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Here is the Complete Code

VendingMachine.java

package statepattern;
import statepattern.exception.MachineWarning;
public class VendingMachine {
    State coninInsertedState = new CoinInsertedState(this);
    State emptyState = new EmptyState(this);
    State noCoinInsertedState = new NoCoinInsertedState(this);
    State dispensingState = new DispensingState(this);
    State machineState = null;
    int capacity = 0;
    public VendingMachine() {
        machineState = noCoinInsertedState;
    }
    public void reFill(int count) {
        capacity += count;
        machineState = noCoinInsertedState;
    }
    /**
     * Two Actions performed by MAchine   
     */
    public void insertCoin() throws MachineWarning {
        machineState.insertCoin();
    }
    
    public void pressButton() throws MachineWarning {
        machineState.pressButton();
        machineState.dispense();
        capacity--;
    }
    
    public boolean isEmpty(){
        if(capacity<=0)
            return true;
        else
            return false;
    }
    
    public void setMachineState(State machineState) {
        this.machineState = machineState;
    }
    public State getMachineState() {
        return machineState;
    }
    public void setConinInsertedState(State coninInsertedState) {
        this.coninInsertedState = coninInsertedState;
    }
    public State getConinInsertedState() {
        return coninInsertedState;
    }
    public void setEmptyState(State emptyState) {
        this.emptyState = emptyState;
    }
    public State getEmptyState() {
        return emptyState;
    }
    public void setNoCoinInsertedState(State noCoinInsertedState) {
        this.noCoinInsertedState = noCoinInsertedState;
    }
    public State getNoCoinInsertedState() {
        return noCoinInsertedState;
    }
    public void setDispensingState(State dispensingState) {
        this.dispensingState = dispensingState;
    }
    public State getDispensingState() {
        return dispensingState;
    }
    public void setCapacity(int capacity) {
        this.capacity = capacity;
    }
    public int getCapacity() {
        return capacity;
    }
}


State interface



import statepattern.exception.MachineWarning;
public interface State {
   
    public void insertCoin()throws MachineWarning;
    public void pressButton()throws MachineWarning;
    public void dispense()throws MachineWarning;
}


Concrete States implementations



ConinINsertedState.java



package statepattern;
import statepattern.exception.MachineWarning;
public class CoinInsertedState implements State{
    VendingMachine machine =null;
    public CoinInsertedState(VendingMachine machine) {
        this.machine =  machine;
    }
    public void insertCoin() throws MachineWarning{
        throw new MachineWarning("Coin is already inserted.");
    }
    public void dispense() throws MachineWarning{
        throw new MachineWarning("Dispense button is not pressed.");
    
    }
    public void pressButton() throws MachineWarning{
        machine.setMachineState(machine.getDispensingState());
    }
}


DispensingState.java



package statepattern;
import statepattern.exception.MachineWarning;
public class DispensingState implements State{
    VendingMachine machine ;
    DispensingState(VendingMachine machine) {
        this.machine = machine;
    }
    public void insertCoin() throws MachineWarning {
        throw new MachineWarning("wait ... previous order is processing");
    }
    public void pressButton() throws MachineWarning {
        throw new MachineWarning("wait ... previous order is processing");
    }
    public void dispense() throws MachineWarning {
        machine.setMachineState(machine.getNoCoinInsertedState());
    }
}


EmptyState.java



package statepattern;
import statepattern.exception.MachineWarning;
public class EmptyState implements State{
    VendingMachine machine;
    public EmptyState(VendingMachine machine) {
        this.machine =  machine;
    }
    public void insertCoin() throws MachineWarning{
        throw new MachineWarning("Can not process the request");
    }
    public void pressButton() throws MachineWarning{
        throw new MachineWarning("Invalid Action");
    }
    public void dispense() throws MachineWarning{
        throw new MachineWarning("Invalid Action");
    }
}


NoCoinInsertedState.java


package statepattern;
import statepattern.exception.MachineWarning;
public class NoCoinInsertedState implements State{
    VendingMachine machine;
    public NoCoinInsertedState(VendingMachine machine) {
        this.machine =  machine;
    }
    public void insertCoin() throws MachineWarning{
        if (!machine.isEmpty()) {
            machine.setMachineState(machine.getConinInsertedState());
        }
        else {
            throw new MachineWarning("Can not process request .. Machine is out of stock");
        }
    }
    public void pressButton() throws MachineWarning{
        throw new MachineWarning("No coin inserted ..");
    }
    public void dispense() throws MachineWarning{
        throw new MachineWarning("Invalid Operation");
    }
}


I have run the Test class to check behavior of Vending MAchine in different states 


