ArrayLists - UNSW Engineering

ArrayListsCOMP1400 Week 8

Wednesday, 12 September 12

Working with arraysThere are a number of common actions we want to do with arrays:

• adding and deleting

• copying

• looking for a particular element

• counting the elements

ArraysArrays are intrinsically fixed-length storage.

Adding or deleting elements from arrays is cumbersome:

• create a new larger/smaller array

• copy the unaffected elements

• add/remove the affected element

AbstractionRather than rewriting methods that perform these actions every time we use an array, we should abstract them into a new object.

What we want is a new class that implements a variable-length list.

The public interface should provide methods to implement the actions listed above.

ArrayListThe ArrayList class in the Java Class Library provides this capability.

http://docs.oracle.com/javase/1.5.0/docs/api/java/util/ArrayList.html

EncapsulationThe ArrayList class abstracts and encapsulates all the methods for implementing a variable-length list using fixed-length arrays.

ArrayList<String> private String[] myList; public void add(String s); public String get(int pos);

Generic classesEach ArrayList holds a specific type (or class) of object.

This class is given as a type parameter in angle brackets <> after the name.

The type of the methods get, set and add depend on the given type.

Parameterised types are called generic classes.

Type parametersWhen we create a variable that holds an ArrayList we also need to provide the type of objects it contains:

ArrayList<String> listOfStrings;

ArrayList<Integer> listOfInts;

ArrayList<Book> listOfBooks;

These are called the type parameters.

collection class

itemclass

collection class

itemclass

anglebrackets

List of Strings ArrayList<String> classRoll = new ArrayList<String>();

classRoll.add("Sim Mautner");

String who = classRoll.get(0);

// who is now “Sim Mautner”

List of intsThis does not work:

ArrayList<int> scores = new ArrayList<int>();

scores.add(1);

int s = scores.get(0);

The type parameter must be a class not a primitive type.

List of intsInstead we use:

ArrayList<Integer> scores = new ArrayList<Integer>();

scores.add(1);

int s = scores.get(0);

The Integer class is an object-wrapper for the primitive type int.

Wrapper ClassesJava implements wrapper classes to convert primitive types into objects automatically.

Integer intObject = 1;

Double doubleObject = 2.0;

Boolean boolObject = true;

Character charObject = ‘x’;

Importing

Since ArrayLists are part of the JCL, we must first import them before use:

import java.util.ArrayList;

ConstructionTo construct an ArrayList we will usually use the simple no-parameter constructor:

// create an empty list

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

There are two other constructors but we won’t be using them much.

Adding itemsA newly constructed list is empty.

We can add items using the add method.

classRoll.add("Malcolm"); classRoll.add("Claude");

The type of objects we add must match the type parameter.

classRoll.add(47); // ERROR

size 0 1

“Malcolm”

“Claude”

Adding duplicatesWe can add something to a list more than once.

classRoll.add("Malcolm"); classRoll.add("Malcolm"); size

0 1 2 3

“Malcolm”

“Claude”

“Malcolm”

SizeWe can use the size method to access the number of objects on the list:

int nStudents = classRoll.size();

// nStudents == 2

classRoll.add("Sim");

nStudents = classRoll.size();

// nStudents == 3

size 0 1

“Malcolm”

“Claude”

SizeWe can use the size method to access the number of objects on the list:

int nStudents = classRoll.size();

// nStudents == 2

classRoll.add("Sim");

nStudents = classRoll.size();

// nStudents == 3

size 0 1

“Malcolm”

“Claude”

size 0 1 2

“Malcolm”

“Claude”

“Sim”

GetWe can access elements on the list by their index using the get method.

String me = classRoll.get(0);

// me == "Malcolm"

String him = classRoll.get(1);

// him == "Claude"

IndexingNote that the indices always start from 0 and end at size-1.

classRoll.size(); // returns 3 classRoll.get(0); // returns "Malcolm" classRoll.get(2); // returns "Sim" classRoll.get(3); // ERROR

size 0 1 2

“Malcolm”

“Claude”

“Sim”

RemoveWe can use the remove method to remove the element at a given index.

The other elements get renumbered.

classRoll.remove(1);

classRoll.size(); // returns 2

classRoll.get(1); // "Sim"

size 0 1 2

“Malcolm”

“Claude”

“Sim”

RemoveWe can use the remove method to remove the element at a given index.

The other elements get renumbered.

classRoll.remove(1);

classRoll.size(); // returns 2

classRoll.get(1); // "Sim"

size 0 1

“Malcolm”

“Sim”

CopyingThere is a special constructor for creating a copy of a list:

ArrayList<String> copy = new ArrayList<String>( classRoll);

copy.add("Troy");

“Malcolm”

“Claude”

size 0 1

“Malcolm”

“Sim”

copy.add("Troy");

“Malcolm”

“Claude”

size 0 1

“Malcolm”

“Sim”

size 0 1

“Malcolm”

“Sim”

copy.add("Troy");

“Malcolm”

“Claude”

size 0 1

“Malcolm”

“Sim”

size 0 1 2

“Malcolm”

“Sim”

“Troy”

Other methodsThere are other methods you can read about in the API documentation:

• clear - remove all elements

• set - set value of a given element

• insert - insert an element at an index

• contains - test if an element is on the list

For-each loopA for-each loop is used to iterate over all the elements of a collection:

for (String who : classRoll) {

// action performed on // each element

System.out.println(who);

element type

element variable

element type

element variable

list variable

Arrays vs ArrayLists// declaring array variables

String[] arrayOfStrings;int[] arrayOfInts;

// declaring list variables

ArrayList<String> listOfStrings;ArrayList<Integer> listOfInts;

Arrays vs ArrayLists// constructing arrays

arrayOfStrings = new String[5]arrayOfInts = new int[10]

// constructing lists (empty)

listOfStrings = new ArrayList<String>();listOfInts = new ArrayList<Integer>();

Arrays vs ArrayLists// measuring size

int arraySize = arrayOfStrings.length;

int listSize = listOfStrings.size();

Arrays vs ArrayLists// getting elements

String s1 = arrayOfStrings[2];int i1 = arrayOfInts[1];

String s2 = listOfStrings.get(2);int i2 = listOfInts.get(1);

Arrays vs ArrayLists// setting elements

arrayOfStrings[2] = "Malcolm";arrayOfInts[1] = 40;

listOfStrings.set(2, "Malcolm");listOfInts.set(1, 40);

Arrays vs ArrayLists// iterating

for (int i = 0; i < arrayOfStrings.length; i++) {

// process arrayOfStrings[i]

foreach (String s : listOfStrings) {

// process s

Arrays vs ArrayLists// Copying

String[] copyArray = new String[arrayOfStrings.length+1];for (int i = 0; i < arrayOfStrings.length; i++) { copyArray[i] = arrayOfStrings[i];}

ArrayList<String> copyList = new ArrayList<String>(listOfStrings);

Arrays vs ArrayLists// Adding an element

String[] copyArray = new String[arrayOfStrings.length+1];for (int i = 0; i < arrayOfStrings.length; i++) { copyArray[i] = arrayOfStrings[i];}copy[arrayOfStrings.length] = value;arrayOfStrings = copyArray;

listOfStrings.add(value);

Other collectionsSets (unordered, no duplicates):

http://docs.oracle.com/javase/1.5.0/docs/api/java/util/HashSet.html

Maps (look-up tables with key/value pairs):

http://docs.oracle.com/javase/1.5.0/docs/api/java/util/HashMap.html

ArrayLists - UNSW Engineering

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