Develop Reference

Turning a Queue into a Priority Queue

In the CarWash program that I have right now there is currently a normal queue that I would like to change into a priority queue. My goal is to take one of the basic server characteristics and use that for priority but I am lost on how to do that. In previous attempts I have tried to change the normal queue into a priority queue and have ran into issues on how I am supposed to base it off a server characteristic.
public class CarWash {
public static void main(String[ ] args) {
Scanner kb = new Scanner (System.in);
System.out.println("Enter wash time: ");
int WASHTIME = kb.nextInt();
System.out.println("Enter arrival probability: ");
double ARRIVALPROB = kb.nextDouble();
System.out.println("enter time for simulation: ");
int TOTALTIME = kb.nextInt();
carWashSimulate(WASHTIME, ARRIVALPROB, TOTALTIME);
}
public static void carWashSimulate(int washTime, double arrivalProb, int totalTime) { //simulates the car wash
Queue<Integer> arrivalTimes = new LinkedList<Integer>( );
int next;
ClientGenerator arrival = new ClientGenerator(arrivalProb);
Server machine = new Server(washTime);
ExpressServer newM = new ExpressServer(washTime);
Averager waitTimes = new Averager( );
Averager lostCustomer = new Averager();
int currentSecond;
// Write the parameters to System.out.
System.out.println("Seconds to wash one car: " + washTime);
System.out.print("Probability of customer arrival during a second: ");
System.out.println(arrivalProb);
System.out.println("Total simulation seconds: " + totalTime);
// Check the precondition:
if (washTime <= 0 || arrivalProb < 0 || arrivalProb > 1 || totalTime < 0)
throw new IllegalArgumentException("Values out of range");
for (currentSecond = 0; currentSecond < totalTime; currentSecond++) {
// Simulate the passage of one second of time
// Check whether a new customer has arrived.
if (arrival.query( )){
System.out.println("Customer arrived at " + currentSecond);
if(arrivalTimes.size() <= 8){
arrivalTimes.add(currentSecond);
}
else{
System.out.println("They left, line was too long");
lostCustomer.addNumber(1);
}
// Check whether we can start washing another car.
if ((!machine.isBusy( )) && (!arrivalTimes.isEmpty( )))
{
next = arrivalTimes.remove( );
waitTimes.addNumber(currentSecond - next);
machine.start( );
System.out.println("Server started at " + currentSecond + " serving customer " + next);
}
// Subtract one second from the remaining time in the current wash cycle.
machine.reduceRemainingTime( );
} // end of for loop
// Write the summary information about the simulation.
System.out.println("Customers served: " + waitTimes.howManyNumbers( ));
if (waitTimes.howManyNumbers( ) > 0)
System.out.println("Average wait for customers served: " + waitTimes.average( ) + " sec");
System.out.println("The number of customers lost was " + lostCustomer);
}
}
}
Client Generator Class:
public class ClientGenerator {
private double probability;
// The approximate probability of query( ) returning true.
public ClientGenerator(double p) {
if ((p < 0) || (1 < p))
throw new IllegalArgumentException("Illegal p: " + p);
probability = p;
}
public void adjust(double a) {
if(a > 0 && a+probability < 1) {
probability = probability + a;
} else if (a < 0 && probability + a > 0) {
probability = probability + a;
}
}
public double getProbability() {
return probability;
}
public boolean query( ) {
return (Math.random( ) < probability);
}
}
Server Class:
public class Server {
private int secondsForService; // Seconds for a single wash
private int timeLeft; // Seconds until this Server is no longer busy
public Server(int s) {
secondsForService = s;
timeLeft =0;
}
public boolean isBusy( ) {
return (timeLeft > 0);
}
public void reduceRemainingTime( ) {
if (timeLeft > 0) timeLeft--;
}
public void start( ) {
if (timeLeft > 0)
throw new IllegalStateException("Server is already busy.");
timeLeft = secondsForService;
}
}
Averager class:
public class Averager
{
private int count; // How many numbers have been given to this averager
private double sum; // Sum of all the numbers given to this averager
public Averager( )
{
count =0;
sum = 0;
}
public void addNumber(double value)
{
if (count == Integer.MAX_VALUE)
throw new IllegalStateException("Too many numbers");
count++;
sum += value;
}
public double average( )
{
if (count == 0)
return Double.NaN;
else
return sum/count;
}
public int howManyNumbers( )
{
return count;
}
}

