Sunday, March 14, 2021

Design a Vending Machine in Java - Interview Question

How do you design a Vending Machine in Java? is one of the good Java interview questions mostly asked at Senior level Java developer Interviews. In a typical coding interview, you will be given a problem statement to develop a vending machine and within a limited time, usually, 2 to 3 hours you need to produce a design document, working code, and unit test in Java. One of the key advantages of such Java interviews is that you can test many essential skills or a candidate in one go. In order to complete the design, coding, and unit testing of a Vending machine, a candidate needs to be really good in all three departments.

By the way, this kind of real-world problem is also a good exercise to improve your object-oriented analysis and design skills (see here), which is very important if you want to become a good application developer.

By designing a vending machine in Java or any other object-oriented language, you not only learn basics e.g. Encapsulation, Polymorphism, or Inheritance but also learns subtle details of how to use an abstract class and interface (see here) while solving a problem or designing an application.

Usually, this kind of problem also gives you an opportunity to utilize Java design patterns, as in this problem we will be using the Factory method pattern for creating different types of Vending Machine. I have talked about this question when I shared 20 software design questions in Java (here), and after that, I receive a lot of feedback to provide a solution for that question.

This two-part series of posts will provide a solution to the Vending machine problem in Java. By the way, this problem can be solved in a different way, and you should try to do that before looking into the solution given here. This is also an opportunity to revisit SOLID and OOPS design principles (see here) and get ready to use them in your code. You'll find many of them are applicable when you design a vending machine in Java.

Btw, If you are serious about learning design patterns and principles, I suggest you take a look at the Design Patterns in Java course on Udemy.  This course covers both SOLID design principles like Open Closed and Liskov substitution, and all-important Object Oriented design patterns like Decorator, Observer, Chain of Responsibility, and much more.





Problem Statement

You need to design a Vending Machine which
  1.  Accepts coins of 1,5,10,25 Cents i.e. penny, nickel, dime, and quarter.
  2.  Allow user to select products Coke(25), Pepsi(35), Soda(45)
  3.  Allow user to take refund by canceling the request.
  4.  Return the selected product and remaining change if any
  5.  Allow reset operation for vending machine supplier.

The requirement statement is the most important part of the problem. You need to read the problem statement multiple times to get a high-level understanding of the problem and what are you trying to solve. Usually, requirements are not very clear and you need to make a list of your own by reading through the problem statement.

I like point based requirement because it's easy to track. Some of the requirements are also implicit but it's better to make them explicit in your list e.g. In this problem, the vending machine should not accept a request if it doesn't have sufficient change to return.

Unfortunately, there is not many book or courses which teach you these skills, you need to develop them by yourself by doing some real-world work. 

Though two of the resources which helped me to improve my object-oriented analysis and design skills are Grokking the Object-Oriented Design Interview, an interactive course on Educative which allows you to practice object-oriented problems on the browser, I highly recommend this course to level up your OOP design skill.

Vending Machine OOP Design Problem and Solution in Java



And, the second resource is the Head First Object-Oriented Design and Analysis 1st edition by Brett D. McLaughlin. One of the best books if you don't have much experience in object-oriented programming.

Java Software Design Problem - Vending Machine Solution


Another resource which is very good on developing application and system design skill is UML for Java Programmers by Robert C. Martin, one of my favorite author. I have read several books about him like  Clean Code, Clean Coder, and a book on software development using Agile. He is one of the best in teaching the OOP concept.

This book has got a similar problem about designing a coffee machine. So, if you want to practice more or try your object-oriented design skill, you can refer to that problem. It's also a very good learning exercise.

Solution and Coding

My implementation of Java Vending Machine has the following classes and interfaces :

VendingMachine
It defines the public API of a vending machine, usually, all high-level functionality should go in this class

VendingMachineImpl
A sample implementation of Vending Machine

VendingMachineFactory
A Factory class to create different kinds of Vending Machine

Item
Java Enum to represent Item served by Vending Machine

Inventory
Java class to represent an Inventory, used for creating the case and item inventory inside Vending Machine

Coin
Another Java Enum to represent Coins supported by Vending Machine

Bucket
A parameterized class to hold two objects. It's kind of Pair class.

