Develop Reference

Recursion in java with objects of courses and prerequisites

Good day, is there a more efficient way to do this problem with recursion than using a switch statement. In my courses class I have a recursive association of course and a prerequisite then a set the preReqs in the PreReqs class. How can I print out all of preReqs of a class when a user enter a class which has preReqs? Right now I am in the process of using a switch statement and printing each preReq individually but is there a better way to do this still using recursion?
An example out of this: If the user types that course, all of the preReqs will print out too.
ACS-3947 Algorithm Design
prereq: ACS-2947 Data Structures and Algorithms
ACS-2947 Data Structures and Algorithms
prereq: ACS-1904 Programming Fundamentals II
ACS-1904 Programming Fundamentals II
prereq: ACS-1903 Programming Fundamentals I
ACS-1903 Programming Fundamentals I
no prereq
Course class:
import java.util.*;
public class Course
{
protected String courseNumber;
protected String courseName;
protected Course prerequisite;
public Course(){
courseNumber = courseName = "Unknown";
prerequisite= null;
}
public Course (String cn, String num){
this.courseNumber=num;
this.courseName=cn;
}
public String getCourseNumber(){
return courseNumber;
}
public String getCourseName(){
return courseName;
}
public Course getPreReq(){
return prerequisite;
}
public void setCourseNumber(String courseNumber){
this.courseNumber=courseNumber;
}
public void setCourseName(String courseName){
this.courseName=courseName;
}
public void setPreReq(Course pr){
prerequisite =pr;
}
}
PreReq class:
import java.util.*;
import java.io.*;
public class Prereqs
{
public static void main (String [] args){
Scanner kb = new Scanner (System.in);
Course nineteen03 = new Course ("Programming Fundamentals I","ACS-1903");
Course nineteen04 = new Course ("Programming Fundamentals II"," ACS-1904");
Course two47 = new Course ("Data Structures and Algorithms","ACS-2947 ");
Course three47 = new Course ("Algorithm Design","ACS-3947");
Course two09 = new Course ("Internet Programming","ACS-2909");
Course three09 = new Course ("Advanced Internet Programming ","ACS-3909");
nineteen04.setPreReq(nineteen03);
two47.setPreReq(nineteen04);
three47.setPreReq(two47);
two09.setPreReq(nineteen03);
three09.setPreReq(nineteen03);
System.out.println("Enter course number with the format: AAA-999");
String input = kb.next();
validate(input);
}
public static void course(Course nineteen04, Course nineteen03,Course two47, Course three47, Course two09, Course three09, String input ){
Course c1 = nineteen04.getPreReq();
Course c2 = two47.getPreReq();
Course c3 = three47.getPreReq();
Course c4 = two09.getPreReq();
Course c5 = three09.getPreReq();
switch (input){
case "ACS-1904":
System.out.println(nineteen04.getCourseName()+" "+nineteen04.getCourseNumber());
System.out.println("preReq: " + c1.getCourseName()+ " "+ c1.getCourseNumber());
}
}
public static String validate (String input)
{
String arg = input;
boolean valid = arg.length()==7;
if (!valid){
throw new IllegalArgumentException("Not the correct format: AAA-999");
}
valid = arg.charAt(3) == '-';
if(!valid) {
throw new IllegalArgumentException("Not the correct format: AAA-999");
}
for(int i=0; i < 3 && valid; i++){
valid = ((i == 3 && Character.isLetter(arg.charAt(i))));
}
for(int i=3; i < 3 && valid; i++){
valid = ((i==6 && Character.isDigit(arg.charAt(i))));
}
return arg;
}
}

