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Polymorphism with Java Interfaces
Rick Mercer
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Outline
 Describe Polymorphism
 Show a few ways that interfaces are used
—
Respond to user interaction with a GUI with
ActionListener
—
—
Compare objects with Comparator
Tag types to have writeable/readable objects with
Serializable
—
—
—
Create our own icons with Icon
Play audio files with AudioClip
Show polymorphic algorithms on List
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Polymorphism
http://www.webopedia.com/TERM/p/polymorphism.html
 In general, polymorphism is the ability to appear in
many forms
 In object-oriented programming, polymorphism
refers to a programming language's ability to
process objects differently depending on their data
type (class)
 Polymorphism is considered to be a requirement of
any true object-oriented programming language
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Polymorphism
from mercer
To understand polymorphism, take an example of a
workday at Franklin, Beedle, and Associates. Kim
brought in pastries and everyone stood around
chatting. When the food was mostly devoured, Jim, the
president of the company, invited everyone to “Get
back to work.” Sue went back to read a new section of
a book she was editing. Tom continued laying out a
book. Stephanie went back to figure out some setting
in her word-processing program. Ian finished the
company catalog.
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Polymorphism
Jeni met with Jim to discuss a new project. Chris began
contacting professors to review a new manuscript. And
Krista continued her Web search to find on whether
colleges are using C++, Python, or Java. Marty went
back to work on the index of his new book. Kim
cleaned up the pastries. Rick's was just visiting so he
went to work on the remaining raspberries.
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Polymorphic Messages
 10 different behaviors with the same message!
 The message “Get back to work” is a
polymorphic message
a message that is understood by many different
types of object (or employees in this case)
— but responded to with different behaviors based
on the type of the employee: Editor, Production,
Marketing, …
—
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Polymorphism
 Polymorphism allows the same message to be
sent to different types to get different behavior
 In Java, polymorphism is possible through
—
inheritance
• Override toString to return different values that are
textual representations of that type.
—
interfaces
• Collections.sort sends compareTo messages to
objects that must have implemented Comparable<T>
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Polymorphism
 The runtime message finds the correct method
same message can invoke different methods
— the reference variable knows the type
—
aString.compareTo(anotherString)
anInteger.compareTo(anotherInteger)
aButton.actionPerformed(anEvent)
aTextField.actionPerformed(anEvent)
aList.add(anObject)
aHashSet.add(anObject)
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The Java Interface
 An interface describes a set of methods
NOT allowed: constructors, instance variables
— static variables and methods are allowed
—
 Interfaces must be implemented by a class
—
646 classes implement >= 1 interfaces (in '02)
 Typically, two or more classes implement the
same interface
Type guaranteed to have the same methods
— Objects can be treated as the same type
— May use different algorithms / instance variables
—
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An interface we'll use soon
 An interface, a reference type, can have
static variables and method headings with ;
public int size(); // no { }
—
 Methods are implemented by 1 or more classes
 Example interface :
public interface ActionListener {
public void actionPerformed(ActionEvent theEvent);
}
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Multiple classes implement
the same interface
 To implement an interface, classes must have
all methods specified as given in the interface
private class Button1Listener implements ActionListener {
}
public void actionPerformed(ActionEvent theEvent) {
// Do this method when button1 is clicked
}
private class Button2Listener implements ActionListener {
}
public void actionPerformed(ActionEvent theEvent) {
// Do this method when button2 is clicked
}
More on ActionListener later
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interface Serializable
 Classes that implement interface Serializable
can have their objects written to and read from
streams with writeObject and readObject
 It is just a tag—no methods
public class BankAccount implements
Comparable<BankAccount>, Serializable
 Notice that a class can implement >1 interfaces
More on Serializable later
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The Comparable interface
A review for most
 Can assign an instance of a class that implements
and interface to a variable of the interface type
Comparable str = new String("abc");
Comparable acct = new BankAccount("B", 1);
Comparable day = new Date();
 Some classes that implement Comparable
BigDecimal BigInteger Byte ByteBuffer Character
CharBuffer Charset CollationKey Date Double
DoubleBuffer File Float FloatBuffer IntBuffer
Integer Long LongBuffer ObjectStreamField
Short ShortBuffer String URI
 Comparable defines the "natural ordering" for
collections
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Implementing Comparable
 Any type can implement Comparable to determine if
one object is less than, equal or greater than another
public interface Comparable<T> {
/**
* Return 0 if two objects are equal; less than
* zero if this object is smaller; greater than
* zero if this object is larger.