The question seems to be about how to configure the priority rules employed by a java.util.PriorityQueue. That's relatively straightforward. Depending on which constructor you use to instantiate one, PriorityQueue relies either on the natural order of its elements (see Comparable) or on the order defined by a specified Comparator. Whenever such a queue contains any elements, its head is the least with respect to the operative ordering, or among the least if there are multiple elements such that no other element is less.
In comments you clarified
my goal is to implement some way of randomly assigning a value that represents the type of car, which will then prioritize the luxury car before the other cars.
Note well that PriorityQueue uses the properties of the enqueued objects to establish their relative order. Right now you are enqueuing integer arrival times, which don't confer an ability to distinguish between classes of car. If you want to carry more information about each vehicle that arrives then you would probably want to create a new class for that, maybe something like this:
class ClientArrival {
enum Category { NORMAL, LUXURY }
Category category;
int arrivalTime;
// ...
}
You would then be able to create one or more implementations of Comparator<ClientArrival> to use to define the priority rule for a PriorityQueue<ClientArrival>. For example,
class LuxuryFirstComparator implements Comparator<ClientArrival> {
int compare(ClientArrival o1, ClientArrival o2) {
if (o1.getCategory() == o2.getCategory()) {
// ... order based on arrival time ...
} else if (o1.getCategory() == ClientArrival.Category.LUXURY) {
return -1;
} else {
return 1;
}
}
}
One might set up a PriorityQueue<ClientArrival> using that to determine priority via
Queue<ClientArrival> arrivals = new PriorityQueue<>(new LuxuryFirstComparator());

How can a User create dates in my Calender (java)?

I m currently trying to code a Calender with java.
I created 3 classes:
1. Date( includes year, month....)
2. Event(includes people, place, the class Date ... + an option to create dates )
3. Mainclass My mainclass that contains the menu.
My problem is that I don't know how the user is able to create his own date, because I have to create the object Termin myself... So, can somebody help me fix this? Thx in advance!
public class Event {
private String mDescription, mPlace, mNames;
private Date mStart, mEnd;
Termin(String description, String place, String names, Date start, Date end) {
mBetreff = description;
mOrt = place;
mNamen = names;
mBeginn = start;
mEnde = end;
}
public void create() {
Scanner read = new Scanner(System.in);
System.out.println("Enter 1. description 2. place 3. names 4. start 5. end ein");
mDescription = read.nextLine();
mPlace = read.nextLine();
mNames = read.nextLine();
}
public String toString() {
return "Description : " + mDescription + "\nPlace: " + mPlace + "\nNames: " + mNames + "\nIts starts at " + mStart
+ " and ends at " + mEnd;
}
}
public class Date {
private int year, day, month, hours, minutes;
Datum(int year, int month, int day, int hours, int minutes) {
this.day= day;
this.year= year;
this.month= month;
this.hours= hours;
this.minutes= minutes;
}
public String toString() {
return "\n" + day + "." + month + "." + year + " um " + hours+ ":" + minutes;
}
public void enterDate() {
}
}
EDIT:
I asked this question 2 years ago, back when I just started coding and had no idea of oop and encapsulation ...
To answer my own question, for every newbie that also tries to create a terminal calender:
Date needs the following methos:
public setDate() {
this.year = read.nextLine();
...
}
for every member.
Event takes the resulting object Date, either in the constructor or in a setter like method.