NotFullPaidException
An Exception is thrown by Vending Machine when a user tries to collect an item, without paying the full amount.

NotSufficientChangeException
Vending Machine throws this exception to indicate that it doesn't have sufficient change to complete this request.

SoldOutExcepiton
Vending Machine throws this exception if the user requests a product that is sold out.

How to design vending machine in Java



How to design Vending Machine in Java

Here is the complete code of Vending Machine in Java, make sure to test this code, and let me know if you face any issues.


VendingMachine.java
The public API of a vending machine, usually all high-level functionality should go in this class

package vending;

import java.util.List;

/**
  * Decleare public API for Vending Machine
  * @author Javin Paul
  */
public interface VendingMachine {   
    public long selectItemAndGetPrice(Item item);
    public void insertCoin(Coin coin);
    public List<Coin> refund();
    public Bucket<Item, List<Coin>> collectItemAndChange();   
    public void reset();
}


VendingMachineImpl.java
A sample implementation of the VendingMachine interface represents a real-world Vending Machine, which you see in your office, bus stand, railway station, and public places.

package vending;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

/**
  * Sample implementation of Vending Machine in Java
  * @author Javin Paul
  */
public class VendingMachineImpl implements VendingMachine {   
    private Inventory<Coin> cashInventory = new Inventory<Coin>();
    private Inventory<Item> itemInventory = new Inventory<Item>();  
    private long totalSales;
    private Item currentItem;
    private long currentBalance; 
   
    public VendingMachineImpl(){
        initialize();
    }
   
    private void initialize(){       
        //initialize machine with 5 coins of each denomination
        //and 5 cans of each Item       
        for(Coin c : Coin.values()){
            cashInventory.put(c, 5);
        }
       
        for(Item i : Item.values()){
            itemInventory.put(i, 5);
        }
       
    }
   
   @Override
    public long selectItemAndGetPrice(Item item) {
        if(itemInventory.hasItem(item)){
            currentItem = item;
            return currentItem.getPrice();
        }
        throw new SoldOutException("Sold Out, Please buy another item");
    }

    @Override
    public void insertCoin(Coin coin) {
        currentBalance = currentBalance + coin.getDenomination();
        cashInventory.add(coin);
    }

    @Override
    public Bucket<Item, List<Coin>> collectItemAndChange() {
        Item item = collectItem();
        totalSales = totalSales + currentItem.getPrice();
       
        List<Coin> change = collectChange();
       
        return new Bucket<Item, List<Coin>>(item, change);
    }
       
    private Item collectItem() throws NotSufficientChangeException,
            NotFullPaidException{
        if(isFullPaid()){
            if(hasSufficientChange()){
                itemInventory.deduct(currentItem);
                return currentItem;
            }           
            throw new NotSufficientChangeException("Not Sufficient change in 
                                                    Inventory");
           
        }
        long remainingBalance = currentItem.getPrice() - currentBalance;
        throw new NotFullPaidException("Price not full paid, remaining : ", 
                                          remainingBalance);
    }
   
    private List<Coin> collectChange() {
        long changeAmount = currentBalance - currentItem.getPrice();
        List<Coin> change = getChange(changeAmount);
        updateCashInventory(change);
        currentBalance = 0;
        currentItem = null;
        return change;
    }
   
    @Override
    public List<Coin> refund(){
        List<Coin> refund = getChange(currentBalance);
        updateCashInventory(refund);
        currentBalance = 0;
        currentItem = null;
        return refund;
    }
   
   
    private boolean isFullPaid() {
        if(currentBalance >= currentItem.getPrice()){
            return true;
        }
        return false;
    }

      
    private List<Coin> getChange(long amount) throws NotSufficientChangeException{
        List<Coin> changes = Collections.EMPTY_LIST;
       
        if(amount > 0){
            changes = new ArrayList<Coin>();
            long balance = amount;
            while(balance > 0){
                if(balance >= Coin.QUARTER.getDenomination() 
                            && cashInventory.hasItem(Coin.QUARTER)){
                    changes.add(Coin.QUARTER);
                    balance = balance - Coin.QUARTER.getDenomination();
                    continue;
                   
