ArrayLists

The lazy man’s array

What’s the matter here?

int[] list = new int[10];

list[0] = 5;

list[2] = “hey”;

list[3] = 15;

list[4] = 23;

What is it? ArrayList is a Java class that already exists (you

don’t have to write it) An ArrayList allows you to store Objects in

sequential order. An ArrayList has functions that do important array

stuff like:– Get the size– Retrieve the element at an index– Insert an element at a given index– Remove the element at a given index– Change the element at a given index– Find a given element

ArrayList functions int size(): returns the number of elements

** Note: regular arrays have a member variable named length. ArrayList: blah.size( ) Regular array: blah.length

boolean add(Object x): appends x to the end of list; returns true

Object get(int index): returns the element at the specified location (without removing it!)

Object set(int index, Object x): replaces the element at index with x and returns the element formerly at index

void add(int index, Object x): inserts x at position index, sliding elements to the right and adjusting size

Object remove(int index): removes the element at index, sliding elements to the left and adjusting size.

** Remember: Indices start at 0

Using ArrayListsArrayList list = new ArrayList();list.add(“like”);list.add(“beautiful”);list.add(“roses”);for(int j=0; j<list.size(); j++)

System.out.print(list.get(j)+ ” ”);System.out.println();list.add(0, “You”);list.add(1, “smell”);for(int j=0; j<list.size(); j++)

System.out.print(list.get(j)+ ” ”);System.out.println();list.remove(3);list.set(3, “POO!”);for(int j=0; j<list.size(); j++)

System.out.print(list.get(j)+ ” ”);

like beautiful rosesYou smell like beautiful roses.You smell like POO!

0 1 2 3 4 5 6

list

like

beautiful

roses

ArrayList list = new ArrayList();list.add(“like”);list.add(“beautiful”);list.add(“roses”);for(int j=0; j<list.size(); j++)

System.out.print(list.get(j)+ ” ”);System.out.println();list.add(0, “You”);list.add(1, “smell”);for(int j=0; j<list.size(); j++)

System.out.print(list.get(j)+ ” ”);System.out.println();list.remove(3);list.set(3, “POO!”);for(int j=0; j<list.size(); j++)

System.out.print(list.get(j)+ ” ”);

ArrayList list = new ArrayList();list.add(“like”);list.add(“beautiful”);list.add(“roses”);for(int j=0; j<list.size(); j++)

System.out.print(list.get(j)+ ” ”);System.out.println();list.add(0, “You”);list.add(1, “smell”);for(int j=0; j<list.size(); j++)

System.out.print(list.get(j)+ ” ”);System.out.println();list.remove(3);list.set(3, “POO!”);for(int j=0; j<list.size(); j++)

System.out.print(list.get(j)+ ” ”);

0 1 2 3 4 5 6

list

like

beautiful

roses

You

ArrayList list = new ArrayList();list.add(“like”);list.add(“beautiful”);list.add(“roses”);for(int j=0; j<list.size(); j++)

System.out.print(list.get(j)+ ” ”);System.out.println();list.add(0, “You”);list.add(1, “smell”);for(int j=0; j<list.size(); j++)

System.out.print(list.get(j)+ ” ”);System.out.println();list.remove(3);list.set(3, “POO!”);for(int j=0; j<list.size(); j++)

System.out.print(list.get(j)+ ” ”);

0 1 2 3 4 5 6

list

like

beautiful

roses

You

smell

ArrayList list = new ArrayList();list.add(“like”);list.add(“beautiful”);list.add(“roses”);for(int j=0; j<list.size(); j++)

System.out.print(list.get(j)+ ” ”);System.out.println();list.add(0, “You”);list.add(1, “smell”);for(int j=0; j<list.size(); j++)

System.out.print(list.get(j)+ ” ”);System.out.println();list.remove(3);list.set(3, “POO!”);for(int j=0; j<list.size(); j++)

System.out.print(list.get(j)+ ” ”);

0 1 2 3 4 5 6

list

like

beautiful

roses

You

smell

ArrayList list = new ArrayList();list.add(“like”);list.add(“beautiful”);list.add(“roses”);for(int j=0; j<list.size(); j++)

System.out.print(list.get(j)+ ” ”);System.out.println();list.add(0, “You”);list.add(1, “smell”);for(int j=0; j<list.size(); j++)

System.out.print(list.get(j)+ ” ”);System.out.println();list.remove(3);list.set(3, “POO!”);for(int j=0; j<list.size(); j++)

System.out.print(list.get(j)+ ” ”);

0 1 2 3 4 5 6

list

like

beautiful

roses

You

smell

PO

O!