Creating an instance-method to create an appointment is kind of... strange since one needs to create an appointment (called Termin in your case) to create an appointment. One possibility would be the builder pattern. By having a public static inner builder class, you can set the constructor(s) private and enforce the use of that builder:
public class Main {
private int value;
private Main(int value) {
this.value = value;
}
public int getValue() {
return (this.value);
}
public static class MainBuilder {
boolean valueWasSet;
int value;
public MainBuilder() {
this.valueWasSet = false;
this.value = -1;
}
public void setValue(int value) {
this.value = value;
this.valueWasSet = true;
}
public Main build() {
if (!this.valueWasSet) {
throw new IllegalStateException("value must be set before a Main can be build.");
}
return (new Main(this.value));
}
}
}
(this is a simplified sketch to show the core mechanism on how to assert that certain values are set before constructing a Main through MainBuilder.
The process of constructing a Main would be:
MainBuilder builder = new MainBuilder();
builder.setValue(100);
// all following Main's will have a value of 100
Main mainOne = builder.build();
Main mainTwo = builder.build();
builder.setValue(200);
// all following Main's will have a value of 200
Main mainThree = builder.build();
Main mainFour = builder.build();

How to fix leaking accessor methods?

so I am having trouble figuring out why my test in JUnit is failing. I have a Bill class, a Money class, and a Date class. A new Bill object is being created in the test and the line
assertTrue( myBill.getAmount().getCents() == 0);
is failing. So I am aware of where it is happening but I'm not exactly sure how to fix it. I have tried changing my mutator methods to things like
return new Date(dueDate);
instead of just
return dueDate;
but it is still failing in JUnit. Please help!
Test code:
#Test
public void testBillConstructorPrivacyLeak()
{
Date date1 = new Date( 1, 1, 2020);
Money money1 = new Money( 10);
Bill myBill = new Bill( money1, date1, "sam");
date1.setYear( 2021);
money1.setMoney( 5, 10);
//Now get values and make sure they have not changed
assertTrue( myBill.getAmount().getCents() == 0);
assertTrue( myBill.getDueDate().getYear() == 2020);
}
My classes:
public class Bill
{
private Money amount;
private Date dueDate;
private Date paidDate;
private String originator;
//paidDate set to null
public Bill (Money amount, Date dueDate, String originator) {
this.amount = amount;
this.dueDate = dueDate;
this.originator = originator;
paidDate = null;
}
//copy constructor
public Bill (Bill toCopy) {
this.amount = toCopy.amount;
this.dueDate = toCopy.dueDate;
this.paidDate = toCopy.paidDate;
this.originator = toCopy.originator;
}
public Money getAmount () {
return new Money(amount);
}
public Date getDueDate () {
return new Date(dueDate);
}
public String getOriginator () {
return originator;
}
//returns true if bill is paid, else false
public boolean isPaid () {
return (paidDate != null);
}
//if datePaid is after the dueDate, the call does not update anything and returns false.
//Else updates the paidDate and returns true
//If already paid, we will attempt to change the paid date.
public boolean setPaid (Date datePaid) {
if (datePaid.isAfter(dueDate)) {
return false;
}
else {
paidDate = new Date(datePaid);
return true;
}
}
//Resets the due date – If the bill is already paid, this call fails and returns false.
//Else it resets the due date and returns true.
public boolean setDueDate (Date newDueDate) {
if (isPaid()) {
return false;
}
else {
dueDate = new Date(newDueDate);
return true;
}
}
//Change the amount owed.
//If already paid returns false and does not change the amount owed else changes
//the amount and returns true.
public boolean setAmount (Money amount) {
if (isPaid()) {
return false;
}
else {
amount = new Money(amount);
return true;
}
}
public void setOriginator (String originator) {
this.originator = originator;
}
//Build a string that reports the amount, when due, to whom, if paid, and if paid
//the date paid
public String toString () {
return "Amount: " + amount + " Due date: " + dueDate + " To: " + "originator" + " Paid?" + isPaid() + "Paid date: " + paidDate;
}
//Equality is defined as each field having the same value.
public boolean equals (Object toCompare) {
if (toCompare instanceof Bill) {
Bill that = (Bill) toCompare;
return this.amount.equals(that.amount) &&
this.dueDate.equals(that.dueDate) &&
this.paidDate.equals(that.paidDate) &&
this.originator.equals(that.originator);
}
return false;
}
}
public class Money
{
private int dollars;
private int cents;
//Constructor which sets the dollar amount, and sets cents to 0
//If the user enters in an amount LT 0, you will throw an IllegalArgumentException
public Money (int dol) {
if (dol < 0) {
throw new IllegalArgumentException ("Must be greater than 0.");
}
this.dollars = dol;
cents = 0;
}
//Constructor which initialized dollars and cents.
//If the user enters in an amount LT 0, you will throw an IllegalArgumentException
public Money (int dol, int cent) {
if (dol < 0 || cent < 0) {
throw new IllegalArgumentException ("Must be greater than 0.");
}
this.dollars = dol;
this.dollars += cent / 100;
this.cents = cent % 100;
}
//Copy constructor
public Money (Money other) {
this.dollars = other.dollars;
this.cents = other.cents;
}
public int getDollars () {
return dollars;
}
public int getCents () {
return cents;
}
//If the user enters in an amount LT 0, you will throw an IllegalArgumentException
public void setMoney (int dollars, int cents) {
if (dollars < 0 || cents < 0) {
throw new IllegalArgumentException ("Must be greater than 0.");
}
this.dollars = dollars;
this.dollars += cents / 100;
this.cents = cents % 100;
}
//Gets the money amount as a double
//For example it might return 5.75
public double getMoney () {
return dollars + (cents / 100.0);
}
//If the user enters in an amount LT 0, you will throw an IllegalArgumentException4
public void add (int dollars) {
if (dollars < 0) {
throw new IllegalArgumentException ("Must be greater than 0.");
}
this.dollars += dollars;
}
//If the user enters in an amount LT 0, you will throw an IllegalArgumentException
public void add (int dollars, int cents) {
if (dollars < 0 || cents < 0) {
throw new IllegalArgumentException ("Must be greater than 0.");
}
this.dollars += dollars;
this.cents += cents;
this.dollars += this.cents / 100;
this.cents = this.cents % 100;
}
//Adds the amounts in other to our money object – reducing cents appropriately.
public void add (Money other) {
this.dollars += other.dollars;
this.cents += other.cents;
this.dollars += this.cents / 100;
this.cents = this.cents % 100;
}
//Two money objects are the same if they have the same value for dollars and cents.
public boolean equals (Object o) {
if( o instanceof Money) {
return this.dollars == ((Money)o).dollars && this.cents == ((Money)o).cents;
}
return false;
}
//Prints out the amount as a string IE “$3.75” or “$4.00” Note the number of digits displayed for cents.
//Again for testing and grading purposes use this EXACT output format
public String toString () {
String c = String.format("%.02d",cents);
return "$" + dollars + "." + c;
}
}

