CS 49J - Lecture 5

Cay S. Horstmann

Homework Show and Tell

Static Puzzle 1

• A singleton is a unique object of a class.
• Example: Want only one random number generator

  public class Random
  {
     private static final long p = 4611686018427388039L; // First prime > 262
     private static final long f = 1729;
     private static final long seed = System.currentTimeMillis;
     private static Random instance = new Random();
     private long last;   
     private Random()
     {
        last = seed;
     }
     public long next()
     {
        last = (last *  f) % p;
        return last;
     }
     public static Random getInstance()
     {
        if (instance == null)
           instance = new Random();
        return random;
     }
  }
  

  How do you get a random number generator?
• What happens when that method is called twice?
• What if it's in two different threads?
• Better approach:

  public static final Random INSTANCE = new Random();
  

Static Puzzle 2

• public class Random {
     public static final Random INSTANCE = new Random();
     private static final long p = 4611686018427388039L;
     private static final long f = 1729;
     private static final long seed = System.currentTimeMillis();
     private long last;   
     private Random()
     {
        last = seed;
     }
     public long next()
     {
        last = (last * f) % p;
        return last;
     }
     public static void main(String[] args)
     {
        System.out.println(Random.INSTANCE.next());
     }
  }
  

• Why does it print zero?
• Moral: Beware of initialization order!

Static Puzzle 3

• public class Random
  {
     private static final long p = 4611686018427388039L;
     private static final long f = 1729;
     private static final long seed = System.currentTimeMillis();
     public static final Random INSTANCE = new Random();
     private long last;   
     private Random()
     {
        last = seed;
     }
     public long next()
     {
        last = (last * f) % p;
        return last;
     }
     public static void main(String[] args)
     {
        Random random = null;
        System.out.println(random.INSTANCE.next());
     }
  }
  

• What if it had been

  null.INSTANCE.next();

Static Puzzle 4

• Now we'll allow multiple random number generators. We want to know how many.

  public class Random {
     private static final long p = 4611686018427388039L;
     private static final long f = 1729;
     private static final long seed = System.currentTimeMillis();
     private static long instances;
  
     private long last;   
     private Random() { last = seed; instances++; }
     public long next()
     {
        last = (last * f) % p;
        return last;
     }
     public static long instances() { return instances; }
     
     public static void main(String[] args)
     {
        int n = 1_000_000;
        for (int i = 0; i < n; i++) 
           long random = new Random().next();
        System.out.println(Random.instances());
     }
  }

• Not threadsafe:

        IntStream.range(0, n).parallel().forEach(i -> new Random().next());
  

Using Libraries

• Marvin—example of library with JAR file
• Download and unzip

  mkdir marvin
  cd marvin
  jar xvf ~/Downloads/MarvinFramework*.zip
  

• Make a file FirstApplication.java with this code. Compile with

  javac -classpath marvin/framework/marvin_1.5.3.jar FirstApplication.java
  

• Add an image res/test.jpg
• Run with

  java -classpath .:marvin/framework/marvin_1.5.3.jar FirstApplication

• In Eclipse, add marvin_1.5.3.jar as an external JAR: Right-click on project → Properties → Java Build Path → Libraries → Add External JARs...

JUnit on the Command Line

• Download and unzip JUnit:

  cd ~
  jar xvf Downloads/junit4.11.zip
  

• Make a directory hw6b and put the files SpinningDisk.java and SpinningDiskTest.java there.
• Compile:

  cd ~/hw6b
  javac -cp .:~/junit4.11/junit-4.11.jar SpinningDiskTest.java
  

• Run:

  java -cp .:~/junit4.11/\* org.junit.runner.JUnitCore SpinningDiskTest
  

• Why not java -cp .:~/junit4.11/junit-4.11.jar . . .? Because JUnit also needs another JAR in that directory to run.