A recursive method needs to contain a condition which terminates the recursion. Your list of courses and their prerequisites remind me of a linked list where each course points to its prerequisite. The list terminates when we reach a course that has no prerequisite. The below code is your Course class with the addition of a main method (imported from your Prereqs class) and the recursive method which I named requirements(). I also added method toString() to make the display of the list of courses and their prerequisites more "human readable". You can experiment by changing the course passed to the initial invocation of method requirements().
public class Course {
protected String courseNumber;
protected String courseName;
protected Course prerequisite;
public Course(){
courseNumber = courseName = "Unknown";
prerequisite= null;
}
public Course (String cn, String num){
this.courseNumber=num;
this.courseName=cn;
}
public String getCourseNumber(){
return courseNumber;
}
public String getCourseName(){
return courseName;
}
public Course getPreReq(){
return prerequisite;
}
public void setCourseNumber(String courseNumber){
this.courseNumber=courseNumber;
}
public void setCourseName(String courseName){
this.courseName=courseName;
}
public void setPreReq(Course pr){
prerequisite =pr;
}
public String toString() {
return courseNumber + " " + courseName;
}
private static void requirements(Course c) {
if (c == null) {
return;
}
else {
System.out.println(c);
requirements(c.getPreReq());
}
}
public static void main(String[] args) {
Course nineteen03 = new Course ("Programming Fundamentals I","ACS-1903");
Course nineteen04 = new Course ("Programming Fundamentals II"," ACS-1904");
Course two47 = new Course ("Data Structures and Algorithms","ACS-2947 ");
Course three47 = new Course ("Algorithm Design","ACS-3947");
Course two09 = new Course ("Internet Programming","ACS-2909");
Course three09 = new Course ("Advanced Internet Programming ","ACS-3909");
nineteen04.setPreReq(nineteen03);
two47.setPreReq(nineteen04);
three47.setPreReq(two47);
two09.setPreReq(nineteen03);
three09.setPreReq(nineteen03);
requirements(three09);
}
}
Running the above code displays the following:
ACS-3909 Advanced Internet Programming
ACS-1903 Programming Fundamentals I

Trying to find if a java list object has an element STRING or INT?

I am trying to find if a list contains a string. I have a list object as follows:
Please note this is just example code, to illustrate my point/question!
import java.util.List;
public class FilterByList {
private String actionHero;
private String actionHero2;
private String move;
private int number;
private String actionHero3;
public FilterByList(String actionHero, String actionHero2, String move, int number, String actionHero3) {
this.actionHero = actionHero;
this.actionHero2 = actionHero2;
this.move = move;
this.number = number;
this.actionHero3 = actionHero3;
}
public String getActionHero() {
return actionHero;
}
public void setActionHero(String actionHero) {
this.actionHero = actionHero;
}
public String getActionHero2() {
return actionHero2;
}
public void setActionHero2(String actionHero2) {
this.actionHero2 = actionHero2;
}
public String getMove() {
return move;
}
public void setMove(String move) {
this.move = move;
}
public int getNumber() {
return number;
}
public void setNumber(int number) {
this.number = number;
}
public String getActionHero3() {
return actionHero3;
}
public void setActionHero3(String actionHero3) {
this.actionHero3 = actionHero3;
}
}
And then:
public static void main(String[] args) {
List<FilterByList> myList = Collections.singletonList(
new FilterByList("superman", "antman", "ACTION", 123, "batman"));
System.out.println(myList);
if (myList.contains("batman")) {
System.out.println("found it!");
} else {
System.out.println("************************* Did not find anything!!1");
}
}
It does NOT find batman in the list object. So what is printed is the follows:
[com.company.FilterByList#7577b641]
************************* Did not find anything!!1
Any idea how I can:
a) print the contents of the list?
b) find an element in the list with as little code as possible?
Would appreciate your help. Please use my code context to answer as it will give me more pointers.

