Develop Reference

Adding multiple authors to an ArrayList from user input

I have two classes and they both form ArrayLists. I have a menu option where the user needs to be able to add a book (title, copyright, ISBN, edition, and AUTHOR). This works entirely, but the problem is I need an option to add multiple authors and I cannot think of a way around this. Here is my working code below for adding a single author:
public void addBook(ArrayList<Book> books, ArrayList<Author> aut) {
Scanner scan = new Scanner(System.in);
System.out.println("Please enter the title:");
String title = scan.nextLine();
System.out.println("Please enter the copyright:");
int copy = scan.nextInt();
System.out.println("Please enter the edition:");
int ed = scan.nextInt();
System.out.println("Please enter the isbn:");
String isbn = scan.nextLine();
scan.nextLine();
System.out.println("Please enter the author: ");
String authors = scan.nextLine();
ArrayList<Author> aut = new ArrayList<>();
String firstName = authors.split(" ")[0];
String lastName = authors.split(" ")[1];
Author bookAuthor = new Author(firstName, lastName);
boolean foundAuthor = false;
for (int i = 0; i < aut.size(); i++) {
Author currentAuthor = aut.get(i);
if (currentAuthor.getFirst().equalsIgnoreCase(firstName)
&& currentAuthor.getLast().equalsIgnoreCase(lastName)) {
bookAuthor = currentAuthor;
foundAuthor = true;
break;
}
}
if (!foundAuthor) {
aut.add(bookAuthor);
}
au.add(bookAuthor);
books.add(new Book(title, copy, ed, isbn, au));
}
What is best approach to support the user entering multiple authors?

This will be fairly long, but hopefully this should help you.
First off, you should put a method in your Book class, that will prompt a user for authors, and add any of entered authors into the list for that object.
Here is what that class would look like along with a main that is just for testing purposes. I have excluded all of your other fields:
public class Book {
ArrayList<Author> authors = new ArrayList<>();
public Book() {
}
public Book(String title, int copyright, int edition, String isbn, ArrayList<Author> authors) {
/*
I left these fields out of this example on purpose.
this.title = title;
this.copyright = copyright;
this.edition = edition;
this.isbn = isbn; */
this.authors = authors;
}
public void promptForAuthors()
{
Scanner scan = new Scanner(System.in);
String response = "Y";
do {
System.out.println("Please enter the author. ");
String input = scan.nextLine();
String [] splitAuthors = input.split(" ");
Author author = new Author(splitAuthors[0], splitAuthors[1]);
if (!this.authors.contains(author)) {
this.authors.add(author);
}
System.out.println("Would you like you add an additional author? (Y or N) ");
response = scan.nextLine();
} while(response.equalsIgnoreCase("Y"));
}
public void printAuthors() {
for (Author a : this.authors) {
System.out.println(a.getFirstName() + " " + a.getLastName());
}
}
public static void main(String[] args) {
Book b = new Book(); //This will normally be the constructor that includes the ISBN, copyright, etc.
b.promptForAuthors();
b.printAuthors();
}
}
You can see from the main how you should use this in your addBooks code. Basically you create your Book out of all the other fields you used as input, then you can call this method on that object.
You will probably need to modify your current constructor so that it no longer requires the list for authors to be input when constructed, so you can use it afterward (you can just have both constructors available as well).
The promptForAuthors() will continue to prompt the user to enter in an Author until they hit anything other than Y or y.
This method will always ask for an author as least a single time, due to the use of a do-while loop. One of the main advantages of doing it in the Book class itself, is this method could be reused at any time to add more authors to the same book down the line if that is functionality eventually needed.
Additionally, the method will use .contains() to check if the Author already exists inside of the list, and only add the Author if it is a new Author. This requires an Override of .hashCode() and .equals() in your Author class which you can find the one I made below:
//IN THE AUTHOR CLASS PLACE THESE METHODS AND MODIFY FOR EXTRA FIELDS
#Override
public boolean equals(Object o) {
if (o == this) return true;
if (!(o instanceof Author)) {
return false;
}
Author user = (Author) o;
return user.firstName.equals(this.firstName) &&
user.lastName.equals(this.lastName);
}
#Override
public int hashCode() {
int result = 17;
result = 31 * result + this.firstName.hashCode();
result = 31 * result + this.lastName.hashCode();
return result;
}
Note: I included a printAuthors() method just as an easy way to verify no extra duplicate Authors were added.
Example Run of adding Authors:
Please enter the author.
Joe Smith
Would you like you add an additional author? (Y or N)
y
Please enter the author.
Billy Bob
Would you like you add an additional author? (Y or N)
n
Joe Smith
Billy Bob

