Lists and the Collection Interface

Chapter 4

2

The List Interface and ArrayList Class

• So far, all we have is an array for storing a collection of elements.

• An array is an indexed structure: can select its elements in arbitrary order using a subscript value

• Elements may be accessed in sequence using a loop that increments the subscript

• You cannot• Increase or decrease the length• Add an element at a specified position without shifting

the other elements to make room• Remove an element at a specified position without

shifting other elements to fill in the resulting gap

3

The List Interface

• Some of the allowed operations on the List interface (java.util.List) include:• Finding a specified target• Adding an element to either end or in the middle• Removing an item from either end or from the middle• Traversing the list structure without a subscript

• Not all classes implementing the List interface perform the allowed operations with the same degree of efficiency

4

The List Interface and ArrayList Class

5

The ArrayList Class

• Simplest class that implements the List interface• Improvement over an array object

• expandable! (ans shrinkable!)• Mostly used when a programmer wants to add new

elements to the end of a list but still needs the capability to access the elements stored in the list in arbitrary order

• How to construct and ArrayList?

ArrayList myList = new ArrayList();

//each element is an Object

6

The ArrayList Class

• How to construct and ArrayList? (Java 5.0)

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

//each element is a String

7

The ArrayList Class• size• int s = myList.size(); //returns 4 for the ArrayList below

8

The ArrayList Class• Add at a specified index

myList.add(2, “Doc”);• Add at the end

myList.add(“Dopey”);

9

The ArrayList Class• Illegal if index > size()

myList.add(8, “Doc”);

10

The ArrayList Class (continued)

• Remove at a certain index

myList.remove(1);

11

The ArrayList Class (continued)

• cannot index like an array

myList[i] ! //illegal

• use

myList.get(i);

myList.set(2, “Sneezy”);

myList.set(6, “Sneezy”); //illegal if index > size()-1

12

The ArrayList Class (continued)

• Search using

myList.indexOf(“Sneezy”); // returns 2

myList.indexOf(“Jumpy”); // returns -1

13

Specification of the ArrayList Class

14

Application of ArrayList

• The ArrayList gives you additional capability beyond what an array provides

• For Java 1.4.2• ArrayList stores items of type Object and can thus

store an object of any class• You cannot store values of the primitive types directly

but must instead use wrapper classes Example??

• When an object is stored in an ArrayList, the programmer must remember the original type

String entry = (String) myList.get(1);

15

Application of ArrayList

• For Java 5.0 and higher:• You cannot store values of the primitive types directly but must

instead use wrapper classes. However, autoboxing/autounboxing are in effect

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

B.add(42); //autoboxing

int temp = B.get(0); //autounboxing

• No casting required when retrieving from the ArrayList

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

C.add(“Hello”);

String entry = myList.get(0);

16

Recall PhoneDirectory?• Instance variable theDirectory

private DirectoryEntry [ ] theDirectory = new DirectoryEntry[capacity];

• ArrayBased addOrChangeEntry methodpublic String addOrChangeEntry(String name, String number) { String oldNumber = null; int index = find(name); if (index > -1) { oldNumber = theDirectory[index].getNumber(); theDirectory[index].setNumber(number); } else { add(name, number); } modified = true; return oldNumber; }

17

Using an ArrayList in PhoneDirectory• Instance variable theDirectory

private ArrayList<DirectoryEntry> theDirectory = new ArrayList<DirectoryEntry>();

• addOrChangeEntry method

public String addOrChangeEntry(String name, String number) { String oldNumber = null; int index = theDirectory.indexOf(new DirectoryEntry(name, “”));//assuming DirectoryEntry class has an equals method that compares entries by name if (index != -1) { DirectoryEntry de = theDirectory.get(index); oldNumber = de.getNumber(); de.setNumber(number); } else { theDirectory.add(new DirectoryEntry(name, number)); } modified = true; return oldNumber;}

18

Assignment #1 (Due Thursday, Sep 24)

• Design and implement the Programming Project #2 described at the end of Chapter 1 (page 56), using an ArrayList as the underlying storage.

• Provide UML diagrams for your design.

• Document your code well.

• Place all your file under a directory called YourNameHW1 and zip the directory. Email a single zipped file to fatalay@sju.edu

• Submit a hardcopy of your code together with your UML diagram in class on the due date.

19

Implementation of an ArrayList Class

• KWArrayList: simple implementation of a ArrayList class• Physical size of array indicated by data field capacity• Number of data items indicated by the data field size

20

Implementation of an ArrayList Class (continued)

21

Performance of KWArrayList and the Vector Class

• Set and get methods execute in constant time• Inserting or removing elements is linear time• Initial release of Java API contained the Vector class

which has similar functionality to the ArrayList• Both contain the same methods

• New applications should use ArrayList rather than Vector