                }else if(balance >= Coin.DIME.getDenomination() 
                                 && cashInventory.hasItem(Coin.DIME)) {
                    changes.add(Coin.DIME);
                    balance = balance - Coin.DIME.getDenomination();
                    continue;
                   
                }else if(balance >= Coin.NICKLE.getDenomination() 
                                 && cashInventory.hasItem(Coin.NICKLE)) {
                    changes.add(Coin.NICKLE);
                    balance = balance - Coin.NICKLE.getDenomination();
                    continue;
                   
                }else if(balance >= Coin.PENNY.getDenomination() 
                                 && cashInventory.hasItem(Coin.PENNY)) {
                    changes.add(Coin.PENNY);
                    balance = balance - Coin.PENNY.getDenomination();
                    continue;
                   
                }else{
                    throw new NotSufficientChangeException("NotSufficientChange,
                                       Please try another product");
                }
            }
        }
       
        return changes;
    }
   
    @Override
    public void reset(){
        cashInventory.clear();
        itemInventory.clear();
        totalSales = 0;
        currentItem = null;
        currentBalance = 0;
    } 
       
    public void printStats(){
        System.out.println("Total Sales : " + totalSales);
        System.out.println("Current Item Inventory : " + itemInventory);
        System.out.println("Current Cash Inventory : " + cashInventory);
    }   
   
  
    private boolean hasSufficientChange(){
        return hasSufficientChangeForAmount(currentBalance - currentItem.getPrice());
    }
   
    private boolean hasSufficientChangeForAmount(long amount){
        boolean hasChange = true;
        try{
            getChange(amount);
        }catch(NotSufficientChangeException nsce){
            return hasChange = false;
        }
       
        return hasChange;
    }

    private void updateCashInventory(List change) {
        for(Coin c : change){
            cashInventory.deduct(c);
        }
    }
   
    public long getTotalSales(){
        return totalSales;
    }
   
}


VendingMachineFactory.java
A Factory class to create different kinds of Vending Machine

package vending;

/**
  * Factory class to create instance of Vending Machine, 
  * this can be extended to create instance of
  * different types of vending machines.
  * @author Javin Paul
  */
public class VendingMachineFactory {      
    public static VendingMachine createVendingMachine() {
        return new VendingMachineImpl();
    }
}


Item.java
Java Enum to represent Item served by Vending Machine

package vending;
/**
  * Items or products supported by Vending Machine.
  * @author Javin Paul
  */
public enum Item{
    COKE("Coke", 25), PEPSI("Pepsi", 35), SODA("Soda", 45);
   
    private String name;
    private int price;
   
    private Item(String name, int price){
        this.name = name;
        this.price = price;
    }
   
    public String getName(){
        return name;
    }
   
    public long getPrice(){
        return price;
    }
}


Coin.java
Another Java Enum to represent Coins supported by Vending Machine

package vending;

/**
  * Coins supported by Vending Machine.
  * @author Javin Paul
  */
public enum Coin {
    PENNY(1), NICKLE(5), DIME(10), QUARTER(25);
   
    private int denomination;
   
    private Coin(int denomination){
        this.denomination = denomination;
    }
   
    public int getDenomination(){
        return denomination;
    }
}

Inventory.java
A Java class to represent an Inventory, used for creating the case and item inventory inside Vending Machine.

package vending;
import java.util.HashMap;
import java.util.Map;

/**
  * An Adapter over Map to create Inventory to hold cash and 
  * Items inside Vending Machine
  * @author Javin Paul
  */
public class Inventory<T> {
    private Map<T, Integer> inventory = new HashMap<T, Integer>();
   
    public int getQuantity(T item){
        Integer value = inventory.get(item);
        return value == null? 0 : value ;
    }
   
    public void add(T item){
        int count = inventory.get(item);
        inventory.put(item, count+1);
    }
   
    public void deduct(T item) {
        if (hasItem(item)) {
            int count = inventory.get(item);
            inventory.put(item, count - 1);
        }
    }
   
    public boolean hasItem(T item){
        return getQuantity(item) > 0;
    }
   
    public void clear(){
        inventory.clear();
    }

    public void put(T item, int quantity) {
        inventory.put(item, quantity);
    }
}


