Develop Reference

Method to Find ArrayList Index to Where the Object Will be Added

I have an ArrayList that is being filled with customer information using a Customer class. In my addCustomerRecord method, I am calling findAddIndex within the addCustomerRecord method so the data entered will be sorted prior to displaying the data. Here is my code and do not mind the fileWhatever method, I don't use it.
public class CustomerDemo
{
//arrayList of customer objects
public static ArrayList<Customer> customerAL = new ArrayList<>();
public static void main (String[] args)
{
//to hold menu choice
String menuChoice = "";
Scanner kb = new Scanner(System.in);
System.out.println("To add a record press 'A': \n"
+ "to display all records press 'D': \n"
+ "to exit press 'Q': \n");
//loop priming read
menuChoice = kb.nextLine();
//make input case insensitive
menuChoice = menuChoice.toLowerCase();
do
{
if(menuChoice.equals("a"))
addCustomerRecord(kb);
else if(menuChoice.equals("d"))
{
displayCustomerRecords();
}
else if(menuChoice.equals("q"))
{
System.out.println("Program exiting..");
System.exit(0);
}
else
{
System.out.println("incorrect entry. Please re-enter a valid entry: \n");
menuChoice = kb.nextLine();
menuChoice = menuChoice.toLowerCase();
}
System.out.println("To add a record press 'A': \n"
+ "to display all records press 'D': \n"
+ "to exit press 'Q': \n");
menuChoice = kb.nextLine();
menuChoice = menuChoice.toLowerCase();
}while(menuChoice.equals("a") || menuChoice.equals("d") || menuChoice.equals("q"));
kb.close();
}
/* public static void displayCustomerRecords()
{
System.out.println();
for (int i = 0; i < customerAL.size(); ++i)
{
System.out.printf("%-15s", customerAL.get(i).getLastName());
System.out.printf("%-15s", customerAL.get(i).getFirstName());
System.out.printf("%-6s", customerAL.get(i).getCustID());
System.out.printf("%15s\n", customerAL.get(i).getPhoneNumber());
}
System.out.println();
}
/**
* prompts to enter customer data and mutator methods called
* with a Scanner object passed as an argument to set data
* #param location index position of where the element will be added.
* #param kb a Scanner object to accept input
*/
public static void addCustomerRecord(Scanner kb)
{
Customer currentCustomerMemoryAddress = new Customer();
System.out.println("Enter first name: \n");
String fName = kb.nextLine();
currentCustomerMemoryAddress.setFirstName(fName);
System.out.println("Enter last name: \n");
String lName = kb.nextLine();
currentCustomerMemoryAddress.setLastName(lName);
System.out.println("Enter customer phone number: \n");
String pNum = kb.nextLine();
currentCustomerMemoryAddress.setPhoneNumber(pNum);
System.out.println("Enter customer ID number: \n");
String ID = kb.nextLine();
currentCustomerMemoryAddress.setCustID(ID);
int addLocation = findAddLocation(currentCustomerMemoryAddress);
customerAL.add(addLocation, currentCustomerMemoryAddress);
currentCustomerMemoryAddress = null;
}
public static int findAddLocation(Customer cust)
{
int location = 0;
if(!customerAL.isEmpty())
{
for(int i = 0; i < customerAL.size(); i++)
{
//Stumped here
}
}
else
return location;
return location;
}
}

It looks like you are reinventing the wheel here William
Replace your code for displayCustomerRecords with this:
public static void displayCustomerRecords()
{
System.out.println();
customerAL.stream().map(c -> String.format("%-15s%-15s%-6s%15s\n",
c.getLastName(), c.getFirstName(), c.getCustID(), c.getPhoneNumber()))
.sorted()
.forEach(System.out::print);
System.out.println();
}
Update
Taking into account your comment you can replace your findAddLocationmethod by the following:
private static Comparator<Customer> comparator = Comparator.comparing(Customer::getLastName)
.thenComparing(Customer::getFirstName)
.thenComparing(Customer::getCustID)
.thenComparing(Customer::getPhoneNumber);
public static int findAddLocation(Customer cust)
{
int location = 0;
if(!customerAL.isEmpty())
{
for(Customer customerInList : customerAL)
{
if(comparator.compare(customerInList, cust) > 0) {
break;
}
location++;
}
}
return location;
}
We are traversing the array using Java's enhanced for-loop and comparing the objects using a Java 8 declared comparator (which I believe is the key to this assignment).
It would be a good idea if you could look into the Comparable interface and implement it in your Customer class. That way you could simply do a simple call to customerInList.compareTo(cust) to compare both objects.
As already stated, this is not a good practice and shouldn't be used in production code.