Your problem results from the fact that in your constructor for Bill you store references to the Money and Date objects. Then, when you modify those objects in the test case you are modifying the same objects.
If you don't want that behavior you have to make a deep copy of the Money and Date objects in the Bill constructor, i.e.:
public Bill (Money amount, Date dueDate, String originator) {
this.amount = new Money(amount);
this.dueDate = new Date(dueDate);
this.originator = originator;
paidDate = null;
}
You don't have to do this for originator because Strings are immutable.

Although you do not show the implementation of your Money class, the fact that it has a setMoney method suggests it's mutable. In that case, your problem is that Bill's constructor isn't making copies of the objects it's passed in, and thus any changes to money1 also change the state of myBill. Similar remarks apply to the Date objects.
Try modifying your code as follows:
public Bill (Money amount, Date dueDate, String originator) {
this.amount = new Money(amount); // needs copy-constructor for Money
this.dueDate = new Date(dueDate); // likewise for Date
this.originator = originator; // no copying needed as String is immutable
paidDate = null;
}
//copy constructor
public Bill (Bill toCopy) {
// Make copies also in the copy-constructor
this.amount = new Money(toCopy.amount);
this.dueDate = new Date(toCopy.dueDate);
this.paidDate = (toCopy.paidDate == null) ? null : new Date(toCopy.paidDate);
this.originator = toCopy.originator;
}
In general, designing your objects to be mutable means you have to copy defensively in constructors and elsewhere.
On the other hand, designing your objects to be immutable is better as it avoids such problems (and is in fact the advice given by Joshua Bloch in his "Effective Java" book), but it turns out that Java doesn't help you a lot with them either and it's likely you'll struggle for quite some time with getting them done right.
My recommendation is for you to explore the http://immutables.github.io/ library for a better starting point with this design approach.

