TheArrayListClass.pdf

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1、The Array List Class Introduction to the Collections Framework or Using the Library My goals with this chapter Be exposed to the Collection Frameworks by looking at the ArrayList class in the library Learn what linked lists are and how to program them Become familiar with the LinkedList class in the

2、 library and see the advantages of using it To become familiar with Iterators and their importance Use the LinkedList class with a generic type The Java Collections Framework An Overview A collection is A group of related data elements,organized into a single object With operations provided to manip

3、ulate the data So far,the only collection we have used is the array.The most common collections are implemented as arrays,linked lists,or trees There are variations on these basic types Using the collections framework reduces your programming effort and increases performance.It implements many usefu

4、l data structures and algorithms The collection framework consists of:Collection Interfaces these interfaces form the basis of the framework They specify methods that must be implemented in classes that implement the interface General-purpose Implementations Classes like ArrayList and LinkedList Abs

5、tract Implementations Partial implementations to make custom collections easier to implement Algorithms Static methods to perform useful functions on collections,such as sorting a list Java predefined Collection interfaces Collection interface One of Collections methods returns an Iterator List inte

6、rface is a sub interface of Collection It has methods that apply specifically to lists Iterator interface It has three easy-to-use methods ListIterator interface is subinterface Iterator The List Interface A list is an expandable collection of elements in which each element has a position or index T

7、here are various ways of storing lists on the computer Some can be accessed in arbitrary order(random access).Some can only be accessed sequentially.Some keep the list sorted by their very nature.Regardless of the way the list is stored,there are certain activities we want to do with every list Add

8、to the list;remove from the list;find an item,etc.Using the general purpose implementations Among others,the library contains a class LinkedList and a class ArrayList Using these predefined classes is easier,faster,and less error prone than writing your own.Much thought has been given to the methods

9、 and their implementation in these prewritten classes And,of course,they are bug free But you must know how to use them Abstract classes,Interfaces and Iterators play a big role in the prewritten classes Using the Array List Class Usually,a list contains one type of data The ArrayList class is expec

10、ts you to say what type of data will be in the class This is done using angle brackets List=new ArrayList();Remember,this is call to the constructor,so dont forget the parentheses.In the Java API the class name is ArrayList The stands for the type of stored in the ArrayList If you do not specify the

11、 type in this way,it will still compile,but the compiler will expect that the array type will be Objects.Generic Collections Generic Collections have several advantages The compiler will pick up incompatible types If you dont specify the type,the ty pe will be Object;then whenever you want to refer

12、to the object,you must downcast from type Object It is recommended that you always use generic collections,UNLESS You have a list that needs to store objects of different types Question:Are Triangle,Square and Circle different types?Implementation of an Array List Class Your text simplifies the Arra

13、yList to look at only a few of the methods and how they are implemented This simplified KWArrayList will be behave the same way as the ArrayList in the Java library Methods to implement Constructors Two overloaded add methods One to add at the end of the array,one to add at a specific index Set and

14、get methods Remove method Data fields of KWArrayList private int INITIAL_CAPACITY private int capacity /the current capacity private int size /the number of actual elements private E the Data;Add to the end of the array The item of type E to be added is the argument.We want the user to know if the a

15、dd was successful,so our method will return true if it was.We also dont want to crash if the array is already full,so must reallocate if it is.Pseudocode Insert the new item at index size Increment size Return true The code to add to the end of an array public boolean add(E anEntry)if(size=capacity)

16、reallocate();theDatasize=anEntry;size+;return true;Add to a specific index Arguments are index and the item to add If we want to add anywhere but the end,we must shift everything after index A couple of housekeeping items If the array is out of bounds,we throw that exception If the array is full,we

17、must reallocate public void add(int index,E item)if(index size)throw new ArrayIndexOutOfBoundsException(index);if(size=capacity)reallocate();/Move data from index to size-1 down for(int i=size;i index;i-)theDatai=theDatai-1;theDataindex=item;+size;The remove method The argument will be the index of

18、the item to be removed After the item is removed,all the items after the item must be moved up The item removed is returned from the method Housekeping If the index is out of bounds,we should throw an exception public E remove(int index)if(index=size)throw new ArrayIndexOutOfBoundsException(index);E

19、 returnValue=theDataindex;for(int i=index+1;i size;i+)theDatai-1=theDatai;size-;return returnValue;The reallocate method This method creates a new array that is twice the size of the current array It then copies the old array into the new one,then resets the reference variable.Constructor public KWA

20、rrayList()capacity=INITIAL_CAPACITY;theData=(E)new Objectcapacity;Notice that this creates an array of type Object,then casts this array to type E E is the type of theData The Vector Class The Vector class comes from older versions of Java It does almost the same thing as the ArrayList class The Vector class is is retained for backwards compatibility In general,it runs slower than the ArrayList class.