AP CS Workshop 2008 1

ArrayList

• It is very common for applications to require us to store a large amount of data.

• Array lists store large amounts of data in a single collection that can be referred to with a single variable.

AP CS Workshop 2008 2

ArrayList

• An ArrayList is a sequence of objects that grows and shrinks as needed.

• The ArrayList class is part of the java.util package of the Java standard class library. – must import java.util.ArrayList

AP CS Workshop 2008 3

ArrayList• Each element in the sequence can be accessed

separately. • We can explicitly overwrite an object at a specified

position in the sequence, thus changing its value.• We can inspect the object at a specified location in the

sequence.• We can add an object into a specified position of the

sequence.• We can add an object to the end of the sequence.

• We can remove an object from a specified location in the sequence.

AP CS Workshop 2008 4

java.util.ArrayList (implements List)

ArrayList is genericArrayList<String>ArrayList<Student>

ArrayList<BankAccount>

AP CS Workshop 2008 5

ArrayLists for AP CS ANotice that the AP CS A Subset does not require the knowledge that ArrayList implements List.

AP CS Workshop 2008 6

java.util.ArrayList<E>for AP CS A

• int size() // returns the number of elements in this list

• boolean add(E x) // appends x to the end of list; returns true

• E get(int index)// returns the element at the specified position in this list.

• E set(int index, E x) // replaces the element at index with x // returns the element formerly at the specified position

AP CS Workshop 2008 7

class java.util.ArrayList<E> for AP CS A

• void add(int index, E x) // inserts x at position index, sliding elements // at position index and higher to the right // (adds 1 to their indices) and adjusts size

• E remove(int index) // removes element from position index, sliding

// subsequent elements to the left (subtracts 1 from their

// indices) and adjusts size// returns the element at the specified position in this list.

AP CS Workshop 2008 8

Course Description AP CS AB

Notice that the AB Subset DOES require the knowledge that ArrayList implements List.iterators

SHOULD BE TAUGHT in A

AP CS Workshop 2008 9

An example:import java.util.ArrayList;

public class ArrayList_01{ public static void main (String [] arg) { System.out.println("ArrayListTest"); ArrayList<String> aList = new ArrayList <String>(); aList.add(new String("Dan")); aList.add("George"); aList.add("Mary"); System.out.println("aList contains:"); for(int x = 0; x < aList.size(); x++) { System.out.println(aList.get(x)); } } }

AP CS Workshop 2008 10

Using enhanced FOR loop:import java.util.ArrayList;

public class ArrayList_01_ForEach{ public static void main (String [] arg) { System.out.println("ArrayListTest"); ArrayList<String> aList = new ArrayList <String>(); aList.add(new String("Dan")); aList.add("George"); aList.add("Mary"); System.out.println("aList contains:");

for(String e:aList)

{

System.out.println(e);

}}

}

AP CS Workshop 2008 11

Other ArrayList methods// ArrayList_02ArrayList<String> students = new ArrayList<String>();students.add("Mary" );students.add("James");students.add("Kevin");students.add(1, "Tanya");String temp = students.get(3);System.out.println(temp); students.remove(2);students.set(1, "John");System.out.println(students.size());

AP CS Workshop 2008 12

What happens?ArrayList_02

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

students.add("Mary");students.add("James");students.add("Kevin");

students.add(1, "Tanya");

String temp = students.get(3);System.out.println(temp);

students.remove(2);

students.set(1, "John");System.out.println(students.size());

Mary James Kevin

Mary Tanya James Kevin

temp

Mary Tanya Kevin

Mary John Kevin

AP CS Workshop 2008 13

An ArrayList is a sequence of objects.

• Array lists can hold any kind of object. For the generic ArrayList, you must include the type of object the ArrayList will hold.– ArrayList<Athlete> athletes = new ArrayList<Athlete>();

– ArrayList<Student> csci6 = new ArrayList<Student>();

– ArrayList<BankAccount> accounts = new ArrayList<BankAccount>();

• The ArrayList method add adds an Object of the type specified in the array list declaration.

