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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.
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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
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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.
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java.util.ArrayList
(implements List)
ArrayList is generic
ArrayList<String>
ArrayList<Student>
ArrayList<BankAccount>
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ArrayLists for AP CS A
Notice that the
AP CS A Subset
does not require
the knowledge
that ArrayList
implements
List.
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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
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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.
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Course Description AP CS AB
Notice that the
AB Subset
DOES require
the knowledge
that ArrayList
implements
List.
iterators
SHOULD BE
TAUGHT in A
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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));
}
}
}
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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);
}
}
}
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Other ArrayList methods
// 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());
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What happens?
ArrayList_02
ArrayList<String> students = new
ArrayList<String>();
students.add("Mary");
students.add("James");
students.add("Kevin");
Mary
Mary
students.add(1, "Tanya");
James
Tanya
Kevin
James
Kevin
temp
String temp = students.get(3);
System.out.println(temp);
students.remove(2);
students.set(1, "John");
System.out.println(students.size());
Mary
Tanya
Kevin
Mary
John
Kevin
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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.
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A bit more about
toString()
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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);
}
}
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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!
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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();
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ArrayLists contain Objects
• Numbers are not objects.
• You can not have an ArrayList of ints
or doubles or booleans or chars.
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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?
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Filling the ArrayList
for (int k = 1; k <= 20; k++)
{
nbrs.add(new Integer(k));
}
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Printing the ArrayList
for(Integer e : nbrs)
{
System.out.println(e);
}
Integer has a toString
method that does the right thing.
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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);
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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);
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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.
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Autoboxing/unboxing
ArrayList<Double> numbers = new ArrayList<Double>();
numbers.add(14.5);
numbers.add(3.4);
double sum = numbers.get(0) + numbers.get(1);
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Auto-boxing and Auto-unboxing
• Integer/int equality tests
int == int
Integer.equals(Integer)
Integer == int
Integer.equals(int)
Integer.compareTo(int)==0
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OK
OK
OK
OK
OK
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Auto-boxing and Auto-unboxing
• Integer/int equality tests
– int .equals(Integer)
– int .equals(int)
– Integer == Integer
– Integer.equals(null)
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NO
NO
NO (as test for value equality)
NO
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Auto-boxing and Auto-unboxing
• NOT part of the AP CS Testable Subset.
• Students need to understand
– intValue()
– doubleValue()
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ArrayList Applications
Finding values, counting, max and min,
average, median, mode…..
Your students should be able to manipulate
all sorts of algorithms for ArrayLists.
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AP CS A 2007 Question 3a
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AP CS A 2007 Question 3b
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AP CS A 2007 Question 3b
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