a collection class is a data type that is capable of holding a group of items. in java, collection...

A A Collection class Collection class is a data is a data type that is capable of type that is capable of holding a group of items.holding a group of items.

In Java, Collection classes In Java, Collection classes can be implemented as a can be implemented as a class, along with methods to class, along with methods to add, remove, and examine add, remove, and examine items.items.

Collection ClassesCollection Classes

Data StructuresData Structuresand Other Objectsand Other ObjectsUsing JavaUsing Java

BagsBags

For the first example, For the first example, think about a bag.think about a bag.

BagsBags

For the first example, For the first example, think about a bag.think about a bag.

Inside the bag are Inside the bag are some numbers.some numbers.

Initial State of a BagInitial State of a Bag

When you first begin When you first begin to use a bag, the bag to use a bag, the bag will be empty.will be empty.

We count on this to be We count on this to be the the initial stateinitial state of any of any bag that we use.bag that we use.

THIS BAGIS

EMPTY.

Adding Numbers into a BagAdding Numbers into a Bag

Numbers may be Numbers may be added into a bag.added into a bag.

I AMPUTTING THE

NUMBER 4INTO THE

BAG.



THE 4 ISIN THEBAG.



The bag can hold The bag can hold many numbers.many numbers.

NOW I'MPUTTINGANOTHER

NUMBER IN THE BAG --

AN 8.

added Numbers into a Bagadded Numbers into a Bag



THE 8 ISALSO IN

THE BAG.

added Numbers into a Bagadded Numbers into a Bag



We can even place We can even place the same number the same number more than once.more than once. NOW I'M

PUTTING ASECOND 4

IN THEBAG.




We can even place We can even place the same number the same number more than once.more than once.

NOW THEBAG HASTWO 4'S

AND AN 8..

Examining a BagExamining a Bag

We may ask about We may ask about the contents of the the contents of the bag. bag.

HAVEYOU GOTANY 4's

?

YES,I HAVETWO OFTHEM.

Removing a Number from a BagRemoving a Number from a Bag

We may remove a We may remove a number from a bag. number from a bag.

THIS4 IS

OUTTAHERE!

Removing a Number from a BagRemoving a Number from a Bag

We may remove a We may remove a number from a bag.number from a bag.

But we remove only But we remove only one number at a one number at a time. time.

ONE 4 ISGONE, BUTTHE OTHER4 REMAINS.

How Many NumbersHow Many Numbers

Another operation is Another operation is to determine how to determine how many numbers are in a many numbers are in a bag.bag.

IN MY OPINION, THERE ARE TOO MANY NUMBERS.

Summary of the Bag OperationsSummary of the Bag Operations

A bag can be put in its A bag can be put in its initial stateinitial state, which , which is an empty bag.is an empty bag.

Numbers can be Numbers can be addedadded into the bag. into the bag. You may check how many You may check how many occurrencesoccurrences of of

a certain number are in the bag.a certain number are in the bag. Numbers can be Numbers can be removedremoved from the bag. from the bag. You can check You can check how manyhow many numbers are in numbers are in

the bag.the bag.

The Bag ClassThe Bag Class

Java classes (introduced in Java classes (introduced in Chapter 2) can be used to Chapter 2) can be used to implement a Collection class implement a Collection class such as a Bag.such as a Bag.

The class definition includes:The class definition includes:

public class Bag

The heading of the definition




class Bag{ public Bag( )...

The heading of the definition A constructor




public class Bag{ public Bag( )... public void add(... public void remove(... ...and so on

The heading of the definition A constructor Public methods



The class definition includes:The class definition includes: The heading of the definitionA constructor Public methods Private instance variables We’ll look at private

instance variables later.

We’ll look at privateinstance variables

later.

public class Bag{ public Bag( )... public void add(... public void remove(... ...and so on

The Bag’s ConstructorThe Bag’s Constructor

Places a bag in the initial state (an empty Places a bag in the initial state (an empty bag)bag)

// Postcondition: The Bag has been initialized// and it is now empty.public Bag( ){ . . .