When I am trying to replicate your code, I am getting error in this line:
public Date getDueDate () {
return new Date(dueDate);
}
Can you please tell what Date constructor you are using. As java.util.date has no such constructor which takes Date as an arguments.
Please elaborate so that I can proceed with debug and answer your query.
Thanks.

Queue not deleting elements (using linkedlist) [java]

I'm trying to get the hang of queues by making a very simple program that simulates a checkout line. For some reason my custom dequeue function (using a linkedlist's removeFirst()) wont actually delete stuff in the queue.
Heres the CustomerQueue Class
public class CustomerQueue<E> {
private LinkedList<E> list = new LinkedList<>();
public void enqueue(E e) { list.addLast(e); }
public E dequeue() { return list.removeFirst(); }
public int size() { return list.size(); }
public E element() { return list.getFirst(); }
#Override
public String toString() { return "Queue: " + list.toString(); }
}
Here is my main:
public static void main(String[] args) {
CustomerQueue<Customer> queue = new CustomerQueue<>();
final int totalTime = 720;
int totalServiced = 0;
int totalQueued = 0;
int totalArrived = 1;
int maxQueuedAtOnce = 0;
Customer next = new Customer();
queue.enqueue(next);
System.out.println(next.getATime() + " <-- ATIME STIME --> " + next.getSTime());
for (int minute = 0; minute < totalTime; minute++) {
System.out.println("-- Minute " + (minute + 1) + " ---------");
if (queue.element().getSTime() == 0) {
totalServiced++;
System.out.println("Current customer (" + queue.dequeue() + ") finished being serviced. (Removed)");
//queue.dequeue();
}
if (next.getATime() == 0) {
System.out.println("Customer ID=" + next.getID() + " has arrived.");
queue.enqueue(next);
System.out.println("Customer ID=" + next.getID() + " in queue.");
next = new Customer();
System.out.println("New Customer generated. ID=" + next.getID() + " ATIME: " + next.getATime() + " STIME: " + next.getSTime());
totalArrived++;
totalQueued++;
}
System.out.println("Customer ID=" + next.getID() + " arrival ticked down.");
next.tickArrivalDown();
queue.element().tickServiceDown();
System.out.println("Current queue customer ID=" + next.getID() + " ticked down.");
if (queue.size() > maxQueuedAtOnce)
maxQueuedAtOnce = queue.size();
}
System.out.println("Total Customers Generated " + next.getNumCustomers());
System.out.println("Total Customers Serviced: " + totalServiced);
System.out.println("Total Customers Arrived: " + totalArrived);
System.out.println("Maximum Customers Queued: " + maxQueuedAtOnce);
System.out.println(queue.element());
}
And of course the Customer Class:
public class Customer {
private int serviceTime;
private int arrivalTime;
private static int numCustomers = 0;
private int ID;
public int getSTime() { return serviceTime; }
public void setSTime(int t) { serviceTime = t; }
public int getATime() { return arrivalTime; }
public void setATime(int t) { arrivalTime = t; }
public int getID() { return ID; }
public static int getNumCustomers() { return numCustomers; }
Customer() {
serviceTime = (int) (Math.random()*3 + 1);
arrivalTime = (int) (Math.random()*3 + 1);
ID = ++numCustomers;
}
public void tickServiceDown() { serviceTime--; }
public void tickServiceUp() { serviceTime++; }
public void tickArrivalDown() { arrivalTime--; }
public void tickArrivalUp() { arrivalTime++; }
#Override
public String toString() {
return "ID: " + ID + " ArrivalTime: " + arrivalTime + " ServiceTime: " + serviceTime;
}
}
The Customer class I have setup generates its own arrival time and completion time when a Customer is instantiated. Basically at the end of the loop, the next customer's arrival time ticks down and the current customer's service time ticks down. When the current customer's service time hits 0, it should dequeue/remove the first element in the LinkedList. For some reason it isnt removing it. Here is the post-loop output:
Total Customers Generated 358
Total Customers Serviced: 1
Total Customers Arrived: 358
Maximum Customers Queued: 357
ID: 1 ArrivalTime: 0 ServiceTime: -717
I'm totally stumped, no amount of googling has helped.