Library Program - Assigning & checking out books

I'm supposed to create a library program in java that allows you to create patrons and check out a maximum of 3 books. I'm really beginner at java so I apologize that my code is all over the place and may not make sense.
Below is the library class that I attempted(i also have a separate Patron, Book and Book Interface class)
My main concerns:
I have 2 ArrayLists, one for a list of inputed Users and another for a list of inputed Books. However how would i be able to assign certain checked out books to a certain user & make sure they borrow no more than 3?
I put a lot of the code in the main method but i end up having a lot of problems with static and non static stuff
How would I be able to create status' for each book? for example if "great expectations" is checked out, how can assign "borrowed" to it and make sure no one else can borrow it?
The program runs so far but its lacking depth because I'm lost as to how to check out/in books under a certain specified patron.
SORRY again for all the inconsistencies in my code and i really really appreciate the help!
import java.awt.List;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Scanner;
import java.util.Collections;
public class Library
{
static ArrayList <Patron> UserList = new ArrayList<Patron>();
static ArrayList <String> BookList = new ArrayList <String> ();
public static String status;
public static String borrower;
public static String borrowDate;
public static String returnDate;
public String status1 = "Available";
public String status2 = "Borrowed";
public static void main(String[] args)
{
Scanner input = new Scanner(System.in);
int choice = 0;
System.out.println("********************Welcome to the Public Library!********************");
System.out.println(" Please Select From The Following Options: ");
System.out.println("**********************************************************************");
while(choice != 9)
{
System.out.println("1: Add new patron");
System.out.println("2: Add new book");
System.out.println("3: Edit patron");
System.out.println("4: Edit book");
System.out.println("5: Display all patrons");
System.out.println("6: Display all books");
System.out.println("7: Check out book");
System.out.println("8: Check in book");
System.out.println("9: Search book");
System.out.println("10: Search Patron");
System.out.println("9: Exit");
choice = input.nextInt();
switch(choice)
{
case 1: //Add new patron
System.out.print("Enter patron first name: ");
String firstName = input.next(); //read name from input
System.out.print("Enter patron last name: ");
String lastName = input.next();
UserList.add(new Patron(firstName, lastName)); //add name to list
System.out.println("-----You have successfully added a new patron!-----");
break;
case 2: //Add new book
System.out.print("Enter book title: ");
String title1 = input.next();
Scanner input1 = new Scanner(System.in);
System.out.print("Enter book author: ");
String author1 = input.next();
Book book1 = new Book(title1);
BookList.add(title1);
FullBookList.add(fullBook);
System.out.println("-----You have successfully added a new book!-----");
status = "available";
borrowDate = "none";
returnDate = "none";
borrower = "none";
break;
case 3: //Edit patron name
System.out.println("Enter original patron name: ");
String originalName = input.next();
System.out.println("Enter edited patron name: ");
String editedName = input.next();
//Collections.replaceAll(UserList, originalName, editedName);
if(UserList.contains(originalName))
{
}
case 4: //edit book
case 5: //display all patrons
System.out.println(UserList);
break;
case 6: //display all books
System.out.println(BookList);
break;
case 7: //check out a book
Patron.CheckOutBook();
break;
case 8: //check in a book
Patron.CheckInBook();
break;
}
}
}
}
import java.util.ArrayList;
import java.util.Scanner;
public class Patron
{
Scanner input = new Scanner(System.in);
private String first;
private String last;
int bookCount = 0; //amount books user has in pocket
int books = 0;
//constructor to "add new patron" by entering their name.
public Patron(String f, String l)
{
first = f;
last = l;
}
public String toString()
{
return first + " " + last;
}
public String getName()
{
return first + " " + last;
}
public static void CheckOutBook()
{
System.out.println("Enter book title to be check out: ");
Scanner input = new Scanner(System.in);
String bookCheckOut = input.next();
if(Library.BookList.contains(bookCheckOut))
{
Library.BookList.remove(bookCheckOut);
System.out.println("-----" + bookCheckOut + " has been checked out!-----");
System.out.println ("-------" + bookCheckOut + " is due in 7 days!-------");
}
else
System.out.println(bookCheckOut + " is not in the library. Please enter "
+ "a different book to be checked out");
}
public static void CheckInBook()
{
System.out.println("Enter book title to be checked in: ");
Scanner input = new Scanner(System.in);
String bookCheckIn = input.next();
if(Library.BookList.contains(bookCheckIn))
{
Library.BookList.add(bookCheckIn);
System.out.println("-----" + bookCheckIn + " has been checked in!-----");
}
else
System.out.println(bookCheckIn + " is not in the library. Please enter "
+ "a different book to be checked out");
}
public boolean canBorrow()
{
if(bookCount <= 3)
{
return true;
}
else
{
return false;
}
}
}