Dynamic Size

No more logical / physical size dilemma!– With ArrayLists, you DO NOT need to

specify a logical size at instantiation.– ArrayLists are always full, but never filled

up.• Physical size is the same as logical size

(There are no “empty” spaces at the end)• ArrayLists will make room for another element

Using ArrayLists:You Try Declare and instantiate an ArrayList Insert 5 of your favorite words Print out the list (you need a loop) Remove the middle word Print out the list again.

Objects Only! ArrayLists can ONLY store Objects. No primitive

data types allowed! Nice for user defined classes:

– Remember that ALL classes inherit from the Object class.

– Therefore any instance of any class that you write is really an instance of Object, and will fit into the ArrayList!

Not nice for primitive data types:– We have to trick the ArrayList into accepting things

like ints and doubles.– This is accomplished by using “wrapper” classes

Integer and Double

Wrapper Classes: they’re very simple

Integer– Integer(int value): constructor that simply stores

the number value into an object

– int intValue( ): returns the stored number value

Double– Double(double value): constructor that simply

stores the number value into an object

– double doubleValue( ): returns the stored number value

Integer bob = new Integer(15);Integer fred = new Integer(bob.intValue() + 5);System.out.println( fred.intValue( ));

Using Wrapper Classes

ArrayList list = new ArrayList();list.add(3);System.out.println(list.get(0)+ 5);

Take 1: ILLEGAL:3 is NOT an object!

ILLEGAL:Can’t add an Object to an intArrayList list = new ArrayList();

Integer myint = new Integer(3);list.add(myint);Object myobj = list.get(0);System.out.println( ((Integer)myobj).intValue()+5);

Take 2:

LEGAL:Wrap the int first

LEGAL:Cast the Object as an Integer.THEN get it’s value.

Using Wrapper Classes:You Try Declare and instantiate an ArrayList Fill it with 5 of your favorite Integers Write a loop to calculate the SUM Output the sum

Store Several Data Types Unlike in an array, all the elements of an

ArrayList DO NOT need to be of the same type.

You can create an array of Students, but it cannot store Fish!

An ArrayList could store both Students and Fish, since they are both Objects

Student[ ] list = new Student[10];list[0] = new Student(“Sakib”);list[1] = new Fish( ); //ERROR ERROR

ArrayList list = new ArrayList( );list.add( new Student(“Sakib”) );list.add( new Fish( ) ); //NO PROBLEM

ArrayList Amnesia Unfortunately, when you add any element to an

ArrayList, he forgets his data type. When you retrieve the element, he only

remembers that he is an instance of the Object class.

The Object class has the following functions:– boolean equals(Object other)– String toString( )**Note: The data type may override these functions.

If you want to call any function besides these two, you will need to cast the Object. Remind him of his original data type.

ArrayList Amnesia: Example

ArrayList list = new ArrayList();list.add(new CD("Ima Artist", "Ima Sings", 15.50, 3));

list.add(new Circle(5));

list.get(0).getArtist();((CD)list.get(0)).getArtist();((Circle)list.get(1)).getRadius();((Shape)list.get(1)).area();((Shape)list.get(0)).area();

Compile-Time Error!

GOOD

GOODGOOD

RUN-Time Error!

Array vs. ArrayList

Fixed Physical Size– Must specify at

instantiation– Limited storage

capacity Only 1 Data Type

– All elements must be of the SAME type

– Primitive data types allowed

Dynamic Size– Never specify size– The ArrayList never

fills up.

Store Several Types– Can store any

combination of objects

– No primitive data types allowed

Array ArrayList