arraylists 1 - ahs-dib.info · arraylists 1 1/26/17 . objective • understand difference between...

ArrayLists 1

1/26/17

Objective

• Understand difference between Arrays and ArrayLists

• Using Array Lists

11-2

Arrays – Objects or Not?

• Why the ambiguity about whether arrays are objects?

– Because for an object, they are pretty lame.

– All they bring to the table is that they are passed by reference and they have one field (length).

– True objects would have plenty of methods to use that would make the clients job much easier.

11-3

So what is an ArrayList?

• An ArrayList is a class that handles all of the common operations done with arrays.

– Its field are:

• Object[] arr

• int capacity

• int size

- It’s methods include:

- Constructors (2), remove(), add(), set(), get(), remove(), indexOf(), size(), contains(), toString()

Array of Strings – removing elements

String[] myStringArray = new String[10];

myStringArray[0] = “hi”;

myStringArray[1] = “bye”;

myStringArray[2] = “hola”;

myStringArray[3]= “adios”;

int size = 4;

System.out.println(myStringArray[2]); //hola

myStringArray[1]=myStringArray[2];

//for loop to find index of “hola”

//to delete index, for loop to move items to the right ove

//keep track of size.

“hi” “bye” “hola” “adios”

ArrayList of Strings – removing elements, much easier for client

Appears the same but easier to manage; ArrayList<String> myArrayList = new ArrayList<String>();

myArrayList.add(“hi”);

myArrayList.add(“bye”);

myArrayList.add(“hola”);

myArrayList.add(“adios”);

System.out.println(myArrayList.get(2)); //hola

int index = myArrayList.indexOf(“hola”);

myArrayList.remove(1); //removes “bye”, shifts hola and

//adios over, decreases size

“hi” “bye” “hola” “adios”

Calling the ArrayList constructor

//constructs an empty ArrayList of strings, default

//capacity is 10

ArrayList<String> myList = new ArrayList<String>();

//constructs an ArrayList of Integers with a capacity of

//50.

ArrayList<Integer>myIntList = new ArrayList<Integer>(50);

*The ArrayList constructor is overloaded. If no arguments passed in, a list with a default capacity of 10 is created.

*If you know your size from the beginning, you should use the second constructor, so time is not wasted resizing later.

11-7

Why use an ArrayList instead of an array?

• Common methods available

• We don’t have to keep track of size, we just call size() method.

• Array will automatically increase in size as needed when adding objects.

• When removing objects, array will decrease and move items over as needed.

• Print name of list, and entire contents is printed

11-8

Why use an Array instead of ArrayList?

• Can use primitive data with arrays

• Can initialize in one line

• Can update values in one line

• Takes up less space (memory), generally more efficient

11-10

java.util.ArrayList<E>

• Implements a list using an array

• Implements java.util.List<E> interface

• So sometimes you will see List x = new ArrayList<String>

«interface»

java.util.List

java.util.ArrayList java.util.LinkedList

11-11

java.util.ArrayList<E> cont’d

• Can only hold objects (of a specified type), not elements of primitive data types.(Why is this?)

• Keeps track of the list capacity (the length of the allocated array) and list size (the number of elements currently in the list)

"Cat" "Hat" "Bat"

capacity

size

...

11-12

ArrayList<E> Constructors

ArrayList<E> ( )

ArrayList<E> (int capacity)

Creates an empty

ArrayList<E> of

default capacity (ten)

Java docs use the letter E as

the type parameter for elements

in generic collections

Creates an empty

ArrayList<E> of the

specified capacity

11-13

ArrayList<E> Methods (a Subset)

int size()

boolean isEmpty ()

boolean add (E obj)

void add (int i, E obj)

E set(int i, E obj)

E get(int i)

E remove(int i)

boolean contains(E obj)

int indexOf(E obj) both use E’s

equals to compare

objects

i must be from 0 to size() -1**(not

capacity)

inserts obj as the

i-th value; i must

be from 0 to size()

returns true

11-14

ArrayList Example

ArrayList<String> names =

new ArrayList<String>( );

names.add("Ben");

names.add("Cat");

names.add(0, "Amy");

System.out.println(names);

[Amy, Ben, Cat]

Output ArrayList’s toString

method returns a string of

all the elements, separated

by commas, within [ ].

11-15

ArrayList<E> - Some things to know

• Automatically increases (doubles) the capacity when the list runs out of space (allocates a bigger array and copies all the values into it).

• get(i) and set(i, obj) are efficient because an array provides random access to its elements.

• Throws IndexOutOfBoundsException when

i < 0 or i size()

(or i > size() in add (i, obj) )

11-16

ArrayList<E> Autoboxing

• If you need to put ints or doubles into a list, use a standard Java array or convert them into Integer or Double objects

• Since Java 5, conversion from int to Integer and from double to Double is, in most cases, automatic (a feature known as autoboxing or autowrapping); the reverse conversion (called autounboxing) is also automatic.

11-17

ArrayList<E> Autoboxing Example

ArrayList<Integer> counts =

new ArrayList<Integer>( );

counts.add(17);

...

int count = counts.get(0);

Autoboxing: compiled as

counts.add(new Integer(17));

Autounboxing: count

gets the value 17

11-18

Look at ArrayListDemo.java

• What will print?

Homework 1-4

1. True or False: a. An ArrayList can contain multiple references to

the same object b. The same object may belong to two different

ArrayLists c. ArrayList’s remove method destroys the object

after it has been removed from the list. d. ArrayList’s add method makes a copy of the

object and adds it to the list. e. Two variables can refer to the same ArrayList

Homework #2 T/F

The ArrayList class has a method trimToSize() which trims the capacity of the list to its current size. T/F, calling this method:

a. Saves space in the program

b. Will make add(x) calls more efficient

c. Will make add(0, x) calls more efficient

d. Will make get(i) and set(i, x) more efficient.

HW #3

Why might you use a simple Java array as opposed to an ArrayList?

HW #4

What is the output: ArrayList<Integer> myList = new ArrayList<Integer>();

myList.add(0);

myList.add(1);

myList.add(2);

myList.add(0,0);

myList.add(1,1);

myList.add(2,2);

System.out.println(myList);

11-23

Lab: Shuffler • The Shuffler program shuffles the lines of

text, then allows the user to restore the right order.

11-24

Lab: Shuffler (cont’d) • Your job is to write the LineList class, with

one field, ArrayList<String>, and the following methods: – int size () — returns the number of lines

– String get (int k) — returns the line at index k

– void add (String line) — appends line to list

– String remove (int k) — removes and returns the k-th line

– void move(int index, int newIndex) — moves the line at index to the position at newIndex

– void shuffle() — shuffles the lines of text

11-25

Lab: Shuffler (cont’d)

• Use the following algorithm for shuffle:

1. Set n to the size of the list;

2. Randomly select an element among the first n

and swap it with the n-th element;

3. Decrement n by one;

4. Repeat Steps 2-3 while n 2.

Homework

• #5, 6, 7, 8, 9

• Finish lab

• Read 11.5