COP 3503 FALL 2012 Shayan Javed Lecture 8

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COP 3503 FALL 2012SHAYAN JAVED

LECTURE 8

Programming Fundamentals using Java

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ArrayList and Java Generics

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Collection

A container object that groups multiple objects

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Collection

A container object that groups multiple objects Example: Arrays

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Collection


Java provides multiple classes/interfaces in the Java Collection Framework

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Collection


Java provides multiple classes/interfaces in the Java Collection Framework Going to look at ArrayList

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Arrays

Hold several elements (primitives/objects) of the same type

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Arrays

Hold several elements (primitives/objects) of the same type

Size = static. Once set, cannot be changed

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Arrays

Advantages: Easy to access elements directly

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Arrays

Advantages: Easy to access elements directly Easy to traverse

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Arrays


Disadvantages: Have to decide on size before creating

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Arrays


Disadvantages: Have to decide on size before creating Inserting/removing elements is troublesome

Shifting elements

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ArrayList (java.util.ArrayList)

Holds several objects in order (like Arrays)

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Unlike Arrays: Size does not need to be specified at creation time

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Unlike Arrays: Size does not need to be specified at creation time Grows in size as items added

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Unlike Arrays: Size does not need to be specified at creation time Grows in size as items added

Provides useful methods for manipulating the list

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Constructors: new ArrayList( ) - no need to pass in size (10)

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Constructors: new ArrayList( ) - no need to pass in size (10) new ArrayList(int) - initial capacity

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Methods: boolean add(Object o): Appends object at the end of the list

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Methods: boolean add(Object o): Appends object at the end of the list void add(int index, Object o): Inserts object at specified index

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Methods: boolean add(Object o): Appends object at the end of the list void add(int index, Object o): Inserts object at specified index void clear(): Removes all elements

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Methods: boolean add(Object o): Appends object at the end of the list void add(int index, Object o): Inserts object at specified index void clear(): Removes all elements boolean contains(Object o): Returns true if object is in the list

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Methods: boolean add(Object o): Appends object at the end of the list void add(int index, Object o): Inserts object at specified index void clear(): Removes all elements boolean contains(Object o): Returns true if object is in the list Object get(int index): Returns object at the index

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Methods: boolean add(Object o): Appends object at the end of the list void add(int index, Object o): Inserts object at specified index void clear(): Removes all elements boolean contains(Object o): Returns true if object is in the list Object get(int index): Returns object at the index boolean remove(Object o): Removes the object from the list

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Methods: boolean add(Object o): Appends object at the end of the list void add(int index, Object o): Inserts object at specified index void clear(): Removes all elements boolean contains(Object o): Returns true if object is in the list Object get(int index): Returns object at the index boolean remove(Object o): Removes the object from the list Object remove(int index): Removes the object at the index

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Methods: boolean add(Object o): Appends object at the end of the list void add(int index, Object o): Inserts object at specified index void clear(): Removes all elements boolean contains(Object o): Returns true if object is in the list Object get(int index): Returns object at the index boolean remove(Object o): Removes the object from the list Object remove(int index): Removes the object at the index int size(): Returns the no. of elements in

the list

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the list int indexOf(Object o): Returns index of first matching element

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the list int indexOf(Object o): Returns index of first matching element Object set(int index, Object o): Sets the object at the index

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the list int indexOf(Object o): Returns index of first matching element Object set(int index, Object o): Sets the object at the index Object clone(): Returns a shallow copy

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ArrayList Example

import java.util.ArrayList;

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ArrayList Example


ArrayList numbers = new ArrayList(); // no size specified initially

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ArrayList Example



numbers.add(new Integer(10));

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ArrayList Example



numbers.add(new Integer(10));numbers.add(3); /* auto-boxing: converted to numbers.add(new

Integer(3)); */

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ArrayList Example




Integer(3)); */

int sum = 0; // sum up all the numbers in the list

int number = 0;

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ArrayList Example




Integer(3)); */


int number = 0;for(int i = 0; i < numbers.size(); i++) {

}

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ArrayList Example




Integer(3)); */



number = (Integer)numbers.get(i); // cast required – why?

}

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ArrayList Example




Integer(3)); */



number = (Integer)numbers.get(i); // cast required – why?sum += number; // auto un-boxing: Integer converted to int

}

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ArrayList Example




Integer(3)); */numbers.add(“A String”); // will compile!int sum = 0; // sum up all the numbers in

the listint number = 0;for(int i = 0; i < numbers.size(); i++) {

number = (Integer)numbers.get(i); // cast required – why?sum += number; // auto un-boxing: Integer converted to int

}

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ArrayList Example




Integer(3)); */numbers.add(“A String”); // will compile!int sum = 0; // sum up all the numbers in

the listint number = 0;for(int i = 0; i < numbers.size(); i++) {

number = (Integer)numbers.get(i); // run-time error when i = 2sum += number; // auto un-boxing: Integer converted to int

}

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ArrayList

How do we make sure we are dealing withobjects of only one type?

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ArrayList

How do we make sure we are dealing withobjects of only one type?

Solution: Java Generics

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Java Generics

Generics is the capability to parameterize types

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Java Generics


Can define class/interface/method with generic types – compiler replaces with concrete types.

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Java Generics


Can define class/interface/method with generic types – compiler replaces with concrete types.