You run contains on List<FilterByList> and that list doesn't have String batman
It has instance of FilterByList, that one of members is field of type string and value 'batman'
Based on your code. You create instance of object FilterByList and you try to compare that object with String
Line
if (myList.contains("batman")) {
Those object are different types, that is the reason, why it is not found
To check if there is batman you can use Stream API
boolean d = myList.stream().map(FilterByList::getActionHero3).anyMatch(hero3 -> hero3.equals("batman"));

Add to your FilterByList class containsHero(String hero) where you compare hero with each of 3 heroes. You don't need to store this single object in List. Just use it.
FilterByList f = new FilterByList("superman", "antman", "ACTION", 123, "batman");
if (f.containsHero("batman")) {
System.out.println("found it!");
} else {
System.out.println("************************* Did not find anything!!1");
}
P.S. imho all the things you are trying to do looks very strange...

I added a new method to the class for searching
public boolean containsHero(String hero) {
return actionHero.equals(hero) || actionHero2.equals(hero) || actionHero3.equals(hero);
}
And then used it with streams like this
if (myList.stream().anyMatch(f -> f.containsHero("batman"))) {
System.out.println("found it!");
}
To get a readable output of your class you can override the toString() method, here is one example
#Override
public String toString() {
StringBuilder builder = new StringBuilder();
builder.append(actionHero);
builder.append(", ");
builder.append(actionHero2);
builder.append(", ");
builder.append(actionHero3);
builder.append(": ");
builder.append(move);
builder.append(": ");
builder.append(number);
return builder.toString();
}
Doing System.out.println(myList);will then output
[superman, antman, batman: ACTION: 123]

You can put into the list two different types of objects, e.g. String and Integer if you will omit the generic type, and implicitly it will be List<Object> myList = new ArrayList<Object>(); for example:
List myList = new ArrayList();
myList.add("superman");
myList.add("antman");
myList.add("ACTION");
myList.add(123);
myList.add("batman");
System.out.println(myList);
if (myList.contains("batman")) {
System.out.println("found it!");
} else {
System.out.println("************************* Did not find anything!!1");
}
But it's not a good way to use collections. For sure, it depends of your particular task, but I'd like to advice to use different collections with different types.
If you want to achieve exactly what you're asking about, you can use Stream API like this:
List<FilterByList> myList = Collections.singletonList(
new FilterByList("superman", "antman", "ACTION", 123, "batman"));
System.out.println(myList);
if (myList.stream().map(FilterByList::getActionHero3).allMatch("batman"::equals)) {
System.out.println("found it!");
} else {
System.out.println("************************* Did not find anything!!1");
}
anyMatch method is more preferable to use, if there will be more than one element in the collection:
(myList.stream().map(FilterByList::getActionHero3).anyMatch("batman"::equals))

Find repeated elements in array list

The following code is suppose to take a string for author and title to check if that exact book is in the array list if it is then it would return the number of copies in the array. So far it's only checking if the book is in the array list, but I was wondering if there is any IPA that I can use to find repeated elements in an array list
public class Book{
private title;
private author;
}
public class Library {
private ArrayList<Book>libraryBooks;
public int checkNumCopies(String title,String author){
int numBookCopies = 0;
for(Book b:libraryBooks){
if((b.equals(title))&& (b.equals(author))){
return "Book is in the library";
}
else
return "Book is not in the library";
}
}

With proper implementation of equals and hashcode implementation in Book
class Book {
private String author;
private String title;
//Getters and Setters
//hashCode and equals impl
//toString impl
}
this can be achieved using streams
List<Book> books = Arrays.asList(new Book("book-1", "title-1"), new Book("book-2", "title-2"), new Book("book-3", "title-3"),
new Book("book-1", "title-1"), new Book("book-2", "title-2"));
Map<Book, Long> bookCount = books.stream().collect(Collectors.groupingBy(b -> b, Collectors.counting()));
System.out.println(bookCount);
output
{Book [author=book-3, title=title-3]=1, Book [author=book-2, title=title-2]=2, Book [author=book-1, title=title-1]=2}