Bucket.java
A parameterized utility class to hold two objects.

package vending;
/**
  * A parameterized utility class to hold two different object.
  * @author Javin Paul
  */
public class Bucket<E1, E2> {
    private E1 first;
    private E2 second;
   
    public Bucket(E1 first, E2 second){
        this.first = first;
        this.second = second;
    }
   
    public E1 getFirst(){
        return first;
    }
   
    public E2 getSecond(){
        return second;
    }
}


NotFullPaidException.java
An Exception, thrown by Vending Machine when a user tries to collect an item, without paying the full amount.

package vending;
public class NotFullPaidException extends RuntimeException {
    private String message;
    private long remaining;
   
    public NotFullPaidException(String message, long remaining) {
        this.message = message;
        this.remaining = remaining;
    }
   
    public long getRemaining(){
        return remaining;
    }
   
    @Override
    public String getMessage(){
        return message + remaining;
    } 
   
}


NotSufficientChangeException.java
Vending Machine throws this exception to indicate that it doesn't have sufficient change to complete this request.

package vending;
public class NotSufficientChangeException extends RuntimeException {
    private String message;
   
    public NotSufficientChangeException(String string) {
        this.message = string;
    }
   
    @Override
    public String getMessage(){
        return message;
    }
   
}


SoldOutException.java
The Vending Machine throws this exception if the user requests a product that is sold out

package vending;
public class SoldOutException extends RuntimeException {
    private String message;
   
    public SoldOutException(String string) {
        this.message = string;
    }
   
    @Override
    public String getMessage(){
        return message;
    }
   
}

That's all in this first part of how to design a vending machine in Java. In this part, we have solved the problem by creating all the classes and writing all code, but the unit test and design document are still pending, which you will see in the second part of this article.

If you want you can try to run this problem by creating the Unit test, or maybe make it an application by using a thread and then use a different thread to act as the user. 

Further Learning
Grokking the System Design Interview
Design Pattern Library
Java Design Patterns - The Complete Masterclass


Other Programming Interview Questions for Java Programmers
  • 10 tricky Java Interview Questions for Experienced developers (list)
  • 25 Software Design Interview Questions for Programmers (design questions)
  • 21 frequently asked SQL Queries from Programming Interviews (list)
  • 18 Java design pattern interviews questions with answers (list)
  • 40 Core Java Interview Questions from Phone Interviews  (list)
  • 7 Best Courses to learn OOP Design Pattern (best courses)
  • 21 Array Concept Interview Questions in Java? (list)
  • 10 Best SQL and Database Courses for Programmers (best courses)
  • 30 OOP Concept Interview Questions with Answers (list)
  • My favorite courses to learn Software Architecture (medium)
  • 12 multi-threading and concurrency Interview Questions (list)
  • 7 Best Courses to learn Data Structure and Algorithms (best courses)
  • 22 Java Inheritance Interview Questions with Answers (list)
  • Top 5 OOP Design Problems for Beginners (questions)
  • 10 Courses to Crack your Programming Interviews (courses)
  • Top 20 System Design Interview Questions for Programmers (questions)
  • 50+ Data Structure and Algorithms Problems from Interviews (article)

Thanks for reading this article so far. If you like this Vending Machine design Problem in Java and my explanation, then please share it with your friends and colleagues. If you have any questions or doubt then, please drop a note,

P. S. - If you need more OOP design questions for practice then, I suggest you check out the Grokking the Object-Oriented Design Interview course on Educative, an interactive learning platform. This course is designed by the hiring managers of Google, Facebook, Microsoft, and Amazon and contains solutions to some of the frequently asked object-oriented design questions from these tech giants.

42 comments :

Anonymous said...

Your getChange() method does not work fine, you just check if the inventory have coin and don't care about quantity of each coin in inventory.
Let i assume that price of COKE is 21, I select COKE and insert a QUARTER, vm return to me 4 PENNY, I do it one more time, at this case NotSufficientChangeException should be throw.