Note: This will likely involve some refactoring to your main loop.
Alright so the way I see it, we have three classes at play here: some Patrons, which can check books out and in, some Books, which have statuses like "available" and "checked out," and a Library, which contains books. So, we need 3 classes:
I'll start with Book and use pseudo code to explain the concepts for you to implement.
class Book
{
//private fields
private final String title;
private final String author;
private Status available = true;
//note--i would prefer using an Enum called status for this,
//but a boolean true/false value works adequately
//Constructor
public Book(string title, string author) {}
//accessors for title, author, available
//setter for available--used for Library only--there are better ways to ensure
//Patrons can't set the status of the book, but for now this is the simplest way
}
As you can see, Books have immutable fields that don't need to change, and one field that tracks it status. A better implementation might make Library track book status, as that makes more logical sense and better code, but this a simple implementation.
Next, Library, which needs lots of books:
class Library
{
private final ArrayList<Book> books;
//Constructor
public Library ()
{
books = loadBooks();
}
//some methods
private ArrayList<Book> loadBooks () {}
//however you want to create all your books (file input, whatever)
public bool isBookAvailable (Book b)
{
if b isn't in library: return false
else return (b in books).isAvailable()
}
public Book checkoutBook (Book b)
{ get book (checking availability, possibly returning a null Book), set status to unavailable, return it }
public Book checkinBook (Book b)
{ check that this the book belongs to library, set status to available }
}
As I said earlier, this isn't perfect. I could spend quite some time going on and on about how to improve the design, but for the sake of simplicity won't.
Now, Patrons. One question is, should Patrons have only one library that the visit? Or do they visit multiple libraries? I'll assume they visit more than one, since sometimes a library doesn't have all the books you want.
class Patron
{
private final String name;
private final Book[] books = new Book[3];//you can see I'm limiting them to 3 books
private int index = 0;
//Constructor
public Patron (String name) {}
//methods
public void checkoutBook (Book b, Library l)
{//could be tricky
check books status in l (l.isBookAvailable(b))
if available:
if space (index < 2) Book newBook = l.checkoutBook(b); books[index++] = newBook;
else: no space
else: not available
}
public void checkinBook (int bookIndex, Library l)
{
if bookIndex < 3:
if books[index] != null:
l.checkinBook (books[index]);
books[index--] = null;
else: no book
else: not valid index
}
}
Of course, other utilities like displaying books (library, patron) and toString methods might be useful. But now the responsibility of the main method is to create some patrons, a library, and give patrons the chance to check out and check in books via a menu. You have the heavy lifting done; you can work on input and output now.
Any questions?