Your Book fields are missing types (I assume you wanted Strings), and you have no accessor (or getters) for those fields. Also, I would probably override equals and use that. Something like,
public class Book {
public Book(String title, String author) {
this.title = title;
this.author = author;
}
private String title;
private String author;
public String getTitle() {
return title;
}
public String getAuthor() {
return author;
}
#Override
public boolean equals(Object obj) {
if (this == obj) {
return true;
}
if (obj instanceof Book) {
Book b = (Book) obj;
return b.title.equals(title) && b.author.equals(author);
}
return false;
}
}
And then to use it, I would do something like
private List<Book> libraryBooks = new ArrayList<>();
public int checkNumCopies(String title, String author) {
Book toFind = new Book(title, author);
int numBookCopies = 0;
for (Book b : libraryBooks) {
if (b.equals(toFind)) {
numBookCopies++;
}
}
return numBookCopies;
}
or in Java 8+, like
Book toFind = new Book(title, author);
return (int) libraryBooks.stream().filter(book -> book.equals(toFind)).count();

Or you could create an equals method on Book and then use a HashSet. Something like
public class Book{
private title;
private author;
//implement hashcode
public boolean equals(Object o) {
if (!(o instanceof Book)){ return false}
Book that = Book.class.cast(o);
// you may want to do a null check here
return this.title.equals(that.title) && this.author.equals(that.author);
}
}
public class Library {
private ArrayList<Book>libraryBooks;
public int checkNumCopies(String title,String author){
return libraryBooks.size() - new HashSet(libraryBooks).size()
}
}
Im not sure entirely on what you need (sorry cant quite understand the question) but hopefully this at least gives you some ideas.
note: I havent compiled this so please excuse any compilation errors.

Here is the solution using java streams API
public class Library {
private ArrayList<Book> libraryBooks;
public long checkNumCopies(String title, String author) {
return libraryBooks
.stream()
.filter(book -> book.getAuthor().equals(author) && book.getTitle().equals(title))
.count();
}
}
I presupposed that there are getter methods in Book class.

If you want to find the number of copies of a book, that would be:
public int checkNumCopies(String title, String author) {
int numBookCopies = 0;
for (Book b : libraryBooks) {
if ((b.title.equals(title)) && (b.author.equals(author))) {
numBookCopies++;
}
}
return numBookCopies;
}
Otherwise, instead of sending title and author individually you can send a book object. That'd be better way :
public int checkNumCopies(Book book) {
int numBookCopies = 0;
for (Book b : libraryBooks) {
if ((b.title.equals(book.title)) && (b.author.equals(book.author))) {
numBookCopies++;
}
}
return numBookCopies;
}
Since you have made the fields private, you need to write getter methods for getting the values from the fields.