Code:
long price = vm.selectItemAndGetPrice(Item.COKE);
System.out.println("Price of COKE:" + price);
vm.insertCoin(Coin.QUARTER);
Bucket> bucket = vm.collectItemAndChange();
System.out.println("[Return] " + bucket);

vm.selectItemAndGetPrice(Item.COKE);
vm.insertCoin(Coin.QUARTER);
bucket = vm.collectItemAndChange();
System.out.println("[Return] " + bucket);

((VendingMachineImpl)vm).printStats();

Your result:
Price of COKE:21
[Return] First: COKE, Second: [PENNY, PENNY, PENNY, PENNY]
[Return] First: COKE, Second: [PENNY, PENNY, PENNY, PENNY]
Total Sales : 42
Current Item Inventory :
- COKE : 3
- SODA : 5
- PEPSI : 5

Current Cash Inventory :
- PENNY : 0
- NICKLE : 5
- DIME : 5
- QUARTER : 7


javin paul said...

@unknown, good catch, in fact this is the reason why one of the unit test fail in the second part. Btw, can you fix the problem? What we need to change in the getChange() method? or else, I'll fix it.

Anonymous said...

We need to solve the problem "Coin change with limited number of coins". And I dont know how to solve this right now. Do you have any ideal?
One more thing in your solution, should the insertCoin() method put the coin to the inventory intermediately??? So I put 5 PENNY, I dont select any item and I refund. I receive 1 NICKLE!

Anonymous said...

Within a limited time, I will use greedy algorithm and code something like below:

private List getChange(long amount) throws NotSufficientChangeException {
List changes = new ArrayList<>();
Inventory changeInventory = new Inventory();
if (amount > 0) {
long balance = amount;
List inventory = cashInventory.getAll();
Collections.sort(inventory, new Comparator() {

@Override
public int compare(Coin o1, Coin o2) {
return o2.getDenomination() - o1.getDenomination();
}
});
while (balance > 0) {
boolean isContinue = false;
for (int i = 0; i < inventory.size(); i++) {
if (balance >= inventory.get(i).getDenomination()
&& cashInventory.hasItem(inventory.get(i), changeInventory.getQuantity(inventory.get(i)) + 1)) {
balance -= inventory.get(i).getDenomination();
changeInventory.add(inventory.get(i));
isContinue = true;
break;
}
}
if (!isContinue) {
break;
}
}

if (balance != 0) {
throw new NotSufficientChangeException("Not sufficient change, please try another product!");
}

for (Coin c : changeInventory.getAll()) {
for (int i = 0; i < changeInventory.getQuantity(c); i++) {
changes.add(c);
}
}
}

return changes;
}

Does this method work fine?

javin paul said...

@Duc Nguyen Tien (3), Yes, greedy algorithm is absolutely fine, I haven't tested your code yet, but there are couple of enhancement need to made e.g. getAll() method which should return list of coins.

Also, I didn't understand the call to hasItem()

&& cashInventory.hasItem(inventory.get(i), changeInventory.getQuantity(inventory.get(i)) + 1))

first one look alright but then I think there is typo because hasItem() only accept one parameter. I am not sure what you are checking with changeInventory there.

Please explain

Anonymous said...

Inventory.getAll() will return list item in inventory
public List getAll() {
return new ArrayList(inventory.keySet());
}
Inventory.hashItem(T item, quantity) check if there are at least "quantity" item in inventory
public boolean hasItem(T item, int quantity) {
return getQuantity(item) >= quantity;
}
I just try to fix the problem, you just check if the inventory has coin and dont care about quantity of this coin.
So every time I pick a coin, I temporaryly put it to a inventory - changeInventory (so i will know how many item i choosed), and I can check if the cash inventory have enough item.
cashInventory.hasItem(inventory.get(i), changeInventory.getQuantity(inventory.get(i)) + 1))

Rudraraju Varma said...

One Question here. Should not the Cash Inventory and the Item Inventory be outside of the Vending Machine implementation class. Since Vending machine is a factory instantiation and serves 1 per user, so the cash and item inventory will always be initialized to its default values each time a user is using a vending machine.

javin paul said...

