Transcript Ch12.5 - ComSciGate

Java
Methods
TM
Maria Litvin
Gary Litvin
An Introduction
to Object-Oriented Programming
Chapter 12½
ArrayList
Copyright © 2003 by Maria Litvin, Gary Litvin, and Skylight Publishing. All rights reserved.
Objectives:

Review interfaces

Learn about Comparable interface

Learn about the java.util.List interface
and java.util.ArrayList class
12½ - 2
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 - 3
Interfaces: example
Assumed
public
public interface Sellable
{
double getPrice ();
void setPrice (double price);
}
implements
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; }
}
12½ - 4
Interfaces (cont’d)

Why do we need them?
– Situation one:
More generic type
of parameter 
more reusable code
(works for any
“Sellable” object)
public method calculateTax (Sellable obj, double rate)
{
return obj.getPrice() * rate;
}
...
GrorceryItem apple = new GroceryItem("Apple", 0.30);
double tax = calculateTax(apple, 0.05);
...
12½ - 5
Interfaces (cont’d)
– Situation two:
Different types of
objects are mixed
together in the
same array or list
private Sellable[ ] items = new Sellable[3];
items[0] = new GroceryItem(...);
items[1] = new Pizza(...);
items[2] = new JewelryItem(...);
12½ - 6
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 - 7
Comparable (cont’d)
«interface»
java.lang.Comparable
java.lang.String
compareTo is
based on
lexicographical
order
java.lang.Integer
java.lang.Double
compareTo is
based on
numerical values
12½ - 8
Comparable example
public class UsedCar
implements Sellable, Comparable
{
private double myPrice;
< ... constructors and other methods... >
public int compareTo(Object other)
{
double diff = getPrice() ((UsedCar) other).getPrice();
Forgetting a
cast or
parentheses
results in a
syntax error
return (int)(100 * diff); // diff in cents
}
}
12½ - 9
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 - 10
List Methods (a Subset)
int size();
boolean isEmpty ();
boolean add (Object obj);
returns true
void add (int i, Object obj);
inserts obj as the
i-th value; i must
be from 0 to size()
Object set(int i, Object obj);
Object get(int i);
Object remove(int i);
boolean contains(Object obj);
int indexOf(Object obj);
i must be from 0 to
size() -1
use equals to
compare objects
12½ - 11
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)
capacity
size
"Cat" "Hat" "Bat"
12½ - 12
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 when
i < 0 or i  size()
12½ - 13
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 - 14
ArrayList (cont’d)

From Java API Docs:
12½ - 15
ArrayList (cont’d)

Example 1
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();
Need a cast to Integer
in order to call intValue
12½ - 16
ArrayList (cont’d)

Example 2
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++;
}
Shifts all the values
after the i-th to the left
and decrements the
size
12½ - 17
Lab: Index Maker
fish.txt
One fish
two fish
Red fish
Blue fish.
Black fish
Blue fish
Old fish
New fish.
This one has
a little star.
This one has a little car.
Say! What a lot
of fish there are.
fishIndex.txt
A 12, 14, 15
ARE 16
BLACK 6
BLUE 4, 7
CAR 14
FISH 1, 2, 3, 4, 6, 7, 8, 9, 16
HAS 11, 14
LITTLE 12, 14
LOT 15
NEW 9
OF 16
OLD 8
ONE 1, 11, 14
RED 3
SAY 15
STAR 12
THERE 16
THIS 11, 14
TWO 2
WHAT 15
12½ - 18
Index Maker (cont’d)
<<interface>>
java.util.List
IndexMaker
extends
DocumentIndex
implements
java.util.ArrayList
has
IndexEntry
12½ - 19
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 - 20
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?
12½ - 21