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Chapter 10 –ArrayLists and an Introduction to
the Java Collections Framework
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The ArrayList Class
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How to Create an ArrayList Object
Adding Elements to an ArrayList Object
How to Access an Element Within an ArrayList
How to Update an ArrayList Object
Additional ArrayList Methods
Printing or Concatenating an ArrayList
Storing Primitives in an ArrayList
ArrayList Example Using Anonymous Objects and
a For-Each Loop
ArrayList Objects Versus Standard Arrays
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Chapter 10 –ArrayLists and an Introduction to
the Java Collections Framework
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The LinkedList Class
The List Interface
Comparing Method Execution Times
Queues, Stacks, and the ArrayDeque Class
Overview of the Java Collections Framework
Collections Example – Information Flow in a Network
of Friends
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The ArrayList Class
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The ArrayList class provides the basic functionality
that comes with a standard array, plus it provides
additional functionality.
The basic functionality: An ArrayList stores an
ordered collection of values and allows access to the
values via an index.
The added functionality: An ArrayList grows and
shrinks dynamically by inserting and deleting
elements at any specified location.
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How to Create an ArrayList Object
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The ArrayList class is defined in the Java API's java.util
package, so for files that use the ArrayList class, import it
like this:
import java.util.ArrayList;
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To initialize an ArrayList reference variable, in a declaration,
use this syntax:
ArrayList<element-type> reference-variable = new ArrayList<>();
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For example, here's how to initialize an ArrayList reference
variable named students:
ArrayList<Student> students = new ArrayList<>();
Use angled brackets to
surround the type for the
elements, and the type must be
a class name (not a primitive).
Use empty angled brackets (the
diamond operator).
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How to Create an ArrayList Object
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Let's compare the syntaxes for creating ArrayList
objects, regular objects, and standard arrays. Here's
the same ArrayList example as before:
ArrayList<Student> students = new ArrayList<>();
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Here's an object example:
Mouse gus = new Mouse();
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Here's a standard-array example:
Student[] students = new Student[100];
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Adding Elements to an ArrayList Object
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To add an element to the end of an ArrayList
object, use this syntax:
ArrayList-reference-variable.add(item);
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The item that's added must be the same type as the
type specified in the ArrayList's declaration.
Write a code fragment that creates this ArrayList
object:
computerScientists
0
"Ada Lovelace"
1
"Grace Hopper"
2
"Marissa Mayer"
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Java API
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API stands for application programming interface.
The Java API is the interface to the huge library of pre-built Java
classes.
As a programmer, you don't need to know the internals of those
classes; you just need to know how to use them. Or said another
way, you just need to know how to interface with them.
To interface with them, you need to use their public methods.
To use a method, you need to know what type of argument(s) to
pass to it and what type of value it returns. A method's API
shows the method's parameters and its return type.
The standard way to show that information is to show the
method's heading. For example, here's the API heading for the
Math class's pow method:
public static double pow(double num, double power)
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How to Access an Element Within an ArrayList
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With standard arrays, you use square brackets to access and
update an element. ArrayList objects don't use square
brackets. Instead, they use a get method to access an element
and a set method to update an element.
Here's the API heading for the ArrayList's get method:
public E get(int index)
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Semantics:
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The index parameter specifies the position of the desired element
within the ArrayList calling object. As with standard arrays, the
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first element is at position 0, the second element is at position 1, etc.
If index refers to a nonexistent element, then a runtime error
occurs.
If index is valid, then get returns the element at the specified
position.
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How to Access an Element Within an ArrayList
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Note the E return type for the ArrayList's get
method:
public E get(int index)
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The E stands for element. It represents the data type
of the ArrayList's elements. It's the same as the
element-type specified in the ArrayList's
initialization:
ArrayList<element-type> reference-variable = new ArrayList<>();
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How to Update an ArrayList Object
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The set method allows you to assign a value to an
existing ArrayList element. Here's its API heading:
public E set(int index, E elem)
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Semantics:
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The index parameter specifies the position of the element
you're interested in.
