Threads in Java

"Dining Philosophers" problem solved with Java and Semaphores.

import java.util.concurrent.Semaphore;
import java.util.Random;
import java.util.Vector;

public class Philosopher extends Thread
{
    private static final Random rand = new Random();
    private static int event=0;
    private static String binary="";
    private int id;
    private Semaphore sem;
    private static Vector vector = new Vector();

    public Philosopher(int i, Semaphore s)
    {
        id = i;
        sem = s;
        binary = binary + "0";
    }

    private void busy()
    {
        try
        {
            sleep(rand.nextInt(1000));
        } catch (InterruptedException e){}
    }

    private void thinking()
    {
        String str = "Philosopher " + id + " is thinking";
        vector.add( this.addToObject(System.currentTimeMillis(), event , str) );
        event++;
        busy();
    }

    private void eating()
    {
        String str ="Philosopher " + id + " is hungry and is trying to pick up his chopsticks";
        vector.add( this.addToObject(System.currentTimeMillis(), event , str) );
        event++;
        busy();
        str = "Philosopher " + id + " is eating";
        this.oneToBinary(id);
        vector.add( this.addToObject(System.currentTimeMillis(), event , str) );
        event++;
        busy();
        str = "Philosopher " + id + " finished eating, and puts away his chopsticks";
        this.zeroToBinary(id);
        vector.add( this.addToObject(System.currentTimeMillis(), event , str) );
        event++;
    }

    private Object[] addToObject(long t, int i,String s ){
        Object[] o = new Object[4];
        o[3] = s;
        o[2] = i;
        o[1] = binary;
        o[0] = t;
        return o;
    }

    private void oneToBinary(int i){
        binary = binary.substring(0,i) + "1" + binary.substring(i+1);
    }

    private void zeroToBinary(int i){
        binary = binary.substring(0,i) + "0" + binary.substring(i+1);
    }

    @Override
    public void run()
    {
        for (int i = 0; i < 10; ++i)
        {
            thinking();
            try
            {
                sem.acquire();
            } catch (InterruptedException e){}
            eating();
            sem.release();
        }
    }

    public static void main(String[] args)
    {
        final int N = 5;
        Semaphore sem = new Semaphore(N, true);
        Philosopher[] philosopher = new Philosopher[N];

        // Start the philosophers
        for (int i = 0; i < N; i++) {
          philosopher[i] = new Philosopher(i, sem);
          philosopher[i].start();
        }
        // Wait for them to finish
        for (int i = 0; i < N; i++) {
          try {
            philosopher[i].join();
          } catch(InterruptedException ex) {
            return;
          }
        }

        for (int i = 0; i < vector.size(); i++) {
            Object[] o = vector.get(i);
            System.out.printf("%d %d %s %s\n", o[0], o[2], o[1], o[3]);
        }
    }
}

Tip of the Day #2 - pointers

Remember

int* ptr;
*ptr = 10;
#but
ptr = new int;
#without ampersand and dereference !!

Constant pointers, pointers to constants ?

What is the difference between:

float fVar = 3.14;
const float* ptr1 = &fVar;

and:

float fVar = 3.14;
float* const ptr2 = &fVar;

? The first one is pointer to constant value, so we can't change the value of variable the pointer points to :

*ptr1 = 10;

The second one in contrary is a constant pointer to a value, and because of that we can't readdress it to point at different variable :

float fVar2;
*ptr2 = &fVar2;

Django Contrib Revealed - Flatpages

Very often while creating big portals there are static pages just wasting your time to be created. Django has a solution for this problem also - the Flatpages application inside Contribution package. This application allows you to manage such static pages from django-admin, and it let's you create templates using Django's template system. How to get started ?
- Add 'django.contrib.flatpages' to INSTALLED_APPS. Also you need to have django.contrib.sites activated since flatpages depend on this package.
- Add 'django.contrib.flatpages.middleware.FlatpageFallbackMiddleware' to MIDDLEWARE_CLASSES.
This is how the flatpage model looks like :

from django.db import models
from django.contrib.sites.models import Site

class FlatPage(models.Model):
    url = models.CharField(max_length=100, db_index=True)
    title = models.CharField(max_length=200)
    content = models.TextField(blank=True)
    enable_comments = models.BooleanField()
    template_name = models.ChorField(max_length=70, blank=True)
    registration_required = models.BooleanField()
    sites = models.ManyToManyField(Site)

Okay so everything looks clear here I guess. For every flatpage we can specify either custom template, or use flatpages/default.html template (still creating the default template is also our responsibility !). Flatpage templates get a single context variable passed - flatpage, so we can use the Flatpage object's fields ie. {{ flatpage.title }}, {{ flatpage.content }}. Simple, but can spare lots of time :)