Try allowing the user to enter a special string to exit the input loop.
Change
String authors = scan.nextLine();
into a loop to populate a list of strings from input
ArrayList<String> authorNames = new ArrayList<>();
System.out.println("Please enter authors. Enter DONE when you are done.");
String nextLine = scan.nextLine();
while (!nextLine.equals("DONE")) {
authorNames.add(nextLine);
nextLine = scan.nextLine();
}
// This doesn't prevent the user from entering zero authors. Idk if you care about that.
alternatively, accept input of authors as only one line with some delimiter and use String.split on that delimiter later.

1 - create a list of lists.(List of Another list author).
2 - create an object of author list.
3 - perform all the action you want to perform on this author list.
4 - add author list to list of author list.
5 - To access author you need syntax like first_list_object.get(index).author_object_variable_or_member.
ArrayList<ArrayList<Author>> authorList = new ArrayList<>();
ArrayList<Author> au = new ArrayList<Author>();
String firstName = authors.split(" ")[0];
String lastName = authors.split(" ")[1];
Author bookAuthor = new Author(firstName, lastName);
au.add(bookAuthor);
authorList.add(au);

I think You're working with the wrong collection. You have a couple of author's. Each author could have written several books. So the right thing would be a Map<Author,ArrayList<Book>>.
It will make Your programming logic for addBook() easier, if You can use a map.

Is there a way i can add multiple variables to one object?

So I've been given the method below and I'm not allowed to change it. What I need is it to create a couple of objects with the variables below but keeps coming up with an error that says "The constructor menu(int, String, String) is undefined." Am I doing something wrong?
import java.util.Scanner;
import java.util.*;
public class menu {
private static void addNewStudent()
{
Scanner scanner = new Scanner(System.in);
System.out.println("Please enter the correct details below");
System.out.println("ID: ");
int userId = scanner.nextInt();
System.out.println("First name: ");
String userFirst = scanner.next();
System.out.println("Last name: ");
String userLast = scanner.next();
System.out.println("English assignment 1 mark: ");
int english1 = scanner.nextInt();
System.out.println("English assignment 2 mark: ");
int english2 = scanner.nextInt();
System.out.println("English assignment 3 mark: ");
int english3 = scanner.nextInt();
System.out.println("Math assignment 1 mark: ");
int math1 = scanner.nextInt();
System.out.println("Math assignment 2 mark: ");
int math2 = scanner.nextInt();;
System.out.println("Math assignment 3 mark: ");
int math3 = scanner.nextInt();
menu userStudentObj = new menu(userId, userFirst, userLast);
menu userEnglishObj = new menu(english1, english2, english3);
menu userMathObj = new menu(math1, math2, math3);
// Asks the user for the student information (ID, First, Last, Assignments)
// Then creates the appropriate objects and adds the students to the student list
I would expect to have created 3 new object that contain the user input variables within the objects if that makes sense.