Test.java


package statepattern.test;
import statepattern.exception.MachineWarning;
import statepattern.VendingMachine;
public class Test {
    public Test() {
    }
    public static void main(String[] args) {
        VendingMachine machine = new VendingMachine();
        machine.reFill(2); // 
        //First despense 
        try {
            machine.insertCoin();
            machine.pressButton();
        } catch (MachineWarning e) {
            System.out.println("Test 1:" + e.getMessage());
        }
        //Second Test 
        try {
            machine.insertCoin();
            machine.insertCoin();
            machine.pressButton();
        } catch (MachineWarning e) {
            System.out.println("Test 2:" + e.getMessage());
            try {
                machine.pressButton();
            } catch (MachineWarning f) {
                // TODO
            }
        }
        //Test THree 
        try {
            machine.pressButton();
        } catch (MachineWarning e) {
            System.out.println("Test 3:" + e.getMessage());
        }
        //Test Four 
        try {
            machine.insertCoin();
        } catch (MachineWarning e) {
            System.out.println("Test 4:" + e.getMessage());
        }
    }
}

public class VendingMachine {

    
     class ItemNode {
        int quantity;
        int name;
        int id;
        double price;
        public ItemNode ( int quantity , int name , int id , double prices)
         {
         	// Fill it .
         }

         public String toString(){
         	return "[ ID: "+id+"name "+name+" prices "+prices+" quantity"+quantity+" ]";
         }
     }

     protected HashMap<String , ItemNode> itemLocater ;
     
     public VendingMachine(){
     	itemLocater = new HashMap<String , ItemNode>();
     }

     public void sendInstruction( String buttonId )
     {

     }

     public boolean isAvailable(String buttonId)  {

     }

     public double settleBill(double amount) {

     }

     protected List<Double> getChange(double amount){

     }

     protected boolean isChangeAvailable() {

     } 

     private List<Double> greedyChange(double amount ){

     }

     private List<Double> minimumChange(double amount)
     {

     }

 	    private List<Double> twentyFiveCentChange(double amount) {

     }
    
     public boolean isEmpty(){

     }

 }

Use state design pattern

classes
class Item < describes the various items in the vending machine>
class ProductCode < to select which drink/snack you want>
class Coin < to accept coins>
class Refund < to return remainder coins>
class Stock < to keep track of the vending machine stock for each product>
class Bagel, Donut, Energy bar, Chocolates extends class Item

import java.util.HashSet;
import java.util.LinkedHashMap;
import java.util.Scanner;
import java.util.Set;

public class Vending {

LinkedHashMap<String,Double> hm = new LinkedHashMap<String, Double>();

Scanner s = new Scanner(System.in);

private double val;

private String key;

private static int quarters;

private static double dollar;

public void snacks() {
HashSet<String> sm = new HashSet<String>();
hm.put("Lays",2.0);
hm.put("Biscut",1.25);
hm.put("Chocklate", 1.5);
sm.add("Exit");
Set <String> keys = hm.keySet();
for(String s : keys) {
System.out.println(s+":"+hm.get(s));

}
for(String s : sm) {
System.out.println(s);
}
System.out.print("Enter Your Snack Item Name:");
key = s.next();
System.out.println("You have selected"+" "+key);
val = dollar - hm.get(key);
System.out.println(val);
if(key == "Exit") {
item();
}


}
public void drinks() {
HashSet<String> ms = new HashSet<String>();
hm.put("Coke", 3.50);
hm.put("Sprite", 2.5);
hm.put("Pepsi", 2.0);
ms.add("Exit");
Set <String> keys = hm.keySet();
for(String s : keys) {
System.out.println(s+":"+hm.get(s));

}
for(String s : ms) {
System.out.println(s);
}
System.out.print("Enter your Drink Item Name:");

key = s.next();
System.out.println("You have selected"+" "+key);
val = dollar - hm.get(key);
if(key == "Exit") {
item();
}
}
public void item() {
HashSet<String> hs = new HashSet<String>();
hs.add("Snacks<1>");
hs.add("Drinks<2>");
hs.add("Exit<3>");
for(String s : hs) {
System.out.println(s);
}
System.out.print("Enter Your Option:");
int m = s.nextInt();


switch (m) {
case 1:
snacks();
break;
case 2:
drinks();
break;
case 3:
System.out.print("Take your Money $"+dollar);
System.exit(0);

}
if(val == 0){

System.out.println("Thank you for coming have fun");



}

else if(val < 0){

System.out.println("You have Insfficient funds");

}

else if(val > 0){

System.out.println(" Balance:$"+val);

repeat();

}
}


private void repeat() {
System.out.print("Do you Want Anything Else(Y/N):");



char c = s.next(".").charAt(0);

if(c == 'y'){

if(val<0){

System.out.println("Insufficient funds");
}

else { dollar = val;}



item(); repeat(); }

else if(c == 'n'){

System.out.println("Take your Balance:$"+val);}
System.exit(0);

}
public static void main(String[] args) {

Vending ven = new Vending();

System.out.println("Welcome to the Locket Vending Machine");

System.out.print("Insert Quarters :");

@SuppressWarnings("resource")
Scanner s = new Scanner(System.in);
Vending.quarters = s.nextInt();
Vending.dollar = 0.25*quarters;
System.out.println("You Have Inserted $"+Vending.dollar);
ven.item();


}

}

// Amount $1 - 99
public int getCoins(int amount){
		
		int total=0,quater=0,dimes=0,nickle=0,cent=0;	
			
		
		if(amount==0){
			return 0;
		}
		
		quater  = amount/25;
		dimes = amount%25/10;
		nickle = amount%25%10/5;
		cent = amount%25%20%5;
		
	System.out.println("quater:"+quater);
	System.out.println("dimes:"+dimes);
	System.out.println("nickle:"+nickle);
	System.out.println("cent:"+cent);
		
	total = quater+dimes+nickle+cent;
	System.out.println("total no.of required coins:"+total);
	return total;
	}