If index refers to a nonexistent element, then a runtime
error occurs.
If index is valid, then set assigns the elem parameter to
the specified element, overlaying whatever was there
originally.
E represents the data type of the ArrayList's elements.
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How to Update an ArrayList Object
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Draw a picture of the colors ArrayList after this
code fragment executes:
String mixedColor;
ArrayList<String> colors = new ArrayList<>();
colors.add("red");
colors.add("green");
colors.add("blue");
mixedColor = colors.get(0) + colors.get(1);
colors.set(2, mixedColor);
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Additional ArrayList Methods
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public void add(int index, E elem)
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public void clear()
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Search for the first occurrence of the elem parameter within the list. If it's
found, return its index position. If it's not found, return -1.
Return true if the list contains no elements.
public E remove(int index)
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Object is a generic class
that can be used as a
class type for any object.
public boolean isEmpty()
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Remove all elements from the list.
public int indexOf(Object elem)
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Starting with the specified index position, shift the original elements to
higher-indexed positions. Then insert the elem parameter at the specified
index position.
Remove the element at the specified index position, shift all higher-indexed
elements to lower-indexed positions, and return the removed element.
public int size()
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Return the number of elements in the list.
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Example ArrayList Program
import java.util.ArrayList;
public class HungerGames
{
public static void main(String[] args)
{
int deceasedIndex; // index of deceased tribute
String deceased;
// name of deceased tribute
ArrayList<String> tributes = new ArrayList<>();
tributes.add("Cato");
tributes.add("Katniss");
tributes.add("Peeta");
tributes.add("Rue");
tributes.add(1, "Finnick");
deceasedIndex = (int) (Math.random() * tributes.size());
deceased = tributes.remove(deceasedIndex);
System.out.println(deceased + " is no longer in the game.");
System.out.println("Remaining: " + tributes);
} // end main
} // end HungerGames
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Printing or Concatenating an ArrayList
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If you attempt to print or concatenate an ArrayList,
the ArrayList returns a comma-separated list of
ArrayList elements surrounded by square brackets,
[].
For example, in the HungerGames program, if Peeta is
removed, the last line prints this:
Remaining: [Cato, Finnick, Katniss, Rue]
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Storing Primitives in an ArrayList
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As mentioned previously, ArrayLists store references. For
example, in the HungerGames program, tribe is an ArrayList
of strings, and strings are reference types.
If you need to store primitives in an ArrayList, you can't do it
directly, but you can do it if the primitives are wrapped in
wrapper classes.
Ever since Java 5.0, the "wrapping" process has been done
behind the scenes. For ArrayLists, it's done automatically if a
wrapper class is used in an ArrayList declaration.
The StockAverage program on the next slide reads int stock
values and stores them in an ArrayList. After all stock values
are entered, the program calculates the average stock value.
Why is an ArrayList appropriate for calculating a stock
average?
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Storing Primitives in an ArrayList
import java.util.Scanner;
import java.util.ArrayList;
public class StockAverage
Must be a
{
wrapper class,
public static void main(String[] args)
not a primitive.
{
Scanner stdIn = new Scanner(System.in);
ArrayList<Double> stocks = new ArrayList<>();
double stock;
// a stock value
double stockSum = 0;
// sum of stock values
System.out.print("Enter a stock value (-1 to quit): ");
stock = stdIn.nextDouble();
while (stock >= 0)
Automatic boxing (“autoboxing”)
{
takes place here.
stocks.add(stock);
System.out.print("Enter a stock value (-1 to quit): ");
stock = stdIn.nextDouble();
} // end while
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Storing Primitives in an ArrayList
for (int i=0; i<stocks.size(); i++)
{
stock = stocks.get(i);
stockSum += stock;
}
Where does
automatic
unboxing take
place?
if (stocks.size() != 0)
{
System.out.printf("\nAverage stock value = $%.2f\n",
stockSum / stocks.size());
}
} // end main
} // end class StockAverage
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ArrayList Example Using Anonymous
Objects and a For-Each Loop
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When storing objects in an ArrayList, it's common to create
an object and add it to the ArrayList all in the same
statement.