*/
public int compareTo(T other);
}
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interface comparator
/**
* Compares its two arguments for order. Returns a
* negative integer, zero, or a positive integer as the
* first argument is less than, equal to, or greater
* than the second argument. Equals not shown here
*/
public interface comparator<T> {
public int compareTo(T other);
}
 Can specify sort order by objects. In the code below
—
—
What class needs to be implemented?
What interface must that class implement?
Comparator<BankAccount> idComparator = new ByID();
Collections.sort(accounts, idComparator);
More on Collections.Sort later
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class OurIcon implements Icon
TBA???
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Playing an Audio File
using an interface
 interface AudioClip has 3 methods
—
loop, play, stop
 The Applet class implements AudioClip
 Supports recording, playback, and synthesis
of sampled audio and Musical Instrument
Digital Interface (MIDI) sequences
Can play .au, .aif, .wav, .midi (sort of)
— For mp3s, need something more complex
—
• We'll see such a libraty later semester
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AudioClip audioClip = null;
URL url = null;
// This assumes songs are in a folder named songfile
// Need "file:" unless you are reading it over the web
String baseFolder = "file:" +
System.getProperty("user.dir") + "/songfiles/";
try {
url = new URL(baseFolder + "Dancing_Queen.au");
audioClip = Applet.newAudioClip(url);
} catch (MalformedURLException e) {
System.out.println("bad url " + url);
}
audioClip.play();
JOptionPane.showMessageDialog(null, "End " + url);
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Java's Collection Framework

Java's Collection Framework
—

Collection framework contains
—
—
—

Unified architecture for representing and manipulating
collections
Interfaces (ADTs): specification not implementation
Concrete implementations as classes
Polymorphic Algorithms to search, sort, find, shuffle, ...
Algorithms are polymorphic:
—
the same method can be used on many different
implementations of the appropriate collection interface.
In essence, algorithms are reusable functionality.
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Collection interfaces in
java.util
Image from the Java Tutorial
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Abstract Data Type
 Abstract data type (ADT) is a specification
of the behaviour (methods) of a type
Specifies method names to add, remove, find
— Specifies if elements are unique, indexed,
accessible from only one location, mapped,...
— An ADT shows no implementation
—
• no structure to store elements, no implemented
algorithms
 What Java construct nicely specifies ADTs?
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Collection Classes
 A collection class the can be instantiated
implements an interface as a Java class
— implements all methods of the interface
— selects appropriate instance variables
—
 Since Java 5: we have concrete collection classes
—
—
—
—
—
Stack<E>
ArrayList<E>, LinkedList<E>
LinkedBlockingQueue<E>, ArrayBlockingQueue<E>
HashSet<E>, TreeSet<E>
TreeMap<K,V>, HashMap<K,V>
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Common Functionality
 Collection classes often have methods for
Adding objects
— Removing an object
— Finding a reference to a particular object find
—
• can then send messages to the object still in the
collection
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List<E>, an ADT written as a
Java interface
 List<E>: a collection with a first element, a last
element, distinct predecessors and successors
—
—
The user of this interface has precise control over
where in the list each element is inserted
duplicates that "equals" each other are allowed
 The List interface is implemented by these three
collection classes
—
—
—
ArrayList<E>
LinkedList<E>
Vector<E>
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import java.util.*; // For List, ArrayList, Linked ...
