ArrayList

JavaJavaMethodsMethods

An Introductionto Object-Oriented Programming

Maria Litvin Gary Litvin

Copyright © 2003 by Maria Litvin, Gary Litvin, and Skylight Publishing. All rights reserved.

TM

Chapter 12½

12½ - 2

Objectives:

Review interfaces Learn about Comparable interface Learn about the java.util.List interface

and java.util.ArrayList class

12½ - 3

Interfaces An interface specifies a list of one or more methods, giving only their signatures, but no code A class implements an interface if it supplies code for all methods of that interface If a class C implements an interface I, objects of C acquire a secondary data type, I A class can implement several interfaces but it can extend only one class

12½ - 4

Interfaces: examplepublic interface Sellable{ double getPrice (); void setPrice (double price);}

public class GroceryItem implements Sellable{ private double myPrice;

< ... constructors and other methods not shown >

public double getPrice () { return myPrice; } public void setPrice (double price) { myPrice = price; }}

Assumed public

implements

12½ - 5

Interfaces (cont’d) Why do we need them?

– Situation one:

public method calculateTax (Sellable obj, double rate) { return obj.getPrice() * rate; }

... GrorceryItem apple = new GroceryItem("Apple", 0.30); double tax = calculateTax(apple, 0.05); ...

More generic type of parameter more reusable code (works for any “Sellable” object)

12½ - 6

Interfaces (cont’d)

– Situation two:

private Sellable[ ] items = new Sellable[3];

items[0] = new GroceryItem(...); items[1] = new Pizza(...); items[2] = new JewelryItem(...);

Different types of objects are mixed together in the same array or list

12½ - 7

Comparable Interface public interface Comparable{ /** * Returns a positive integer if this is * "greater than" other, a negative integer if * this is "less than" other, and 0 if this is * "equal" to other */ int compareTo(Object other);}

Kind of like this “minus” other

12½ - 8

Comparable (cont’d)

«interface»java.lang.Comparable

java.lang.String java.lang.Integer java.lang.Double

compareTo is based on

lexicographical order

compareTo is based on

numerical values

12½ - 9

Comparable examplepublic class UsedCar implements Sellable, Comparable{ private double myPrice;

< ... constructors and other methods... >

public int compareTo(Object other) { double diff = getPrice() - ((UsedCar) other).getPrice();

return (int)(100 * diff); // diff in cents }}

Forgetting a cast or parentheses results in a syntax error

12½ - 10

java.util.List Interface The List library interface describes a list of objects in

abstract terms In a list, objects are arranged in sequence

obj0, obj1, ..., objn-1

In Java, a list holds references to objects A list can contain duplicate objects (both

obj1.equals(obj2) and obj1 == obj2)

12½ - 11

List Methods (a Subset)int size();boolean isEmpty ();boolean add (Object obj);void add (int i, Object obj);Object set(int i, Object obj);Object get(int i);Object remove(int i);boolean contains(Object obj);int indexOf(Object obj);

returns true

inserts obj as the i-th value; i must be from 0 to size()

i must be from 0 to size() -1

use equals to compare objects

12½ - 12

java.util.ArrayList Implements List using an array Keeps track of the list capacity (the length of the

allocated array) and list size (the number of elements currently in the list)

"Cat" "Hat" "Bat"

capacitysize

12½ - 13

ArrayList (cont’d) Automatically increases (doubles) the capacity when

the list runs out of space; allocates a bigger array and copies all the values into it

get(i) and set(i, obj) are efficient because an array provides random access to its elements

Throws IndexOutOfBoundsException wheni < 0 or i size()

12½ - 14

ArrayList (cont’d) ArrayList holds objects (of any type) If you need to put ints or doubles into a list, use a

standard Java array or convert them into Integer or Double objects

You have to remember what types of objects your put into the list and may need to cast a retrieved object back into its type

12½ - 15

ArrayList (cont’d) From Java API Docs:

12½ - 16

ArrayList list = new ArrayList (); list.add (new Integer(1)); list.add (new Integer(2)); list.add (new Integer(3));

int sum = 0; for (int i = 0; i < list.size(); i++) sum += ((Integer) list.get(i)) . intValue();

ArrayList (cont’d) Example 1

Need a cast to Integer in order to call intValue

12½ - 17

ArrayList words = new ArrayList (4); words.add ("One"); words.add ("Fish"); words.add ("Two"); words.add ("Fish"); int i = 0;

while (i < words.size() ) { if ( ”Fish".equals (words.get(i)) ) words.remove(i); else i++; }

ArrayList (cont’d) Example 2

Shifts all the values after the i-th to the left and decrements the size

12½ - 18

Lab: Index Maker

One fishtwo fishRed fishBlue fish.

Black fishBlue fishOld fishNew fish.

This one hasa little star.

This one has a little car.Say! What a lotof fish there are.

A 12, 14, 15ARE 16BLACK 6BLUE 4, 7CAR 14FISH 1, 2, 3, 4, 6, 7, 8, 9, 16HAS 11, 14LITTLE 12, 14LOT 15NEW 9OF 16OLD 8ONE 1, 11, 14RED 3SAY 15STAR 12THERE 16THIS 11, 14TWO 2WHAT 15

fish.txt fishIndex.txt

12½ - 19

Index Maker (cont’d)

IndexMaker

IndexEntry

DocumentIndex

<<interface>>java.util.List

java.util.ArrayList

implementsextends

has

12½ - 20

Review: How is an interface different from a class? Can a class implement several interfaces? Can an interface have more than one method? If C is a class, when is the following statement

valid?Comparable x = new C();

What are the methods of Comparable?

12½ - 21

Review (cont’d): Name the List methods that can add a value to

the list. Name the List methods that can tell you whether

a given value is in the list. In an ArrayList, should the indices be less than

the size or less than the capacity? What happens when the size of a List equals the

capacity and you try to add a value?