@Rudraraju, Vending machine is not per user. There supposed to be only one Vending Machine per application. May be you can consider Making Vending Machine Singleton.

Rudraraju Varma said...

@Javin,
If The Vending machine is not per user then why do we require Factory class To retrieve the Vending Machine

javin paul said...

@Rudraraju, Factory pattern is used to encapsulate creation of Vending machine, right now its returning the default implementation but it could return a different type of Vending machine in future.

Unknown said...

Hi,
Is this Code Updated

Anonymous said...

how to run this program

Unknown said...

import java.util.*;

class Sample
{
int total=0;
public int input(int coins)
{
switch(coins)
{
case 1:
total = total +1;
break;
case 5:
total = total + 5;
break;
case 10:
total = total + 10;
break;
case 25:
total = total + 25;
break;
case 50:
total = total + 50;
break;
case 100:
total = total + 100;
break;
case 200:
total = total + 200;
break;
default :
System.out.println("Wrong Input");
break;
}
return 0;
}

public int select(int choice)
{

switch(choice)
{
case 1:
System.out.println("You have selected CANDY(10)");
total = total - 10;
System.out.println("Your Remaining change: "+total);
break;
case 2:
System.out.println("You have selected SNACK(50)");
total = total - 50;
System.out.println("Your Remaining change: "+total);
break;
case 3:
System.out.println("You have selected NUTS(90)");
total = total - 90;
System.out.println("Your Remaining change: "+total);
break;
case 4:
System.out.println("You have selected COKE(25)");
total = total - 25;
System.out.println("Your Remaining change: "+total);
break;
case 5:
System.out.println("You have selected PEPSI(35)");
total = total - 35;
System.out.println("Your Remaining change: "+total);
break;
case 6:
System.out.println("You have selected SODA(45)");
total = total - 45;
System.out.println("Your Remaining change: "+total);
break;
case 7:
System.out.println("You have cancelled your Item.");
total = total - 0;
System.out.println("Your Remaining change: "+total);
break;
case 8:
Machine.main(null);
break;

default:
System.out.println("Wrong Choice");

}
return 0;
}
}

class Machine
{
public static void main(String args[])
{
Sample sp = new Sample();
Scanner sc = new Scanner(System.in);
System.out.println("\t\t\tWelcome to Vending Machine.");
System.out.println("Put coins in the denomination of: 1,5,10,25,50,100,200");
int coins = sc.nextInt();
sp.input(coins);
System.out.println("Select one of the Items: \n");
System.out.println("1.CANDY(10)");
System.out.println("2.SNACK(50)");
System.out.println("3.NUTS(90)");
System.out.println("4.COKE(25)");
System.out.println("5.PEPSI(35)");
System.out.println("6.SODA(45)");
System.out.println("7.CANCEL");
System.out.println("8.RESET");
int choice = sc.nextInt();
sp.select(choice);

}
}

peyali said...

Hi All,

As per above comments chain there is a issue in getChange() method, which is not considering quantity of coins.Below is my attempt to fix it works for me.