import static org.junit.Assert.*;
import org.junit.Test;
public class ThreeClassesImplementList {
@Test
public void showThreeImplementationsOfList() {
// Interface name: List
// Three classes that implement the List interface:
List<String> bigList = new ArrayList<String>();
List<String> littleList = new LinkedList<String>();
List<String> sharedList = new Vector<String>();
// All three have an add method
bigList.add("in array list");
littleList.add("in linked list");
sharedList.add("in vector");
// All three have a get method
assertEquals("in array list", bigList.get(0));
assertEquals("in linked list", littleList.get(0));
assertEquals("in vector", sharedList.get(0));
}
}
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Iterators
 Iterators provide a general way to traverse
all elements in a collection
ArrayList<String> list = new ArrayList<String>();
list.add("1-FiRsT");
list.add("2-SeCoND");
list.add("3-ThIrD");
Iterator<String> itr = list.iterator();
while (itr.hasNext()) {
System.out.println(itr.next().toLowerCase());
}
Output
1-first
2-second
3-third
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Newest way to visit elements:
Java's Enhanced for Loop
 The for loop has been enhanced to iterate
over collections
 General form
for (Type element : collection) {
element is the next thing visited each iteration
}
for (String str : list) {
System.out.println(str + " ");
}
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Can't add the wrong type
 Java 5 generics checks the type at compile time
—
—
See errors early--a good thing
"type safe" because you can't add different types
ArrayList<GregorianCalendar> dates =
new ArrayList<GregorianCalendar>();
dates.add(new GregorianCalendar()); // Okay
dates.add("String not a GregorianCalendar"); // Error
ArrayList<Integer> ints = new ArrayList<Integer>();
ints.add(1); // Okay. Same as add(new Integer(1))
ints.add("Pat not an int")); // Error
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Algorithms
 Java has polymorphic algorithms to provide
functionality for different types of collections
—
—
—
—
—
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Sorting (e.g. sort)
Shuffling (e.g. shuffle)
Routine Data Manipulation (e.g. reverse, addAll)
Searching (e.g. binarySearch)
Composition (e.g. frequency)
Finding Extreme Values (e.g. max)
 Demo a few with ArrayList
—
Override toString and equals for DayCounter
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TreeSet implements Set
 Set<E> An interface for collections with no
duplicates. More formally, sets contain no pair
of elements e1 and e2 such that
e1.equals(e2)
 TreeSet<E>: This class implements the Set
interface, backed by a balanced binary search
tree. This class guarantees that the sorted set
will be in ascending element order, sorted
according to the natural order of the elements
as defined by Comparable<T>
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Set and SortedSet
 The Set<E> interface
—
add, addAll, remove, size, but no get!
 Two classes that implement Set<E>
TreeSet: values stored in order, O(log n)
— HashSet: values in a hash table, no order, O(1)
—
 SortedSet extends Set by adding methods E
first(), SortedSet<E> tailSet(E fromElement),
SortedSet<E> headSet(E fromElement), E last(),
SortedSet<E> subSet(E fromElement, E toElement)
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TreeSet elements are in order
Set<String> names = new TreeSet<String>();
names.add("Sandeep");
names.add("Chris");
names.add("Kim");
names.add("Chris"); // not added
names.add("Devon");
for (String name : names)
System.out.println(name);
 Output?
 Change to HashSet
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The Map Interface (ADT)
 Map describes a type that stores a collection
of elements that consists of a key and a value
 A Map associates (maps) a key the it's value
 The keys must be unique
the values need not be unique
— put destroys one with same key
—
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Map Operations
 Java's HashMap<K, V>
—
—
—
—
—
public V put(K key, V value)
• associates key to value and stores mapping
public V get(Object key)
• associates the value to which key is mapped or null
public boolean containsKey(Object key)
• returns true if the Map already uses the key
public V remove(Object key)
• Returns previous value associated with specified key, or
null if there was no mapping for key.
Collection<V> values()
• get a collection you can iterate over
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Code Demo
Rick: Put in a file named HashMapDemo.java
 Add some mappings to a HashMap and
iterate over all elements with
Collection<V> values() and all keys
with Set<K> keySet()
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Queue<E>
boolean add(E e) Inserts e into this queue
E element() Retrieves, but does not remove, the head of
this queue
boolean offer(E e)Inserts e into this queue
E peek() Retrieves, but does not remove, the head of this
queue, or returns null if this queue is empty
E poll() Retrieves and removes the head of this queue, or
returns null if this queue is empty
E remove() Retrieves and removes the head of this queue
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ArrayBlockingQueue<E>
a FIFO queue
ArrayBlockingQueue<Double> numberQ =
new ArrayBlockingQueue<Double>(40);
numberQ.add(3.3);
numberQ.add(2.2);
numberQ.add(5.5);
numberQ.add(4.4);
numberQ.add(7.7);
assertEquals(3.3, numberQ.peek(), 0.1);
assertEquals(3.3, numberQ.remove(), 0.1);
assertEquals(2.2, numberQ.remove(), 0.1);
assertEquals(5.5, numberQ.peek(), 0.1);
assertEquals(3, numberQ.size());
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