The main problem is that you didn't specify any non default constructor, to create objects so compiler have no ideas what do you want to create if object is not specified for that. But there is another problem. Your object is designed in not the best way. So if you want to handle it correctly you could change your class design so you handle it in right way.
For example you could make it a bit different, like in this example (it's not perfect, it still can be more separate and better designed):
class User {
int userId;
String userFirstName;
String userSecondName;
private ArrayList<Integer> mathSums = new ArrayList<>();
private ArrayList<Integer> englishSums = new ArrayList<>();
User(int userId, String userFirstName, String userSecondName) {
this.userId = userId;
this.userFirstName = userFirstName;
this.userSecondName = userSecondName;
}
public void addMathSums(int mathSums) {
this.mathSums.add(mathSums);
}
public void addEnglishSums(int englishSums) {
this.englishSums.add(englishSums);
}
}
So by the help of this class you could change the way, how you define and set the values for your object and it would look like that:
1) You define constructor with arguments so you're able to use it for object creation:
User user = new User(userId, userFirst, userLast);
2) Then you can add sums into ArrayLists that are member variables (fields) in User class:
user.addMathSums(math1);
user.addMathSums(math2);
user.addMathSums(math3);
user.addEnglishSums(english1);
user.addEnglishSums(english2);
user.addEnglishSums(english3);
It could be performed in different ways using arrays, lists or other data structure, but you really need to pay attention for your class design. More time you spend on designing object less modifications you will need later.
3) You probably want to interact with this object so you need to get a reference for it somehow. You could change the method signature so you will "return" created object after you finish your input. So you need to make some changes:
On the method change the signature return type to User:
private static User addNewStudent() { ... }
Then add return for that and return created user:
private static User addNewStudent() {
/* code */
User user = new User(userId, userFirst, userLast);
/* add sums */
return user;
}
4) Use this method in your main():
public static void main(String[] args) {
User createdUser = User.addNewStudent();
}

Java - Calling private arraylist from class [duplicate]

This question already has answers here:
Is it possible in Java to access private fields via reflection [duplicate]
(3 answers)
Closed 7 years ago.
My template opens with a menu of options and the user inputs something between 1-3 to select one of the three options.
When the user chooses option 1, it asks them to input a number teamNumber. One must instantiate the class Team, then it writes it to an arraylist.
If there is at least one number in numberList, the user can select option 2. It asks them to input any of the numbers from the arraylist and searches it. If the number they input is found, then you input a String teamMemberFirstName and a char firstInitialLastName. Then it will write the input to a private arraylist located in another class TeamMember.
Once they have input the info in option 1 and 2, they can choose option 3. It allows you to print the list of inputted names based on which team number you put them on.
I am not sure how, in option 3, to call the private arraylist from the TeamMember class teamList. Any guidance on how to proceed with this step? My code is below.
Main class:
public class Main {
public static void main(String[] args) {
int choosing;
Scanner scan = new Scanner(System.in);
String input;
int teamNumber;
boolean stayInLoop;
ArrayList<Team> numberList = new ArrayList<Team>();
do {
stayInLoop = true;
System.out.println("1. Add a new team");
System.out.println("2. Add a new team member");
System.out.println("3. View teams");
input = scan.nextLine();
if (input.equals("1")) {
System.out.println("Enter a team number:");
teamNumber = scan.nextInt();
scan.nextLine();
Team addTeam = new Team(teamNumber);
numberList.add(addTeam);
}
if (input.equals("2")){
boolean foundIt = false;
boolean valid = true;
System.out.println("Team number:");
teamNumber = scan.nextInt();
scan.nextLine();
for (int a = 0; a < numberList.size() && foundIt == false; a++){
Team addTeam = numberList.get(a);
if (addTeam.findTeam() == teamNumber) {
foundIt = true;
System.out.println("Enter first name of team member:");
String teamMemberFirstName = scan.nextLine();
System.out.println("Enter first initial of last name:");
char firstInitialLastName = scan.nextLine().charAt(0);
TeamMember inputTeamMember = new TeamMember(teamMemberFirstName, firstInitialLastName);
inputTeamMember.addMember(inputTeamMember, valid = true);
System.out.println("Success!");
}
}
if (foundIt == false) {
System.out.println("Try again.");
}
}
if (input.equals("3")){
for (int a = 0; a < numberList.size(); a++) {
Team addTeam = numberList.get(a);
//Not sure what to put where there are ????'s - I tried a few ideas and stuff I found online, but nothing worked
//I assume I call the method/class here????
System.out.println("Team: " + addTeam.findTeam() + " Members: " +
"I will put the member called from the arraylist here????");
}
}
}while (stayInLoop == true;)
}}
TeamMember class:
public class TeamMember {
private final String teamMemberFirstName;
private final char firstInitialLastName;
private ArrayList<TeamMember> teamList = new ArrayList<>();
public TeamMember(String teamMemberFirstName, char firstInitialLastName) {
this.teamMemberFirstName = teamMemberFirstName;
this.firstInitialLastName = firstInitialLastName;
}
public int addMember(TeamMember member, boolean valid) {
valid = teamList.add(member);
return teamList.size();
}
}