private List getChange(long changeAmount) throws NotSufficientChangeException {
List changes = Collections.emptyList();
int CHANGE_PENNY = 0;
int CHANGE_NICKEL = 0;
int CHANGE_DIME = 0;
int CHANGE_QUARTER = 0;
int CHANGE_DOLLAR = 0;
if(changeAmount > 0)
{
changes = new ArrayList();
long remaining = changeAmount;
while (remaining > 0) {
if (remaining >= Coin.DOLLAR.getCents()
&& coinsInventory.isInStock(Coin.DOLLAR) && CHANGE_DOLLAR < coinsInventory.getQuantity(Coin.DOLLAR)) {
changes.add(Coin.DOLLAR);
remaining = remaining - Coin.DOLLAR.getCents();
CHANGE_DOLLAR++;
continue;
}else if (remaining >= Coin.QUARTER.getCents()
&& coinsInventory.isInStock(Coin.QUARTER) && CHANGE_QUARTER < coinsInventory.getQuantity(Coin.QUARTER) ) {
changes.add(Coin.QUARTER);
remaining = remaining - Coin.QUARTER.getCents();
CHANGE_QUARTER++;
continue;
} else if (remaining >= Coin.DIME.getCents()
&& coinsInventory.isInStock(Coin.DIME) && CHANGE_DIME < coinsInventory.getQuantity(Coin.DIME)) {
changes.add(Coin.DIME);
remaining = remaining - Coin.DIME.getCents();
CHANGE_DIME++;
continue;
} else if (remaining >= Coin.NICKEL.getCents()
&& coinsInventory.isInStock(Coin.NICKEL) && CHANGE_NICKEL < coinsInventory.getQuantity(Coin.NICKEL)) {
changes.add(Coin.NICKEL);
remaining = remaining - Coin.NICKEL.getCents();
CHANGE_NICKEL++;
continue;
} else if (remaining >= Coin.PENNY.getCents()
&& coinsInventory.isInStock(Coin.PENNY) && CHANGE_PENNY < coinsInventory.getQuantity(Coin.PENNY)) {
changes.add(Coin.PENNY);
remaining = remaining - Coin.PENNY.getCents();
CHANGE_PENNY++;
continue;
} else {
throw new NotSufficientChangeException(
"Not Sufficient Change, Please try another product");
}
}

}
return changes;
}

qwerty said...

shouldn't the reset method reinitialize the inventories rather than clearing it. I mean when i hear reset, it would generally mean at the EOD, the machine's items will be refilled, totalsales will be calculated and reset to 0, and the coin inventory is reinitialized for the next day. If the inventories are reset how is it going to be used again?

qwerty said...

By the way here's my getChange method:
public HashMap getChange(long remaining) throws NotSufficientChangeException{
HashMap coinMap = initializeHashMap();
long balance = remaining;
while (balance > 0) {
if (balance >= Coin.QUARTER.getDenominator() && coinInventory.hasItem(Coin.QUARTER) && coinInventory.getQuantity(Coin.QUARTER) >= balance/Coin.QUARTER.getDenominator()) {
coinMap.put(Coin.QUARTER,coinMap.get(Coin.QUARTER) == null ? 0 : coinMap.get(Coin.QUARTER)+1);
balance-=Coin.QUARTER.getDenominator();
continue;
} else if (balance >= Coin.DIME.getDenominator() && coinInventory.hasItem(Coin.DIME) && coinInventory.getQuantity(Coin.DIME) >= balance/Coin.DIME.getDenominator()) {
coinMap.put(Coin.DIME,coinMap.get(Coin.DIME) == null ? 0 : coinMap.get(Coin.DIME)+1);
balance-=Coin.DIME.getDenominator();
continue;
} else if (balance >= Coin.NICKEL.getDenominator() && coinInventory.hasItem(Coin.NICKEL) && coinInventory.getQuantity(Coin.NICKEL) >= balance/Coin.NICKEL.getDenominator()) {
coinMap.put(Coin.NICKEL,coinMap.get(Coin.NICKEL) == null ? 0 : coinMap.get(Coin.NICKEL)+1);
balance-=Coin.NICKEL.getDenominator();
continue;
} else if (balance > Coin.PENNY.getDenominator() && coinInventory.hasItem(Coin.PENNY) && coinInventory.getQuantity(Coin.PENNY) >= balance/Coin.PENNY.getDenominator()) {
coinMap.put(Coin.PENNY,coinMap.get(Coin.PENNY) == null ? 1 : coinMap.get(Coin.PENNY)+1);
balance-=Coin.PENNY.getDenominator();
continue;
} else {
throw new NotSufficientChangeException("Not sufficient Change Exception, please try another product");
}
}
return coinMap;
}

I have changed List to HashMap everywhere

javin paul said...

@qwery, very well thought. Yes, reset should behave like this in practice. You can modify reset method accordingly.

Anonymous said...

I'm no expert, but I don't like how long and complicated the main class (the vending machine itself) is. Shouldn't it have a single responsibility? Why not delegate handling the coins and handling the inventory to separate objects like InventoryManager and Register for example?

javin paul said...

@Anonymous, , well you can always do refactoring to improve the code. Though I like your suggestion to make main class shorter.

anonym said...