Looking at your output
ID: 1 ArrivalTime: 0 ServiceTime: -717
and ServiceTime being what is being returned in getSTime()
then as 0 != -717 I think the below will be fine
How about if (queue.element().getSTime() <= 0) {

I figured out the answer. The initial if statement:
if (queue.element().getSTime() == 0) needed to become if (queue.size() > 0 && queue.element().getSTime() == 0)
Also needed to make the queue.element().tickServiceDown(); enclosed with the following if-statement if (queue.size() > 0)

Beginner Java: How do I write a constructor to call methods?

I am trying to figure out how to write a constructor that calls methods. I have been given the following instructions for a Java project. The emboldened ones are relevant to this step. Step 3 I have completed, but I can't confirm if I completed it correctly. The code for Step 3 is the second Date constructor within the Date class.
Uncomment line 1 from DateTest (don’t forget to delete the “Line 1.” part) and build and run the project. What is the output? Why is this the output?
Create a default constructor for Date which sets the date to 1/1/2000. Build and run the project. What is the output?
Create a constructor that has three int parameters for the month, day, and year and sets the values of these instance variables to the values passed in. Uncomment lines 2 and 3. Build and run the project. What is the output?
Rewrite the constructor from question 3 so that it calls setMonth(), setDay(), and setYear(). Build and run the project. What is the output?
Write a set() method that has three parameters for the month, day, and year. Uncomment lines 4 and 5. Build and run the project. What is the output?
Rewrite the constructor from question 3 so that it calls set (). Build and run the project. What is the output?
Below is the code for Date class and DateTest class.
package datetest;
import java.util.Scanner;
public class Date
{
public Date() {
month = 1;
day = 1;
year = 2000;
}
public Date(int m, int d, int y) {
month = m;
day = d;
year = y;
}
private int month;
private int day;
private int year; //a four digit number.
public void setYear(int newYear)
{
year = newYear;
}
public void setMonth(int newMonth)
{
if ((newMonth <= 0) || (newMonth > 12))
{
month=newMonth;
}
else
month = newMonth;
}
public void setDay(int newDay)
{
if ((newDay <= 0) || (newDay > 31))
{
day=1;
}
else
day = newDay;
}
public int getMonth( )
{
return month;
}
public int getDay( )
{
return day;
}
public int getYear( )
{
return year;
}
public void printDate( )
{
System.out.print(getMonth() + "/" + getDay() + "/" + getYear());
}
public void readInput( )
{
boolean tryAgain = true;
Scanner keyboard = new Scanner(System.in);
System.out.println("Enter month, day, and year.");
System.out.println("Do not use a comma.");
month = keyboard.nextInt( );
day = keyboard.nextInt( );
year = keyboard.nextInt( );
}
}
This is the DateTest class.
package datetest;
public class DateTest {
public static void main(String[] args) {
Date today = new Date();
System.out.println("Today’s date is " + today.getMonth() + "/" + today.getDay() + "/" + today.getYear());
//Line 2. today = new Date(55, 55, 2011);
//Line 3. System.out.println("Today’s date is " + today.getMonth() + "/" + today.getDay() + "/" + today.getYear());
//Line 4. today.set(10, 5, 2011);
//Line 5. System.out.println("Today’s date is " + today.getMonth() + "/" + today.getDay() + "/" + today.getYear());
}
}
I have attempted to write the code to call the methods in step 4. Would the following code be the correct way to write a constructor to call methods?
public Date (int m, int d, int y) {
this.setMonth(month);
this.setDay(day);
this.setYear(year);
}

Would the following code be the correct way to write a constructor to call methods?
public Date (int m, int d, int y) {
this.setMonth(month);
this.setDay(day);
this.setYear(year);
}
Yes, if you used your m, d, and y arguments instead of month, day, and year:
public Date (int m, int d, int y) {
this.setMonth(m);
this.setDay(d);
this.setYear(y);
}
With your code, you're actually just setting the instance members (month and so on) to their existing values (because month in the constructor is automatically resolved to the instance data member month using an implied this.). So I'm guessing when you tried it, you ended up with zeroes and didn't understand why. (int members are auto-initialized to zero before the code in the constructor runs.)