You cannot directly access private fields from other classes. Either move your list to the Team class or create a getter to retrieve the list.

In a public class, you can return a private object in a public method. This seems like the easiest way in this project. Add a new method to your TeamMember class, and have it return teamList:
//inside your TeamMember class, anywhere after you assign the private variable
public static ArrayList show(){
//the static keyword, in short, will make the method callable without a class instance.
return teamList;
}
Since the TeamMember method show() is now static, you should be able to simply call TeamMember.show() and get the ArrayList.
Important note: In order for this to work, you must make the private arraylist static too. A static object cannot call a non-static object.
This will turn it into private static ArrayList<TeamMember> teamList = new ArrayList<>();
In the Main class, like I said above, simply call TeamMember.show(). You do not need to create an instance.

If you change your teamList to public instead of private your Main class will be able to access the variable. When you make something private in Java you're basically making that instance variable accessible only through the class that it's instantiated in. If you want the variable to be visible to other classes for reference you should make it public
Since the assignment calls for it, you're going to need to define a getter and setter for your 'teamList' variable.
public void setArray(ArrayList newTeamList){
teamList = newTeamList;
}
public ArrayList getArray(){
return teamList;
}
This'll allow you to access the private variable through the methods

Java target/select class/object - confused (sorry for bad title)

What I have below is producing the desired results by print some employee details along with weekly / monthly wages as appropriate.
However I understand that I should not be inputting data in the constructor as I've done.
I need to prompt for a hours worked value only for "PartTimeEmployees", just not the way I've done it.
I've tested with For-Each loops, Enhanced For loops and using the instanceOf operator.
If I could get some guidance/hints or examples of how to accomplish what is currently being done in the constructor, but in the TestEmployee class instead that would be great.
Mostly I'm not sure how to even describe what I'm trying to achieve. This hinders Googling somewhat. (Help with a better title would also be great)
Thanks in advance.
public class TestEmployee
{
public static void main(String[] args)
{
int size;
Employee[] employees = new Employee[4];
employees[0] = new FullTimeEmployee("Jane", 26000);
employees[1] = new PartTimeEmployee("Jack");
employees[2] = new FullTimeEmployee("Lucy", 52000);
employees[3] = new PartTimeEmployee("Lenny");
for(int i = 0; i < employees.length; i++)
{
employees[i].print();
}
}
}
Class: PartTimeEmployee - Constructor:
public PartTimeEmployee(String thisName)
{
super(thisName);
System.out.println("Please enter the number of hours worked by " + thisName + ": ");
numHours = keyboard.nextInt();
setHours(numHours);
}

If I get your question, below might fit with your need -
First of all create generic Employee class -
class Employee {
private String name;
private int workingHours;
private final boolean IS_PART_TIME_EMP;
public Employee(String name, int workingHours) {
this.name = name;
this.workingHours = workingHours;
this.IS_PART_TIME_EMP = false;
}
public Employee(String name) {
this.name = name;
this.IS_PART_TIME_EMP = true;
}
public String getName() {
return name;
}
public int getWorkingHours() {
return workingHours;
}
public void setWorkingHours(int workingHours) {
this.workingHours = workingHours;
}
public boolean isPartTimeEmployee() {
return IS_PART_TIME_EMP;
}
}
Now you can use it as per your requirement.
Employee[] employees = new Employee[4];
employees[0] = new Employee("Jane", 26000);
employees[1] = new Employee("Jack");
employees[2] = new Employee("Lucy", 52000);
employees[3] = new Employee("Lenny");
Scanner sc = new Scanner(System.in);
for (Employee employee : employees) {
if(employee.isPartTimeEmployee()) {
System.out.println("Please enter working hours by " + employee.getName() + ": ");
int numHours = sc.nextInt();
employee.setWorkingHours(numHours);
}
}

