Transcript ArrayList - UniMAP Portal

ARRAYLIST AND VECTOR
List
Class ArrayList and class Vector belong to List
interface. List is sub class of Collections.
Collections Class provides static methods
that manipulate collections polymorphically. A
List (sometimes known as sequence) is an
ordered Collection that can contain
duplicate elements.
List indices are zero
based. Class ArrayList and Vector are
resizable-array implementation of List.
With various methods available in the
ArrayList class, empty list can be created,
elements can be added, removed, accessed
and we can know the size of the list.
Similarly with Vector class, various methods
are available to create list and manipulate it as
necessary
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Elements stored in ArrayList and
Vector are objects of a subclass, whose
superclass is Object class.
Elements loose all type information. Thus,
before elements can be used, they must be
cast first to a correct type
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Autoboxing occurs when primitive-type
values are inserted into the list of type
ArrayList or Vector
ArrayList and Vector classes are located in
java.util. Hence they must first be imported
before the associated classes can be utilized in the
Java program. To import ArrayList use import
java.util.ArrayList;. To import class Vector use
this; import java.util.Vector; or we can use import
java.util.*;.
The ArrayList Class
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Similar to an array, an ArrayList allows object
storage
Unlike an array, an ArrayList object:
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Automatically expands when a new item is added
Automatically shrinks when items are removed
Requires:
•import java.util.ArrayList;
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Creating and Using an ArrayList
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Create an ArrayList object with no-args
constructor
– ArrayList nameList = new ArrayList();
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To populate the ArrayList, use the add
method:
– nameList.add("James");
– nameList.add("Catherine");
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To get the current size, call the size method
– nameList.size();
// returns 2
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Creating and Using an ArrayList
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To access items in an ArrayList, use the get method
nameList.get(1);
In this statement 1 is the index of the item to get.
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Example: ArrayListDemo1.java
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Using an ArrayList
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The ArrayList class's toString method returns a
string representing all items in the ArrayList
System.out.println(nameList);
This statement yields :
[ James, Catherine ]
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The ArrayList class's remove method removes
designated item from the ArrayList
nameList.remove(1);
This statement removes the second item.
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See example: ArrayListDemo3.java
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Using an ArrayList
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The ArrayList class's add method with one
argument adds new items to the end of the
ArrayList
To insert items at a location of choice, use the add
method with two arguments:
nameList.add(1, "Mary");
This statement inserts the String "Mary" at index 1
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To replace an existing item, use the set method:
nameList.set(1, "Becky");
This statement replaces “Mary” with “Becky”
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See example: ArrayListDemo4.java
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Using an ArrayList
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An ArrayList has a capacity, which is the number of
items it can hold without increasing its size.
The default capacity of an ArrayList is 10 items.
To designate a different capacity, use a
parameterized constructor:
ArrayList list = new ArrayList(100);
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Using a Cast Operator with the get Method
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An ArrayList object is not typed
To retrieve items from an ArrayList, you
must cast the item to the appropriate type
ArrayList nameList = new ArrayList();
nameList.add("Mary"); // Inserts an item
String str = (String)nameList.get(0);
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Try get without the cast to see the effect.
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Example: ArrayListDemo6.java
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Using ArrayList as a Generic Data Type
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You can create a type-safe ArrayList object by
using generics.
For example an ArrayList object for Strings:
ArrayList<String> nameList = new ArrayList<String>();
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The get method no longer requires casts to work.
Example: GenericArrayListDemo1.java
Example: GenericArrayListDemo2.java
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