Loop for ArrayList printing out null

I'm new to Java so sorry for all the mistakes!
Im creating a Library program consisting of 4 classes: Library, Book, BookInterface & Patron.
In the Book class I have a method that prints out all the books in the library and their status' (in or out). Instead I keep getting something like this:
Great Gatsby: null
Withering Heights: null
Does it have something to do with the setStatus() method?
Every time a user adds a new book, it creates a new Book instance and then I do setStatus("IN"). So how come it is not saving and instead printing out null?
Thank you very much for the help!!
Book class:
import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;
import java.util.Scanner;
public class Book implements BookInterface
{
Scanner input = new Scanner(System.in);
static ArrayList <String> UserList = new ArrayList<String>();
static ArrayList <String> BookList = new ArrayList <String> (); //display just titles// use when checking out books
static ArrayList <String> OrigBookList = new ArrayList <String> (); //keep track of all titles ever entered
public String title;
public String author;
public String book;
public boolean checkIn;
private String status;
private String borrower;
public Book(String t, String a)
{
title = t;
author = a;
}
//constructor create new book
public Book(String newTitle)
{
title = newTitle;
}
public String toString()
{
return title + " " + author;
}
public String getTitle()
{
return title;
}
public void setTitle(String title)
{
this.title = title;
}
public String getAuthor()
{
return author;
}
public void setAuthor(String author)
{
this.author = author;
}
public String getStatus(String book)
{
return status;
}
public void setStatus(String status)
{
this.status = status;
}
public void setBorrower(String borrower)
{
this.borrower = borrower;
}
public String getBorrower(String checkPatron)
{
return borrower;
}
public String getBook(String checkPatron)
{
return book;
}
public void setBook(String bookCheckOut)
{
this.book = bookCheckOut;
}
public void addBook()
{
Scanner input = new Scanner(System.in);
Scanner inputread = new Scanner(System.in);
System.out.print("Enter book title: ");
String title1 = inputread.nextLine();
Scanner input1 = new Scanner(System.in);
System.out.print("Enter book author: ");
String author1 = inputread.next();
Book fullBook = new Book(title1, author1); //create constructor w/ title & author
Book book1 = new Book(title1); //constructor w/ just title to be used to display all books
OrigBookList.add(title1);
book1.setStatus("IN");
System.out.println("-----------------------------------------------");
System.out.println("-----" + title1 + " is now in the library!-----");
System.out.println("-----------------------------------------------");
}
public void editBook()
{
Scanner inputread = new Scanner(System.in);
System.out.println("Enter original book title: ");
String origTitle = inputread.nextLine();
System.out.println("Enter edited book title: ");
String editedTitle = inputread.nextLine();
Collections.replaceAll(Book.UserList, origTitle, editedTitle);
System.out.println("------------------------------------------------------");
System.out.println(origTitle + " has been changed to " + editedTitle + "!");
System.out.println("------------------------------------------------------");
}
public void libraryInventory()
{
System.out.println("------------------ Library Inventory: ---------------");
for(int i =0; i<= OrigBookList.size()-1; i++)
{
//Book Title: checked in/out
System.out.println(OrigBookList.get(i) + ":" + getStatus(OrigBookList.get(i)));
}
System.out.println("-----------------------------------------------------");
}
}

getStatus(OrigBookList.get(i)) ignores the parameter you pass to it and just returns the status of the Book for which you called the libraryInventory method. Obviously, that Book instance doesn't have the status field initialized, but even if it did, it will give you the status of just one Book.
Instead of having a static list of book titles (static ArrayList <String>), perhaps you should maintain a list of the books themselves (static ArrayList <Book>), or even better, put that list in a separate class (you can call it Library).
Methods such as libraryInventory shouldn't be in the Book class (and if you insist on keeping them in the Book class, make them static, since they don't refer to a single Book instance).

Your whole program seems to be running inside an instance of the class Book. In it, you are making and discarding new instances of Book, called fullBook and book1, and for fullBook you set its status. When you call getStatus on the main Book in your program, it just returns its own status, which was never set to anything.
If you want to save a sequence of instances of Book, you need to put the instances somewhere, not just instantiate them and then add the title to a list.