Constructor is not meant for user input.Its main intention is to initialize object of that class.
Instead of doing that in constructor,you can try something like this
employees[1] = new PartTimeEmployee("Jack");
System.out.println("Please enter the number of hours worked by " + employees[1].getName()+ ": ");
numHours = keyboard.nextInt();
employees[1].setHours(numHours);

You most likely will have some logical main loop in your program, like
while(!quit) {
// 1. ask if you want to add part time or full time employee
// 2. ask proper questions
// 3. call correct constructor
}
Writing such small pseudo code algorithm should be self explanatory and get you going.
Step one: presentation of options available for user and reading user input.
Step two: performing actions depending on user input from step 1
Step three: final call to proper constructor depending on results from steps 1 and 2

If I understood your question correctly (which I'm really not sure of) you want to prompt for the employee data in the main method.
In that case I'd use a loop and query the following things:
name of the employee
does the employee work full time? (y/n)
if yes: what is the wage? (assume hours = whatever a full time employee works a day)
if no: how many hours? (and probably the hourly wage as well)
Then use that information to construct an Employee object (I don't see any need for the subclasses here).

Declaring an object array in java (code included)?

public class Pig {
private int pigss;
private Pig[] pigs;
public Pig[] pigNumber (int pigss)
{
pigs = new Pig [pigss];
return pigs;
}
Code that includes main method:
public class animals{
public static void main(String[] args){
Pig cool = new Pig();
Scanner keyboard = new Scanner (System.in);
System.out.println("How many pigs are there?");
int pigss = Integer.parseInt( keyboard.nextLine() );
cool.pigNumber(pigss);
//This is where I have trouble. I want to use the array pigs here in the main method, this is what i tried:
Pig[] pigs = cool.pigNumber(pigss);
I then tried to use a for loop and assign values (String) to the index of arrays (pigs[]). But the error that gives me is: cannot convert from String to Pig. Any tips are appreciated. THank you.
for(int j = 0; j < pigs.length; j++)
{
System.out.println("What is the pig " + (j+1) + "'s name");
pigs[j] = keyboard.nextLine();
}

Your pigs will need an attribute to contain the string values you are trying to pass:
public class Pig {
private String name;
public void setName(String n) {
name = n;
}
public String getName() {
return name;
}
Then when you want to assign this string value to your pig:
int indexOfPig = 0; // Or whatever it is supposed to be
pigs[indexOfPig].setName("I am a string");
In java you can only use ints as the indexes of arrays

It is saying 'cannot convert from String to Pig' because you can't do that!
If you want somehow convert a String to a Pig, you are going to need to write some code to do the conversion. For example, you might write a constructor that creates a new Pig from some kind of description. Or you might write a method that looks up a Pig by name or number or something.
It is hard to offer any more concrete advice because you don't tell us what is in the string values ... or how you expect the strings to become pigs. (The only suggestion I have is to try Macrame :-) )

Pig doesn't have a name member or even method that accepts a string. Also you are trying to assign a String(keyboard.nextline() to a Pig(pigs[j].
Add an attribute name to your pig.
class Pig{
public String name:
public void Pig(String name){
this.name = name;
}
}
Then assign a new instance of Pig in the loop.
pigs[j] = new Pig(keyboard.nextLine());
Also get rid of the useless class pigNumber. All you need is an ArrayList of Pigs. The array list can be dynanically sized.
List<Pig> pigs = new ArrayList<Pig>
so your loop could be something like
String name = ""
while(true){
name = keyboard.readline();
if(name== "stop"){
break;
}
pigs.add(new Pig(names);
}
Then getting the number of pigs is a simple
System.out.println(pigs.length());