1. Computer Science 209
The Singleton Pattern

2. Random Numbers
System.out.println((int)(Math.random() * 6) + 1);
Math.random() uses a single generator that is seeded at the startup of the JVM. The seed is calculated as a function of the computer’s clock
Kind of a pain to use to obtain random integers

3. java.util.Random The Random class is much more convenient to use
java.util.Random generator = new java.util.Random();
System.out.println(generator.nextInt(6) + 1);
System.out.println(generator.nextDouble());
System.out.println(generator.nextBoolean());
The Random class is much more convenient to use
A Random object is also seeded from the clock

4. java.util.Random
java.util.Random generator1 = new java.util.Random();
java.util.Random generator2 = new java.util.Random();
System.out.println(generator1.nextInt(6) + 1);
System.out.println(generator2.nextInt(6) + 1);
If distinct Random objects are seeded during the same millisecond, they will get the same seed and generate the same pseudo-random sequence

5. Rolling a Die import java.util.Random; public class Die{
private int value;
private Random generator;
public Die(){
value = 0;
generator = new Random(); }
public void roll(){
value = generator.nextInt(6) + 1;
public String toString(){
return "" + value;

6. Rolling a Die private Die die1 = new Die();
public DiceApp(){
setTitle("Roll the Dice");
diceField1.setEditable(false);
diceField2.setEditable(false);
rollButton.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent e){
die1.roll();
die2.roll();
diceField1.setText(die1.toString());
diceField2.setText(die2.toString()); }
});
Container c = getContentPane();
JPanel panel = new JPanel();
panel.add(diceField1);
panel.add(diceField2);
c.add("Center", panel);
c.add("South", rollButton);

7. Two Won’t Work
The two dice are created within a millisecond of each other, so their random number generators produce the same sequence of numbers
We need a way of sharing one instance of Random among all dice

8. The Context of the Singleton Pattern
All clients need to share a single instance of a class
No additional instances can be created accidentally

9. Solution of the Singleton Pattern
Define a class with a private constructor
The class constructs a single instance of itself
The class includes a static method to return the single instance

10. A Better Random import java.util.Random; public class SingleRandom{
private static SingleRandom instance = new SingleRandom();
private Random generator;
private SingleRandom(){
generator = new Random(); }
public int nextInt(int limit){
return generator.nextInt(limit);
public static SingleRandom getInstance(){
return instance;

11. A Better Die public class BetterDie{ private int value;
private SingleRandom generator;
public BetterDie(){
value = 0;
generator = SingleRandom.getInstance(); }
public void roll(){
value = generator.nextInt(6) + 1;
public String toString(){
return "" + value;

12. Playing Cards Suits are strings, but should not be just any strings
import javax.swing.*;
public class Card implementsComparable<Card>{
private String suit;
private int rank;
public Card(String suit, int rank){
this.suit = suit;
this.rank = rank; }
Suits are strings, but should not be just any strings
Must check and throw exceptions to enforce proper use
Card queenOfSpades = new Card("spades", 12);
Card twoOfHearts = new Card("hearts", 2);

13. Define a New Type for Suits
import javax.swing.*;
public class Card implementsComparable<Card>{
private Suit suit;
private int rank;
public Card(Suit suit, int rank){
this.suit = suit;
this.rank = rank; }
The type Suit is restricted to just 4 values
The compiler enforces their proper use
Card queenOfSpades = new Card(Suit.spade, 12);
Card twoOfHearts = new Card(Suit.heart, 2);

14. Similar to the singleton, but has 4 instances
public class Suit implements Comparable<Suit>{
static public final Suit spade = new Suit(4, "spades");
static public final Suit heart = new Suit(3, "hearts");
static public final Suit diamond = new Suit(2, "diamonds");
static public final Suit club = new Suit(1, "clubs");
private int order;
private String name;
private Suit(int ord, String nm){
name = nm;
order = ord; }
public int compareTo(Suit other){
return order - other.order;
public String toString(){
return name;
Similar to the singleton, but has 4 instances

15. Problem with Decks of Cards
Every time you create a new deck, the card images must be reloaded from disk
If you do this in an applet, they’re reloaded from a remote Web server
Decks are mutable, but cards are not

16. Solution: 52 Singleton Cards
The Card class maintains a static list of all cards, which are instantiated just once, when the first card is instantiated
When new decks are instantiated, they receive references to cards in this list
Card queenOfSpades = Card.newInstance(Suit.spade, 12);
Card twoOfHearts = Card.newInstance(Suit.heart, 2);