What if I want to add some more items to inventory ? What changes would you suggest then ?

java tech said...

please update the latest running code of getChange()

Michal tech said...

another possible implementation for getChange:

/**
* @return the list of coins to be returned as change, or null if there is not the right amount of coins
*/
private ArrayList checkChange() throws NotSufficientChangeException{
int changeNeeded = curBalance - curProduct.getCost();
ArrayList allChange = new ArrayList<>();
while (changeNeeded > 0){
if (changeNeeded > Coin.QUARTER.getValue() && allCoins.contains(Coin.QUARTER)){
allChange.add(Coin.QUARTER); //add to the change
allCoins.remove(Coin.QUARTER); //remove from all coins
changeNeeded -= Coin.QUARTER.getValue(); //update the change needed
}
else if (changeNeeded > Coin.DIME.getValue() && allCoins.contains(Coin.DIME)){
allChange.add(Coin.DIME); //add to the change
allCoins.remove(Coin.DIME); //remove from all coins
changeNeeded -= Coin.DIME.getValue(); //update the change needed
}
else if (changeNeeded > Coin.NICKLE.getValue() && allCoins.contains(Coin.NICKLE)){
allChange.add(Coin.NICKLE); //add to the change
allCoins.remove(Coin.NICKLE); //remove from all coins
changeNeeded -= Coin.NICKLE.getValue(); //update the change needed
}
else if (changeNeeded > Coin.PENNY.getValue() && allCoins.contains(Coin.PENNY)){
allChange.add(Coin.PENNY); //add to the change
allCoins.remove(Coin.PENNY); //remove from all coins
changeNeeded -= Coin.PENNY.getValue(); //update the change needed
}
else throw new NotSufficientChangeException();
allCoins.addAll(allChange);
refund();
}
return allChange;
}

Unknown said...

Hi Thanks for solution. It's very useful. But, still i'm very much confuse, Firstly I tried by own, I was just adding more and more methods, variables, conditional statements and sitch cases.
How should I achieve solution approach like your solution(I'm having exp of 1 yr :( ), how can i improve logic implementation.

yakkala1 said...

I would be using State Pattern for this problem

Unknown said...

well explained design and implementation!!

Santhosh dk said...

The better approach would be to use State Design pattern.

javin paul said...

Indeed, you can implement this solution using State design pattern as well.

Wondering said...

I was looking a your code for this design,.I noticed that you define classes/methods calling other classes/methods, before defining the called classes/methods. For example you define the Enums after they are used. So I'm wondering if that is your personal preference or is there a reason for doing it this way?

javin paul said...

It was because I have put all code in one page, in reality they are in different source file, one file per class.

Wondering said...

I see. How about the method placements with the VendingMachineImpl class? What I am asking here, is that are you grouping methods by functionality or as they are called, or some other system. I would appreciate any elaboration.

maxim said...

We have another problem. In some countries there is 3 nominal coins. So suppose our machine has [5, 3 3] for change. This code will not able to give me 6(3+3). It will fail after 5+3

javin paul said...

That's interesting? how you going to fix that maxim? Do you have solution for your problem?

AbdelkarimLYOUSSI said...

I agree with you Wondering, the way Paul is coding is not helpful especially for beginners who want to follow with him. Method invocation before its implementation is something that confuses even senior programmers...

javin paul said...

The key here is to understand how Java program is executed, like the execution starts from main and method is called using objects. If you know that, you won't be trouble, order doesn't matter if your code has compiled fine.

Unknown said...

Hi
I havent read throught all the coments, so i don't know wheather its point out already or not, but shouldnt the initialize() method also put items into itemInventory?

javin paul said...

@Unknown, yes it does that by putting cash and items into respective inventory.

Anonymous said...

I'm new to java, if i use these class how to execute, if anyone has sample main method?

Anonymous said...

how to run this progran

javin paul said...

You need to create unit test to run this program, check out the second part of this article here where I have shared unit tests for this Vending machine program.

Unknown said...

very good work. Thanks for this,very clear.

javin paul said...

Thank you @Unknown, glad you like it.

Unknown said...

By making item a enum, aren't we making it difficult to change item price in future?

Post a Comment