0 bob name balance GiftCard alice 75.0 name balance GiftCard String "Macy's" String "Target"

• Alice buys a $75 gift card from Target.

Object References

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GiftCard alice = new GiftCard("Macy's", 100.0); GiftCard bob = alice; bob.buyGoods(60.0); alice = new GiftCard("Target", 75.0); bob = alice;

40.0 bob name balance GiftCard alice 75.0 name balance GiftCard String "Macy's" String "Target"

• Bob takes Alice’s Target card and loses Macy’s card.

Object References

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40.0 bob name balance GiftCard alice 75.0 name balance GiftCard String "Macy's" String "Target"

• But now the program cannot access the Macy’s gift

card any more.

• Such objects are considered “garbage” because they

still take up memory space.

Garbage

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String "Macy's" 40.0 GiftCard bob alice 75.0 store balance GiftCard String "Target"

• To reclaim the memory space, Java has a garbage

collector that periodically “cleans up” memory so that it can be reused.

• (C doesn’t have garbage collection and programs can easily

have a “memory leak” if not programmed with extreme care.)

Garbage Collector

name balance

Compliments of the garbage collector!

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Object Types as Parameters

• An object type parameter is an alias of the argument.

GiftCard alice = new GiftCard("Macy's", 50.0); goShopping(alice); public static void goShopping(Giftcard card) { while (card.getBalance > 0) { card.buyGoods(10.0) } }

50.0 "Macy's"

alice card copies the reference

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The null Pointer

null sue If we do not instantiate an object, the variable holds a special value null that represents a nonexisting

• bject.

GiftCard sue; sue.addMoney(30);

If we try to use the variable as an object, we get a NullPointerException at runtime.

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The equals Method Revisited

• The == operator tests whether two variables have the

same references (identity);

• Whereas the equals method tests whether two

variables refer to objects that have the same state (content). store balance GiftCard

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Arrays of Objects

Array of Objects

• Arrays can store references to objects in addition to

primitive values.

• E.g.,

GiftCard[] cards = new GiftCard[4];

• Creating an array does not create the objects.
• Instead each element of the array is initialized to

null. [0] [1] [2] [3]

null null null null

cards

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Creating the Objects in the Array

• To fill the array, you need to create the objects in

addition to creating the array.

cards[0] = new GiftCard("Target", 25.0); cards[1] = new GiftCard("Macy’s”, 50.0);

[0] [1] [2] [3]

null null

cards 25.0 "Target" 50.0 "Macy's" store balance

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Using an Array of Objects

• You can use an an array element (array name with

an index) in the same way you use a variable that is a reference to an object.

• E.g., Add $20 to the gift card at index 1.

cards[1].addMoney(20.0);

That is, cards[1] is a reference to a GiftCard

• bject on which to invoke the addMoney() method.

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• If program attempts to use null when an object is

required, Java throws a NullPointerException:

• Invoke a method of a null object:

cards[3].buyGoods(30.0); (Error)

• Access or modify a field of a null object:

b = cards[2].balance; (Error)

• Access length or element of a null as if it were

an array:

int[] data; if (data.length > 0) (Error)

NullPointerException

null null

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Arrays as a Field of a Class

• When a class has a field that is an array, do not create

the array when you declare the field.

• The constructor should create the array.

public class Wallet { GiftCard[] cards; // declare the field … public Wallet(int numCards, String store) { cards = new GiftCard[numCards]; for (i = 0; i < numCards; i++) { cards[i] = new GiftCard(store); } …

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ContactList

A class to maintain a list of contacts.

Suppose we want to manage a list of contacts.

What is a contact?

• Let's start with a Person class that holds (name-

number) pairs How should we store a list of contacts?

• In an array of Person objects

How long should the array be?

• As long as the number contacts we have

What if we want to be able to add and remove contacts?

• Make the array bigger than the biggest expected

size

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Use a partially filled array to give the allusion the array can grow and shrink

What does a ContactList class have to remember (fields)?

• an array of contacts
• the number of contacts in the array

What will be the ContactList class invariants?

• All the contacts are at the beginning of the array.
• There are no gaps between contacts.

(If we remove a contact we must make sure the beginning of the array stays full.)

• The number of contacts matches the number of

entries in the contacts array.

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Let's implement a ContactList class

What operations might we want?

• read a file of contact data
• add a contact
• look up phone number given a name
• edit a contact
• remove a contact

For each public method we need to be sure that the class invariants are maintained by the end of the method.

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