• The ArrayList method get returns an Object of the type specified in the array list declaration.

AP CS Workshop 2008 14

A bit more about

toString()

AP CS Workshop 2008 15

What happens?public class StringTest{

public static void main(String[] args) {

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

stringList.add("Fran");stringList.add("Marie");stringList.add("Joe");

System.out.println(stringList);}

}

AP CS Workshop 2008 16

WHY????public class StringTest{

public static void main(String[] args) {

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

stringList.add("Fran");stringList.add("Marie");stringList.add("Joe");

System.out.println(stringList);}

} [Fran, Marie, Joe] Let's look at the API!

AP CS Workshop 2008 17

The ArrayList is defined using generics .

• A generic is a method that is recompiled with different types as the need arises. List<Employee> employeeList = new ArrayList<Employee>();

• Use of generics provides compile-time checking to make sure you are using the correct type Employee emp = employeeList.get(i).getName();

AP CS Workshop 2008 18

ArrayLists contain Objects

• Numbers are not objects.

• You can not have an ArrayList of ints or doubles or booleans or chars.

AP CS Workshop 2008 19

Again:

An ArrayList is a sequence of objects.

• Array lists can hold any kind of object.

• The ArrayList method add adds an Object of specified type.

• The ArrayList method get returns an Object of specified type.

• What if we want our ArrayList to hold ints or doubles?

AP CS Workshop 2008 20

Filling the ArrayList

for (int k = 1; k <= 20; k++)

{

nbrs.add(new Integer(k));

}

AP CS Workshop 2008 21

Printing the ArrayList

for(Integer e : nbrs)

{

System.out.println(e);

}

Integer has a toString method that does the right thing.

AP CS Workshop 2008 22

Printing the sum of the integer values in the ArrayList

int sum = 0;Integer temp; for (int k = 0; k < nbrs.size(); k++){ temp = nbrs.get(k); sum += temp.intValue();}System.out.println("sum equals: " + sum);

AP CS Workshop 2008 23

Printing the sum of the integer values in the ArrayList

//alternate method using enhanced for and auto

//boxing/unboxing

sum = 0;

for (Integer e:nbrs)

{

sum += e;

}

System.out.println("sum equals: " + sum);

AP CS Workshop 2008 24

Java 5.0 introduced

• Auto-boxing and Auto-unboxing (not tested)– The idea of auto-boxing and auto-unboxing is

to make it easier to convert between primitive data types like int, double, and boolean and their equivalent classes, like Integer, Double, and Boolean. It is sometimes frustrating and tedious to have to do such a conversion, especially if results must be converted back to their original form.

AP CS Workshop 2008 25

Autoboxing/unboxing

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

numbers.add(14.5);

numbers.add(3.4);

double sum = numbers.get(0) + numbers.get(1);

AP CS Workshop 2008 26

Auto-boxing and Auto-unboxing

• Integer/int equality tests

int == int OK

Integer.equals(Integer) OK

Integer == int OK

Integer.equals(int) OK

Integer.compareTo(int)==0 OK

AP CS Workshop 2008 27

Auto-boxing and Auto-unboxing

• Integer/int equality tests– int .equals(Integer) NO– int .equals(int) NO– Integer == Integer NO (as test for value

equality)

– Integer.equals(null) NO

AP CS Workshop 2008 28

Auto-boxing and Auto-unboxing

• NOT part of the AP CS Testable Subset.

• Students need to understand– intValue()– doubleValue()

AP CS Workshop 2008 29

ArrayList Applications

Finding values, counting, max and min, average, median, mode…..

Your students should be able to manipulate all sorts of algorithms for ArrayLists.

AP CS Workshop 2008 30

AP CS A 2007 Question 3a

AP CS Workshop 2008 31

AP CS A 2007 Question 3b

AP CS Workshop 2008 32

AP CS A 2007 Question 3b