Cracking the Technical Interview

The Finite-Source Cookie Jar Problem

Question: A cookie jar is being shared by two sisters, Tina and Judy, using the following rule: Judy can get a cookie from the jar only after Tina (being the older sister) gets a cookie in at least two separate occasions, whereas Tina gets a cookie from the jar whenever she wants to. The mother is alerted by Judy or Tina whenever they want to eat a cookie and the jar is empty, in which case the mother fills the jar up (from a hidden source), and the kids go on eating. Treating Tina, Judy and the mother as three independently executing processes and the Cookie Jar as a shared resource, develop a concurrent algorithm to solve the Finite-Source Cookie Jar Problem.

Solution:

package cookiejar.finitesource;

import java.util.Random;

public class CookieJar {

static Object o = new Object();
static Object mother = new Object();
static Object children = new Object();

static int tinaCount = 0;
static int cookieCount = 25;
static Random random = new Random();

public static void main(String... args) {
new Thread(new Mother()).start();
new Thread(new Tina()).start();
new Thread(new Judy()).start();
}

private static void checkCookieCount() {
if (cookieCount == 0) {
synchronized (mother) {
System.out.println("Oops! no cookie mother!!! Please help.");
mother.notifyAll();
}
try {
System.out
.println("We are hungry mother. Where are other cookies? We are waiting.");
synchronized (children) {
children.wait();
}
} catch (InterruptedException e) {
}

}

}

private static void randomWait() {
int millisecs = random.nextInt(100);
try {
Thread.sleep(millisecs);
} catch (InterruptedException e) {
}
}

static class Tina implements Runnable {

@Override
public void run() {
while (true) {
randomWait();
boolean empty = false;
synchronized (o) {
if (cookieCount == 0) {
empty = true;
synchronized (mother) {
System.out
.println("Oops! no cookie mother!!! Please help.");
mother.notifyAll();
}
}
}
if (empty) {
try {
System.out
.println("We are hungry mother. Where are other cookies? We are waiting.");
synchronized (children) {
children.wait();
}
} catch (InterruptedException e) {
}
}
synchronized (o) {

cookieCount--;
tinaCount++;
System.out.println("TinaCount = " + tinaCount);
o.notify();
}
System.out.println("Tina is eating cookie.");
}

}

}

static class Judy implements Runnable {

@Override
public void run() {
while (true) {
randomWait();
boolean empty = false;
synchronized (o) {
System.out.println("Judy tries to get a cookie.");

while (tinaCount < 2) {
System.out
.println("Judy is waiting since Tina has not eaten 2 times.");
try {
o.wait();
System.out
.println("Judy is now awake and again checking for tinaCount.");
} catch (InterruptedException e) {
}
}
// start of method

if (cookieCount == 0) {
empty = true;
synchronized (mother) {
System.out
.println("Oops! no cookie mother!!! Please help.");
mother.notify();
}
}
}

if (empty) {
try {
System.out
.println("We are hungry mother. Where are other cookies? We are waiting.");
synchronized (children) {
children.wait();
}
} catch (InterruptedException e) {
}
}
// end of method
synchronized (o) {

}
System.out.println("Judy is eating cookie.");
synchronized (o) {
cookieCount--;
tinaCount = 0;
System.out.println("Judy has now reset the tinaCount.");
}
}

}

}

static class Mother implements Runnable {

@Override
public void run() {
while (true) {
synchronized (mother) {
try {
System.out
.println("Mother is sleeping children. Wake me up when you finish your cookies.");
mother.wait();
} catch (InterruptedException e) {
}
}
synchronized (o) {
cookieCount = 25;
System.out
.println("Mother has filled the cookie jar with good cookies.");
}
synchronized (children) {
System.out
.println("Get the cookies dear! I've filled up the jar.");
children.notifyAll();
}
}

}

}

}

The Cookie Jar Problem

Question: A cookie jar that never becomes empty (the best type!) is being shared by two sisters, Tina and Judy, using the following (not-so-good) rule: Judy can get a cookie from the jar only after Tina (being the older sister) gets a cookie in at least two separate occasions, whereas Tina gets a cookie from the jar whenever she wants to. Treating Tina and Judy as two independently executing processes and the Cookie Jar as a shared resource, develop a concurrent algorithm to solve the Cookie Jar Problem.

Solution:
package cookiejar;

import java.util.Random;

public class CookieJar {

static Object o = new Object();
static int tinaCount = 0;
static Random random = new Random();

public static void main(String... args) {
new Thread(new Tina()).start();
new Thread(new Judy()).start();
}

private static void randomWait() {
int millisecs = random.nextInt(2);
try {
Thread.sleep(millisecs);
} catch (InterruptedException e) {
}
}

static class Tina implements Runnable {

@Override
public void run() {
while (true) {
randomWait();
synchronized (o) {
tinaCount++;
System.out.println("TinaCount = " + tinaCount);
o.notifyAll();
}
System.out.println("Tina is eating cookie.");
}

}

}

static class Judy implements Runnable {

@Override
public void run() {
while (true) {
randomWait();
synchronized (o) {
System.out.println("Judy tries to get a cookie.");
while (tinaCount < 2) {
System.out.println("Judy is waiting since Tina has not eaten 2 times.");
try {
o.wait();
System.out.println("Judy is now awake and again checking for tinaCount.");
} catch (InterruptedException e) {
}
}
}
System.out.println("Judy is eating cookie.");
synchronized (o) {
tinaCount = 0;
System.out.println("Judy has now reset the tinaCount.");
}
}

}

}

}

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What the bleep do we know!?

KG Lesson

Share everything.

Play fair.

Don’t hit people.

Put things back where you found them.

Clean up your own mess.

Don’t take things that aren’t yours.

Say you’re sorry when you hurt somebody.

Wash your hands before you eat.

Flush.

Warm cookies and cold milk are good for you.

Live a balanced life – learn some and think some and draw and paint and sing and

dance and play and work every day some.

Take a nap every afternoon.

When you go out into the world, watch out for traffic, hold hands and stick together.

Be aware of wonder.