Peter Sander ESSI-Université de Nice Sophia Antipolis 4 java util ArrayList ❍ Solution ○ la classe java util ArrayList ○ c'est la classe la plus utilisée
ArrayList Exemple avec ArrayList package eclipse classes; import java util ArrayList; public class Main { public static void main(String[] args) { ArrayList list
An ArrayList is an object that can store a group of other objects for us and allow us to manipulate those objects one by one For example, we could use an ArrayList to store all the String names of the pizza toppings offered by a restaurant, or we could store all the URLs that make up a user's favorite sites
En java, les chaînes de caractères sont des objets Nous allons apprendre dans ce chapitre à mieux les utiliser La seconde classe s'appelle ArrayList
code will remove all the elements of ArrayList whose object is obj obj clear(); Examples package com tutorialspoint; import java util ArrayList; public class
The ArrayList class provided by Java is essentially a way to create an array that can grow For example, in our trivia game we declared the array to be of size 5
10 déc 2018 · add some elements to them; and then print out the collections: ArrayList quick example: List listStrings = new ArrayList();
Example: reading words from a file, but you don't ArrayList is a class in Java java lang ClassCastException: line xx • To get a "String" from ArrayList, we
Example: // Removes all plural words from the given list public static void removePlural(ArrayList list)
value stored in this field, in the example above, by writing a length ArrayLists java util Class ArrayList java lang Object java util AbstractCollection
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ArrayList
James Brucker
Limitations of Arrays
You allocate space for array when you create it:
numWords = console.nextInt();
String [ ] words = new String[numWords];
What if you don't know the size of data in
advance? Example: reading words from a ifile, but you don't know how many words are in the ifile?
After you create an array, you cannot change the
size.
ArrayList
ArrayList food = new ArrayList( );
food.size(); // returns 0. Its empty food.add("Apple"); food.add("Banana"); food.size(); // returns 2
System.out.println( food.get(0) ); // Apple
System.out.println( food.get(1) ); // BananaArrayList is an alternative for variable size data yArrayList is an ordered collection of elements yArrayList grows and shrinks as needed! ycan add, delete, replace objects anywhere yArrayList is a class in Java List and ArrayList List is a basic data type (not a class). yin Java,
List is an interface, not a class
yyou cannot create "List" objects
ArrayList is a class that behaves like a
Listyyou can ignore "List" for now
Untyped ArrayList is a zoo
ArrayList list = new ArrayList( );
list.add( "Apple" ); // a string list.add( LocalDate.now() ); // LocalDate object list.add( new Double(3.14) ); // another object // Get something from arraylist
Object obj = list.get(1);
String fruit = (String) list.get(0);•A plain "ArrayList" accepts any kind of Object •When you "get" an element, it always returns type Object
Using a cast is dangerous
// Get Strings. Must cast the result
String fruit = (String) list.get(0);
// If result is not a String, an Exception occurs
String fruit2 = (String) list.get(1);
java.lang.ClassCastException: line xx•To get a "String" from ArrayList, we must cast the result to String. •What if the element is not a String?
Typed ArrayList
ArrayList fruit =
new ArrayList
( ); list.add( "Apple" ); // a string list.add( "Orange" ); // string list.add( new Double(3.14) ); // Compile Error // Compiler will not allow to add a Double // No cast! Result is automatically String String s = list.get(1); // No cast! •Arraylist for String (only): ArrayList • is called a type parameter. •Type can be any class name, but not primitive "ArrayList of String" ArrayList means "ArrayList of Strings"
ArrayList means "ArrayList of Food"
Common operations
ArrayList fruit =
new ArrayList( ); list.add( "Apple" ); // add at end of list (0) list.add( "Orange" ); // add at end of list (1) list.add( 1, "Banana" ); // add at index 1 list.size( ); // 3 things in list list.get(1); // "Banana" was inserted list.get(2); // "Orange" was pushed down list.contains("Fig") // false list.remove("Apple") // remove first occurence list.get(0) // "Banana" Demo View and inspect an ArrayList using BlueJ.
Notice what happens when number of items in
ArrayList increases.
Useful ArrayList Methods
int size( )returns # items in ArrayList add( T obj ) add an object to ArrayList (at end) add( int k, T obj )add obj at position k (push others down) T get(int index)get object at given index T remove(int index)delete item from ArrayList & return it clear( )remove all items from List set(int index, T obj) replace the object at index contains( T obj )"true" if obj is in ArrayList ensureCapacity(int size)make sure ArrayList can hold at least this many elements without resizing T = the type used to create ArrayList, can be String, Person, Food,...
ensureCapacity( ) improved eiÌifiÌiciency when you are adding a lot of items to an Arraylist. Working with ArrayList
Some useful methods
Iterate over all the elements
ArrayList menu = Restaurant.getMenu( );
for(int k=0; kSystem.out.println( list.get(k) ); }•Print everything in the restaurant menu •Print the menu using a for-each loop ArrayList list = Restaurant.getMenu( );
for( String menuItem: menu ) { System.out.println( menuItem );
Copying ArrayList to Array
ArrayList list = new ArrayList( ); ... read all the data and save in list // create an array large enough to store the data String [ ] words = new String[ list.size( ) ];
// copy ArrayList to Array list.toArray( words );•Use an ArrayList to save data when you don't know how big the data set is. •list.toArray( array ) - copy to Array Sorting
ArrayList list = Restaurant.getMenu( );
Collections.sort( list ); // sorts the menu Sort an ArrayList using the java.util.Collections class yCollections.sort( anyList ) yanyList must contain objects that are Comparable y String, Double, Long, Int, Date... y any class that has a compareTo method Summary
ArrayList is a collection that:
elements are ordered can add, remove, or set elements at any pos'n duplicate values are allowed size grows/shrinks automatically ArrayList is not an array.
More Information
Big Java, Chapter 7 or Core Java, Volume 1. Java Tutorial - has examples Java API documentation.quotesdbs_dbs2.pdfusesText_2
[PDF] Collections Collections Collections javautilArrayList