For example, the upcoming BearStore program stores Bear
objects in an ArrayList. In storing a Bear object, the
program creates a Bear object and adds it to the bears
ArrayList, all in the same statement:
bears.add(new Bear("Acme", "brown teddy"));
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An anonymous object is an object that's instantiated, but it's
not stored in a variable (and with no variable, there's no name
for it; thus, we say it's "anonymous").
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ArrayList Example Using Anonymous
Objects and a For-Each Loop
import java.util.Scanner;
import java.util.ArrayList;
public class BearStore
{
ArrayList<Bear> bears = new ArrayList<>();
//**********************************************************
// Fill store with specified number of standard teddy bears.
public void addStdBears(int num)
{
anonymous object
for (int i=0; i<num; i++)
{
bears.add(new Bear("Acme", "brown teddy"));
}
} // end addStdBears
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ArrayList Example Using Anonymous
Objects and a For-Each Loop
//**********************************************************
// Fill store with specified number of customized bears.
public void addUserSpecifiedBears(int num)
{
for (int i=0; i<num; i++)
{
bears.add(getUserSpecifiedBear());
}
} // end addUserSpecifiedBears
//**********************************************************
// Prompt user for a customized bear name and return bear.
private Bear getUserSpecifiedBear()
{
Scanner stdIn = new Scanner(System.in);
String maker, type;
System.out.print("Enter bear's maker: ");
maker = stdIn.nextLine();
System.out.print("Enter bear's type: ");
type = stdIn.nextLine();
return new Bear(maker, type);
anonymous
} // end getUserSpecifiedBear
object
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ArrayList Example Using Anonymous
Objects and a For-Each Loop
//**********************************************************
// Print all the bears in the store.
public void displayInventory()
{
for (Bear bear : bears)
{
bear.display();
}
} // end displayInventory
//**********************************************************
public static void main(String[] args)
{
BearStore store = new BearStore();
store.addStdBears(3);
store.addUserSpecifiedBears(2);
store.displayInventory();
} // end main
} // end BearStore class
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ArrayList Example Using Anonymous
Objects and a For-Each Loop
public class Bear
{
private final String MAKER; // bear's manufacturer
private final String TYPE; // type of bear
//**********************************************************
public Bear(String maker, String type)
{
MAKER = maker;
TYPE = type;
}
public void display()
{
System.out.println(MAKER + " " + TYPE);
}
} // end Bear class
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Anonymous Objects
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The bear store program contains several examples of
using anonymous objects. In general, you'll see
anonymous objects being used in two circumstances:
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Passing a newly created object into a method or
constructor. For example:
bears.add(new Bear("Acme", "brown teddy"));
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Returning a newly created object from a method. For
example:
return new Bear(maker, type);
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For-Each Loop
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Note the for-each loop in the BearStore's
displayInventory method:
public void displayInventory()
{
for (Bear bear : bears)
{
bear.display();
}
} // end displayInventory
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Read this as "for each
bear in bears, …"
For each iteration
through the loop,
bear accesses the
next element in the
bears ArrayList.
For-each loop syntax for an ArrayList:
for (<element-type> <element-name> : <ArrayList-reference-variable>)
For-Each Loop
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Note that using the for-each loop is an option, not a
requirement. Here's an alternative
displayInventory implementation that uses a
standard for loop:
public void displayInventory()
{
for (int i=0; i<bears.size(); i++)
{
bears.get(i).display();
}
} // end displayInventory
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The for-each loop implementation is preferred
because it is simpler.
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ArrayList Objects Versus Standard Arrays
Benefits of an ArrayList Over Benefits of a Standard Array
Over an ArrayList
a Standard Array
1.
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It's easy to increase the size
of an ArrayList – just call
add.
It's easy for a programmer
to insert or remove an
element to or from the
interior of an ArrayList –
just call add or remove and
specify the element's index
position.
1.
2.
A standard array uses []'s to
access array elements
(which is easier than using
get and set methods).
A standard array is more
efficient with storing
primitive values.