Java Swing Object[][] getData() confusion

Hi I'm having some trouble getting started with a problem in a Java course learning Swing and starting on JTables and getting data into them. It's going to be hard to explain so I'm just going to post the code I was given, along with the question.
The question is:
The getData() method needs to return an Object[][] containing the data represented by the class.
The first class is MusicAlbum
class MusicAlbum {
private String id;
private String name;
private String genre;
private boolean isCompilation;
private int track_count;
public MusicAlbum(String id, String name, String genre, boolean isCompilation, int track_count) {
this.id = id;
this.name = name;
this.genre = genre;
this.isCompilation = isCompilation;
this.track_count = track_count;
}
public String getId() {
return id;
}
public String getName() {
return name;
}
public String getGenre() {
return genre;
}
public boolean isCompilation() {
return isCompilation;
}
public int getTrackCount() {
return track_count;
}
public boolean equals(Object obj) {
if (obj instanceof MusicAlbum)
return this.id.equalsIgnoreCase(((MusicAlbum)obj).id);
return super.equals(obj);
}
}
The class I have to implement the methods in is MusicDataObject (at the bottom)
import java.util.Random;
import java.util.Scanner;
public class MusicDataObject {
private List<MusicAlbum> albums = new ArrayList<>();
private Random random = new Random(); // for generating IDs
public void addAlbum(MusicAlbum album) throws IllegalArgumentException {
if (searchAlbum(album.getId()) != null)
throw new IllegalArgumentException("Album ID is not new!");
albums.add(album);
}
public MusicAlbum searchAlbum(String id) {
for (MusicAlbum album : albums) {
if (album.getId().equalsIgnoreCase(id)) {
return album;
}
}
return null;
}
public MusicAlbum removeAlbum(String id) {
MusicAlbum album = searchAlbum(id);
albums.remove(album);
return album;
}
public void updateAlbum(MusicAlbum album)
throws IllegalArgumentException {
if (removeAlbum(album.getId()) == null)
throw new IllegalArgumentException("Album ID does not exist!");
addAlbum(album);
}
public String generateID() {
String formatter = "A%0" + (int)Math.ceil(Math.log10(albums.size() * 2) + 1) + "d";
String ID;
do {
ID = String.format(formatter, random.nextInt(albums.size() * 2 + 1));
} while (searchAlbum(ID) != null);
return ID;
}
public void saveData(String fileName) throws IOException {
// make sure that the file exists or try to create it
File fout = new File(fileName);
if (!fout.exists() && !fout.createNewFile())
return;
PrintWriter out = new PrintWriter(fout);
for (MusicAlbum album: albums) {
out.println(serializeAlbum(album));
}
out.close();
}
public String serializeAlbum(MusicAlbum album) {
return String.format(
"%s;%s;%s;%b;%d",
album.getId(),
album.getName(),
album.getGenre(),
album.isCompilation(),
album.getTrackCount());
}
public void loadFile(String fileName) throws FileNotFoundException {
albums = new ArrayList<>();
Scanner in = new Scanner(new File(fileName));
while (in.hasNext()) {
// --- split the next line with the character ";"
String line = in.nextLine();
String[] tokens = line.split(";");
// --- construct a new MusicAlbum using the resulting tokens. NOTE: This isn't very robust.
// If a line doesn't contain enough data or the data is invalid, this will crash
albums.add(new MusicAlbum(
tokens[0],
tokens[1],
tokens[2],
Boolean.parseBoolean(tokens[3]),
Integer.parseInt(tokens[4])
));
}
}
// ----- these methods need to be implemented
public Object[][] getData() {
// TODO
}
public String[] getColumnNames() {
// TODO
}
}
The sample data being used is in a txt file, formatted as so:
A01;Defiance;Soundtrack;true;24
A02;Insomniac;Punk Rock;false;14
A03;A Great Day For The Race;Gypsy Jazz;false;10
A04;Viva La Internet;Ska;false;31
A05;New Surrender;Rock;false;17
So basically it's this getData() method they want me to implement that is giving me grief. I don't fully understand what they want me to do, nor do I fully understand what the Object[][] does.
I hope I have been clear enough, and I will appreciate all help given. Also please try to explain things as best you can and dumb them down as much as possible, I'm new to a lot of this :)
Thanks for your time.

Object[][] is a 2-dimensional array. Each of its element is an Object[], a one-dimensional array.
Your task is to create a 2 dimensional array, having one element (Object[]) for each of your MusicAlbum. An Object[] should hold the properties of a MusicAlbum like id, name, genre, isCompilation and track_count.
You can create an object array like this:
Object[] arr = new Object[] { "some", "values", 23, true };
You can create a 2 dimensional array like this:
Object[][] arr2d = new Object[size][];
And you can iterate over all your MusicAlbums, create an Object[] for each of them containing the properties of that music album, and set it in the arr2d.
You can set/get elements of a 2-dimensional array just like any other arrays:
// Set first element:
arr2d[0] = arr;
// Get first element:
Object[] firstElement = arr2d[0];
The getColumnNames() method should just return a String[] (a String array) containing the column names, the names of the properties.
And it might be obvious but note that the order you return the column names and the order of the property values (in the elements of the Object[]) should be the same.