lecture01-arraylists

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CSE 2123

ArrayLists

Jeremy Morris


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A programming problem

Consider the following task:

You have a file containing a list of purchases

Maybe from an online ordering system

You are writing an application that needs to beable to compute:

The mediandollar amount of the purchases

The number of purchases where more money was spent

than median We cant do this as a running total we need to store

the individual purchases to do the computation

How can we store a sequence of data values?

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We need to use an array

Recall what we know about arrays

A sequence of variables of the same data type (e.g. int,

double, String, boolean, etc.) Each variable in the array is called an element

Array elements are accessed through their index

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A programming snippet

Scanner keyboar d = new Scanner ( Syst em. in);

Syst em. out.print("Enter a filename: ");

St r i ng f name = keyboar d. next Li ne( ) ;

Scanner i nFi l e = new Scanner ( new Fi l e( f name) ) ;int numPur chases= i nFi l e. next I nt ( ) ;

double[] pur chases= new double[numPur chases];

int count = 0;

while ( count


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Now consider the following task:

You have a file containing a list of purchases

Maybe from an online ordering system

Your application needs to do what it did before Except you dont know the number of elements in the file

before we read it

We just need to keep reading elements from the file until

we reach the end

How are we going to do that?

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A programming snippet



St r i ng f name = keyboar d. next Li ne( ) ;

Scanner i nFi l e = new Scanner ( new Fi l e( f name) ) ;int numPur chases= i nFi l e. next I nt ( ) ;

double[] pur chases= new double[numPur chases];

int count = 0;

while ( count


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We can use a dynamic array

Just like an array, except it grows as we need it

In Java we can use anArrayListto hold this information

No need to pre-determine the number of elements upfront, just add to the array as we need it

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Classes A Light Introduction

ArrayList is an example of a class

Youve seen other examples of classes before:

String

Scanner File

Well talk (a lot) more about classes in the coming

weeks

The thing to understand now is that a class is acombination of dataand behavior(methods)

We call this encapsulation(more on that later too)

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You already know how to use classes:Scanner keyboar d = new Scanner ( Syst em. i n)

Like any other variable you need to declareit

Give it a name and a type

Unlike a primitive type you also need to instantiate

it

Thats what that magic word newdoes

It constructs a new instance of the class

More about that later too

For now, just keep the syntax in mind

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Unlike primitive types, classes are a combination

of dataand methods

Primitive types (int, char, boolean, double) only contain

data

Methods are all implemented separately

Each class has a number of associated methods

Youve already seen this behavior in action:

St r i ng i nput = keyboar d. next Li ne( ) ;

next Li ne( ) is a method, one defined by the Scanner

class

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ArrayLists

So what does this have to do with ArrayLists?

ArrayList is a Java class

Part of the Java Standard Library

It implements a dynamic array

To use an ArrayList we need to know how to use

it:

How do we declare it?

How can we put things into an ArrayList?

How can we read things from an ArrayList?

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ArrayList Some Sample codeimport j ava. ut i l . Ar r ayLi st ;

public static void mai n( St r i ng [ ] ar gs) {

Ar r ayLi st st r i ngLi st = new Ar r ayLi st ( ) ;

Scanner keyboar d = new Scanner ( Syst em. i n) ;

St r i ng i nput = ;

while ( i nput . equal s( st op) == false) {

i nput = keyboar d. next Li ne( ) ;

st r i ngLi st . add( i nput ) ;

}

int i = 0;

whi l e ( i


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ArrayList - Declaration

Lets start with this thing:

This syntax indicates that the class you are using

is a genericclass

Ar r ayLi st - is the placeholder for the class Can be used with any class

For now, you can think of it as replacing the array type

declaration:St r i ng[ ] st r i ngAr r = new St r i ng[ 10] ;

Ar r ayLi st st r i ngL = new Ar r ayLi st ( ) ;

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ArrayList Some Sample codei mpor t j ava. ut i l . Ar r ayLi st ;








}

int i = 0;

whi l e ( i


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ArrayList - Methods

Add is the main method used to addelements to an ArrayList:

bool ean add( E obj )

Adds an element to the end of the ArrayList

Returns true if it is able to add it, false if not

The type E is a generic type must match the type used to

declare the ArrayList

No real equivalent in arrays, since arrays arent dynamic

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}

int i = 0;

whi l e ( i


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ArrayList - Methods

The method used to access elements:

E get ( i nt i ndex)

Access an element at position index in the ArrayList



Equivalent to accessing an array via a subscript:

Syst em. out . pr i nt l n( st r i ngAr r [ i ndex] ) ;

Syst em. out . pr i nt l n( st r i ngL. get ( i ndex) ) ;

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}

int i = 0;

whi l e ( i


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ArrayList - Methods

We also need to have a way to check howlong our ArrayList is:int si ze( )

Returns back the count of elements in the ArrayList

Equivalent to using the array length attribute:

int c = st r i ngAr r . l engt h;

int c = st r i ngL. si ze( ) ;

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ArrayList - Methods

Since ArrayLists are dynamic, theres asecond method for adding elements:

voi d add( i nt i ndex, E obj )

Adds an element at position index

Moves everything currently in the ArrayList from index on to

the right one position to insert the new element into place

The type E is a generic type must match the type used todeclare the ArrayList

No real equivalent in arrays, since arrays arent dynamic

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ArrayList - Methods

We also want some way of changingelements at a position:

E set ( i nt i ndex, E obj )

Changes the element at position index to the new value

obj



Returns back what was previously stored at that index Equivalent to using the array subscript:

st r i ngAr r [ i ndex] = obj ;

st r i ngL. set ( i ndex, obj ) ;

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ArrayList - Methods

Because ArrayLists are dynamic, we candelete elements from them:

E r emove( i nt i ndex)

Removes the element at index from the list

Everything to the right of index is shifted one position left to

fill the gap

Returns back what weve removed

No real equivalent in arrays because arrays are notdynamic

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ArrayList Example



St r i ng f name = keyboar d. nextLi ne( ) ;

Scanner i nFi l e = new Scanner ( new Fi l e( f name) ) ;

ArrayList pur chases= new ArrayList();

while ( i nFi l e. hasNext ( ) ) {

doubl e pur ch =i nFi l e. next Doubl e( ) ;

pur chases. add( pur ch) ;

}

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Collections - ArrayList

ArrayLists are used to hold objects

We cannot use them to hold primitive types

Ar r ayLi st i nt Li st = new Ar r ayLi st ( ) ;

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Collections - ArrayList




However, each primitive type has a wrapper

class that we can use

i nt classI nt eger char classChar act er doubl e classDoubl e

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Digression Wrapper classes

Used to transform a primitive typeinto a class

Useful for generic classes(like collections) that

require a class and cant use a primitive

Additionally, wrapper classes have their own

useful methods

Example: converting a String to an Integer

St r i ng r at i ngSt r i ng = 3;I nt eger r at i ngI nt eger = new I nt eger ( r at i ngSt r i ng) ;

Example: extracting a primitive typei nt pr i mi t i veRat i ng = r at i ngI nt eger . i nt Val ue( ) ;

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Digression Wrapper Classes

Similar methods exist for Double:St r i ng r at i ngSt r i ng = 3. 0;

Doubl e r at i ngDoubl e = new Doubl e( r at i ngSt r i ng) ;

doubl e pr i mi t i veDoubl e = r at i ngDoubl e. doubl eVal ue( ) ;

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Digression Wrapper Classes

Taking a primitive data element and wrappingit up in a wrapper class is known as boxingI nt eger myI nt = new I nt eger ( 6) ;

Taking the primitive out of the wrapper is

known as unboxingi nt myPr i mi t i ve = myI nt . i nt Val ue( ) ;

Java will (usually) automatically do the right

thing on its own (autoboxing)i nt myPr i mi t i ve = myI nt ;I nt eger myI nt = 6;

i nt sum = myI nt + 3;

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ArrayLists




Instead we use the wrapper object:


But we can use autoboxingto add elements:i nt Li st . add( 12) ;

i nt val ue = i nt Li st . get ( 0) ;

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ArrayLists

If we need an ArrayList of primitives, we use

the corresponding class instead

But we can use autoboxingto make our life

easier adding and examining elements


Ar r ayLi st i nt Li st =

new Ar r ayLi st ( ) ;

i nt Li st . add( 12) ;

int myI nt = i nt Li st . get ( 0) ;

}

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In-class Examplepublic class Ar r ayExampl e {

public static int count Occur r ences( char [ ] ar r ay, char ch) {

int chCount =0;

for ( int l oopCount =0; l oopCount

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