}

The Add MethodThe Add Method

Adds a new number to the bagAdds a new number to the bag

public void add(int newEntry){

. . .}

The Size MethodThe Size Method

Counts how many integers are in the bag.Counts how many integers are in the bag.

// Postcondition: The return value is the number// of integers in the Bag.public int size( ){

. . .}

The countOccurrences MethodThe countOccurrences Method

Counts how many copies of a number occurCounts how many copies of a number occur

// Postcondition: The return value is the number// of copies of target in the Bag.public int countOccurrences(int target){

. . .}

The Remove MethodThe Remove Method

Removes one copy of a numberRemoves one copy of a number

// Postcondition: If target was in the Bag, then// one copy of target has been removed from the// Bag, and the return value is true; otherwise the// Bag is unchanged and the return value is false.public boolean remove(int target){

. . .}

Using the Bag in a ProgramUsing the Bag in a Program

Here is typical code from a Here is typical code from a program that uses the new program that uses the new Bag class:Bag class:

Bag ages = new Bag( );

// Record the ages of three children:ages.add(4);ages.add(8);ages.add(4);

Documentation for the Bag ClassDocumentation for the Bag Class

The documentation gives The documentation gives specificationsspecifications for the bag for the bag methods.methods.

Specifications are written as Specifications are written as precondition/postcondition precondition/postcondition contracts.contracts.

Everything needed to Everything needed to useuse the the Bag class is included in this Bag class is included in this documentation.documentation.

Bag’s documentationcan be automatically

created with the Javadoc tooldescribed inAppendix H.

A QuizA Quiz

Suppose that a Mysterious Suppose that a Mysterious Benefactor provides you Benefactor provides you with the Bag class, but you with the Bag class, but you are only permitted to read are only permitted to read the documentation. You the documentation. You cannot read the class cannot read the class implementation or .java implementation or .java file. Can you write a file. Can you write a program that uses the Bag program that uses the Bag data type ?data type ?

Yes I can.Yes I can. No. Not unless I see the No. Not unless I see the

class implementation for class implementation for the Bag.the Bag.

A QuizA Quiz

Suppose that a Mysterious Suppose that a Mysterious Benefactor provides you Benefactor provides you with the Bag class, but you with the Bag class, but you are only permitted to read are only permitted to read the documentation. You the documentation. You cannot read the class cannot read the class implementation or .java implementation or .java file. Can you write a file. Can you write a program that uses the Bag program that uses the Bag data type ?data type ?

Yes I can.Yes I can.

You know the name of the You know the name of the new data type, and you new data type, and you also know the headings also know the headings and specifications of each and specifications of each of the operations. This is of the operations. This is enough for you to create enough for you to create and use Bags. and use Bags.

Implementation DetailsImplementation Details

The entries of a bag The entries of a bag will be stored in the will be stored in the front part of an array, front part of an array, as shown in this as shown in this example. example.

[ 0 ][ 0 ] [1][1] [ 2 ][ 2 ] [ 3 ][ 3 ] [ 4 ][ 4 ] [ 5 ][ 5 ] . . .. . .

An array of integersAn array of integers

4 8 4

We don't care what's inWe don't care what's inthis part of the array.this part of the array.


The entries may The entries may appear in any order. appear in any order. This represents the This represents the same bag as the same bag as the previous one. . . previous one. . .


4 4 8


[ 0 ][ 0 ] [1][1] [ 2 ][ 2 ] [ 3 ][ 3 ] [ 4 ][ 4 ] [ 5 ][ 5 ] . . .. . .


. . . and this also . . . and this also represents the same represents the same bag.bag.

An array of integersAn array of integersWe don't care what's inWe don't care what's inthis part of the array.this part of the array.

[ 0 ][ 0 ] [1][1] [ 2 ][ 2 ] [ 3 ][ 3 ] [ 4 ][ 4 ] [ 5 ][ 5 ] . . .. . .

4 4 8


We also need to keep track of how We also need to keep track of how many numbers are in the bag.many numbers are in the bag.


8 4 4


An integer to keepAn integer to keeptrack of the bag's sizetrack of the bag's size

3

[ 0 ][ 0 ] [1][1] [ 2 ][ 2 ] [ 3 ][ 3 ] [ 4 ][ 4 ] [ 5 ][ 5 ] . . .. . .