Introduced in JDK 1.5

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java.util.ArrayList<E>

Methods: boolean add(E o): Appends object at the end of the

list void add(int index, E o): Inserts object at specified index void clear(): Removes all elements boolean contains(E o): Returns true if object is in the list E get(int index): Returns object at the index boolean remove(E o): Removes the object from the list E remove(int index): Removes the object at the index int size(): Returns the no. of elements in

the list int indexOf(E o): Returns index of first matching element E set(int index, E o): Sets the object at the index E clone(): Returns a shallow copy

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Java Generics

Type of objects it can hold is specified at declaration

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Java Generics

Type of objects it can hold is specified at declaration

// will only hold Integer objectsArrayList<Integer> numbers = new

ArrayList<Integer>();

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ArrayList Example


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

numbers.add(new Integer(10));numbers.add(3); /* auto-boxing: */numbers.add(“A String”); // WON’T COMPILE!



number = (Integer)numbers.get(i); // cast NOT required – why?sum += number; // auto un-boxing: Integer converted to int

}

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ArrayList Example


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

numbers.add(new Integer(10));numbers.add(3); /* auto-boxing: */numbers.add(“A String”); // WON’T COMPILE!



number = numbers.get(i); // cast NOT required – why?sum += number; // auto un-boxing: Integer converted to int

}

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Java Generics

More strictness

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Java Generics

More strictness Fewer errors

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Generic Classes

public class AClass<E> {// properties and methods can use the “E” type

}

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Generic Classes

public class AClass<E> {// properties and methods can use the “E” typeArrayList<E> numbers = new ArrayList<E>();

}

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Generic Methods

public class GenericMethodDemo { public static void main(String[] args ) {

Integer[] integers = {1, 2, 3, 4, 5}; String[] strings = {"London", "Paris", "New York”};

}

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Generic Methods



}

public static <E> void print(E[] list) { for (int i = 0; i < list.length; i++)

System.out.print(list[i] + " "); System.out.println(); }

}

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Generic Methods



GenericMethodDemo.<Integer>print(integers); GenericMethodDemo.<String>print(strings);

}

public static <E> void print(E[] list) { for (int i = 0; i < list.length; i++)

System.out.print(list[i] + " "); System.out.println(); }

}

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Generic Interfaces

public interface AnInterface<E> {// properties and methods can use the “E” type

public void aMethod(E object);}

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Generic Interfaces

public interface Comparable<E> {

public int compareTo(E object);

}

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Generic Interfaces

public interface Comparable<E> {

public int compareTo(E object);

}

public interface Comparator<E> {

public int compare(E object1, E object2);

}

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Generic Interfaces

public Book implements Comparable{

// comparison based on pricepublic int compareTo(Object o) {Book b = (Book)o;if (price > b.getPrice())return 1;else if (price < b.getPrice())return -1;elsereturn 0;}

}

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Generic Interfaces

public Book implements Comparable{

// comparison based on pricepublic int compareTo(Object o) {Book b = (Book)o;if (price > b.getPrice())return 1;else if (price < b.getPrice())return -1;elsereturn 0;}

} HOW WOULD YOU USE JAVA GENERICS?

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Generic Interfaces

public Book implements Comparable<Book>{

// comparison based on pricepublic int compareTo(Book b) {Book b = (Book)o;if (price > b.getPrice())return 1;else if (price < b.getPrice())return -1;elsereturn 0;}

}

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Generic Interfaces


// comparison based on pricepublic int compareTo(Book b) {Book b = (Book)o; // no more casting!if (price > b.getPrice())return 1;else if (price < b.getPrice())return -1;elsereturn 0;}

}

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Generic Interfaces


// comparison based on pricepublic int compareTo(Book b) {Book b = (Book)o; // no more casting!if (price > b.getPrice())return 1;else if (price < b.getPrice())return -1;elsereturn 0;}

}

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Generic Interfaces

public class ReviewComparison implements Comparator {public int compare (Object o1, Object o2) {

Book b1 = (Book)o1;Book b2 = (Book)o2;if (b1.getACR() > b2.getACR())

return 1;if (b1.getACR() < b2.getACR())

return -1;else

return 0;}

}

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Generic Interfaces

public class ReviewComparison implements Comparator<Book> {public int compare (Book b1, Book b2) {Book b1 = (Book)o1;Book b2 = (Book)o2;if (b1.getACR() > b2.getACR())return 1;if (b1.getACR() < b2.getACR())return -1;elsereturn 0;}

}

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Generic Interfaces

public class ReviewComparison implements Comparator<Book> {public int compare (Book b1, Book b2) {Book b1 = (Book)o1;Book b2 = (Book)o2;if (b1.getACR() > b2.getACR())return 1;if (b1.getACR() < b2.getACR())return -1;elsereturn 0;}

}

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Java Generics - Summary

Allows you to create a “general” template

Classes/methods/interfaces

Versatile and strict

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Back to ArrayList...

How does it work “behind the scenes”?

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Elements are stored inside an array private array

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Array has an initial capacity

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Array has an initial capacity Empty constructor [new ArrayList()]: capacity 10

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Array has an initial capacity Empty constructor [new ArrayList()]: capacity 10 Otherwise specify capacity [new ArrayList(capacity)]

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ArrayList

When number of elements exceeds capacity (the add/insert method):

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ArrayList

When number of elements exceeds capacity (the add/insert method): Internal array replaced by new one

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ArrayList

When number of elements exceeds capacity (the add/insert method): Internal array replaced by new one Elements from the old array copied over

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ArrayList


Insert and remove require shifting of elements

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ArrayList


Insert and remove require shifting of elements (Recall add/remove in Project1)

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Efficiency of ArrayLists

Reallocation of underlying array = when capacity reached and need to add more elements

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Add/Insert/Remove = costly because requires shifting of elements in array

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Accessing element (get) = fast just like an array

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Accessing element (get) = fast just like an array

Problems overcome with Linked Lists.

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Other Java Collections

LinkedList

Stack

Queue

Will look at these later

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COP 3503 FALL 2012 Shayan Javed Lecture 8

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