An ExerciseAn Exercise

Use these ideas to write a Use these ideas to write a list of private instance list of private instance variables could implement variables could implement the Bag class. You should the Bag class. You should have two instance have two instance variables. variables.

You have 60 secondsto write the declaration.

An ExerciseAn Exercise

public class Bag{ private int[ ] data; private int manyItems; ...}

One solution:One solution:

An Example of Calling AddAn Example of Calling Add

public void add(int newEntry)

Before calling add, weBefore calling add, wemight have this bag b:might have this bag b:

2

[ 0 ][ 0 ] [ 1 ][ 1 ] [2][2] . . .. . .

8 4b.datab.data

b.manyItemsb.manyItems


void add(int newEntry)

b.datab.data


We activateWe activateb.add(17)b.add(17)

What values will be inb.data and b.manyItemsafter the methodfinishes ?

[ 0 ][ 0 ] [ 1 ][ 1 ] [2][2] . . .. . .

8 4


2


void add(int newEntry)

After activating b.add(17),After activating b.add(17),we will have this bag b:we will have this bag b:

33

[ 0 ][ 0 ] [1][1] [ 2 ][ 2 ] . . .. . .

8 4 1717


b.datab.data


[ 0 ][ 0 ] [ 1 ][ 1 ] [2][2] . . .. . .

8 4

2

Pseudocode for addPseudocode for add

Make sure there is room for a new entry in Make sure there is room for a new entry in the array.the array.

Place Place newEntrynewEntry in the appropriate location in the appropriate location of the of the datadata array. array.

Add one to the instance variable Add one to the instance variable manyItemsmanyItems.. What is the “appropriate

location” of the data array ?


data[manyItems] = newEntry;manyItems++;



Add one to the instance variable Add one to the instance variable manyItemsmanyItems..


data[ manyItems++] = newEntry;



Add one to the instance variable Add one to the instance variable manyItemsmanyItems..

The Other Bag OperationsThe Other Bag Operations

Read Section 3.2 for the implementations of the Read Section 3.2 for the implementations of the other bag methods.other bag methods.

Remember: If you are just Remember: If you are just usingusing the Bag class, the Bag class, then you don’t need to know how the operations then you don’t need to know how the operations are implemented.are implemented.

Later we will Later we will reimplementreimplement the bag using more the bag using more efficient algorithms. efficient algorithms.

We’ll also have a few other operations to We’ll also have a few other operations to manipulate bags.manipulate bags.

Other Kinds of BagsOther Kinds of Bags

In this example, we have implemented a In this example, we have implemented a bag containing bag containing integersintegers..

But we could have had a bag of But we could have had a bag of float float numbersnumbers, a bag of , a bag of characterscharacters, a bag of , a bag of StringsStrings . . . . . .Suppose you wanted one of these other bags. How much would you need to change in the implementation ?

A Collection class is a class that can hold a group A Collection class is a class that can hold a group of items.of items.

Collection classes can be implemented with a Java Collection classes can be implemented with a Java class.class.

The author of the class should provide The author of the class should provide documentation that another programmer can read documentation that another programmer can read to use the class.to use the class.

Other details are given in Section 3.2, which you Other details are given in Section 3.2, which you should read.should read.

Summary Summary

THE ENDTHE END

Presentation copyright 1999, Addison Wesley LongmanPresentation copyright 1999, Addison Wesley LongmanFor use with For use with Data Structures and Other Objects Using JavaData Structures and Other Objects Using Javaby Michael Main.by Michael Main.

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Students and instructors who use Students and instructors who use Data Structures and Other ObjectsData Structures and Other Objects Using Java Using Java arearewelcome to use this presentation however they see fit, so long as this copyright notice welcome to use this presentation however they see fit, so long as this copyright notice remains intact.remains intact.

a collection class is a data type that is capable of holding a group of items. in java, collection...

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bag operationsa bag

public bag

public class bag

bag postcondition

isalso inthe bag

class definition

new number

certain number