A Beginner Level "Student Library Program" in JAVA, which interacts the Students and the Books. This Library Program can do following functions:
1-Adding a Book to Library.
2-Update Book Quantity.
3-Search a Book with its Serial number.
4-Search Books With Author Name.
5-Show all Books and their related Information.
6-Registering a Student.
7-Show All Registered Students.
8-Student can Check Out Book From Library(if registered).
:- Student can not Check Out more than 3 Books
:- You can only borrow a Book If it is Available in Library
9-Student can Check In Book to Library.
10-You can also see the Books which a Student has Checked Out(only while checking in)
Note: At the time it can store only 50 books for simlicity in program
I Have created this program with the maximum skill and knowledge i had in java. As I'm a Beginner so I couldn't do more
Kindly give reviews about Program
Also tell me refinements which are to be made in program
Kindly Tell me the better way to do this Program
package library;
import java.util.Scanner;
public class book {
public int sNo;
public String bookName;
public String authorName;
public int bookQty;
public int bookQtyCopy;
Scanner input = new Scanner(System.in);
public book(){
System.out.println("Enter Serial No of Book:");
this.sNo = input.nextInt();
input.nextLine();
System.out.println("Enter Book Name:");
this.bookName = input.nextLine();
System.out.println("Enter Author Name:");
this.authorName = input.nextLine();
System.out.println("Enter Quantity of Books:");
this.bookQty = input.nextInt();
bookQtyCopy = this.bookQty;
}
}
package library;
import java.util.Scanner;
public class books {
book theBooks[] = new book[50]; // Array that stores 'book' Objects.
public static int count; // Counter for No of book objects Added in Array.
Scanner input = new Scanner(System.in);
public int compareBookObjects(book b1, book b2){
if (b1.bookName.equalsIgnoreCase(b2.bookName)){
System.out.println("Book of this Name Already Exists.");
return 0;
}
if (b1.sNo==b2.sNo){
System.out.println("Book of this Serial No Already Exists.");
return 0;
}
return 1;
}
public void addBook(book b){
for (int i=0; i<count; i++){
if (this.compareBookObjects(b, this.theBooks[i]) == 0)
return;
}
if (count<50){
theBooks[count] = b;
count++;
}
else{
System.out.println("No Space to Add More Books.");
}
}
public void searchBySno(){
System.out.println("\t\t\t\tSEARCH BY SERIAL NUMBER\n");
int sNo;
System.out.println("Enter Serial No of Book:");
sNo = input.nextInt();
int flag = 0;
System.out.println("S.No\t\tName\t\tAuthor\t\tAvailable Qty\t\tTotal Qty");
for (int i=0; i<count; i++){
if (sNo == theBooks[i].sNo){
System.out.println(theBooks[i].sNo + "\t\t" + theBooks[i].bookName + "\t\t" + theBooks[i].authorName + "\t\t" +
theBooks[i].bookQtyCopy + "\t\t" + theBooks[i].bookQty);
flag++;
return;
}
}
if (flag == 0)
System.out.println("No Book for Serial No " + sNo + " Found.");
}
public void searchByAuthorName(){
System.out.println("\t\t\t\tSEARCH BY AUTHOR'S NAME");
input.nextLine();
System.out.println("Enter Author Name:");
String authorName = input.nextLine();
int flag = 0;
System.out.println("S.No\t\tName\t\tAuthor\t\tAvailable Qty\t\tTotal Qty");
for (int i=0; i<count; i++){
if (authorName.equalsIgnoreCase(theBooks[i].authorName)){
System.out.println(theBooks[i].sNo + "\t\t" + theBooks[i].bookName + "\t\t" + theBooks[i].authorName + "\t\t" +
theBooks[i].bookQtyCopy + "\t\t" + theBooks[i].bookQty);
flag++;
}
}
if (flag == 0)
System.out.println("No Books of " + authorName + " Found.");
}
public void showAllBooks(){
System.out.println("\t\t\t\tSHOWING ALL BOOKS\n");
System.out.println("S.No\t\tName\t\tAuthor\t\tAvailable Qty\t\tTotal Qty");
for (int i=0; i<count; i++){
System.out.println(theBooks[i].sNo + "\t\t" + theBooks[i].bookName + "\t\t" + theBooks[i].authorName + "\t\t" +
theBooks[i].bookQtyCopy + "\t\t" + theBooks[i].bookQty);
}
}
public void upgradeBookQty(){
System.out.println("\t\t\t\tUPGRADE QUANTITY OF A BOOK\n");
System.out.println("Enter Serial No of Book");
int sNo = input.nextInt();
for (int i=0; i<count; i++){
if (sNo == theBooks[i].sNo){
System.out.println("Enter No of Books to be Added:");
int addingQty = input.nextInt();
theBooks[i].bookQty += addingQty;
theBooks[i].bookQtyCopy += addingQty;
return;
}
}
}
public void dispMenu(){
System.out.println("----------------------------------------------------------------------------------------------------------");
System.out.println("Enter 0 to Exit Application.");
System.out.println("Enter 1 to Add new Book.");
System.out.println("Enter 2 to Upgrade Quantity of a Book.");
System.out.println("Enter 3 to Search a Book.");
System.out.println("Enter 4 to Show All Books.");
System.out.println("Enter 5 to Register Student.");
System.out.println("Enter 6 to Show All Registered Students.");
System.out.println("Enter 7 to Check Out Book. ");
System.out.println("Enter 8 to Check In Book");
System.out.println("-------------------------------------------------------------
---------------------------------------------");
}
public int isAvailable(int sNo){
//returns the index number if available
for (int i=0; i<count; i++){
if (sNo == theBooks[i].sNo){
if(theBooks[i].bookQtyCopy > 0){
System.out.println("Book is Available.");
return i;
}
System.out.println("Book is Unavailable");
return -1;
}
}
System.out.println("No Book of Serial Number " + " Available in Library.");
return -1;
}
public book checkOutBook(){
System.out.println("Enter Serial No of Book to be Checked Out.");
int sNo = input.nextInt();
int bookIndex =isAvailable(sNo);
if (bookIndex!=-1){
//int bookIndex = isAvailable(sNo);
theBooks[bookIndex].bookQtyCopy--;
return theBooks[bookIndex];
}
return null;
}
public void checkInBook(book b){
for (int i=0; i<count; i++){
if (b.equals(theBooks[i]) ){
theBooks[i].bookQtyCopy++;
return;
}
}
}
}
package library;
import java.util.Scanner;
public class student {
String studentName;
String regNum;
book borrowedBooks[] = new book[3];
public int booksCount = 0;
Scanner input = new Scanner(System.in);
public student(){
System.out.println("Enter Student Name:");
this.studentName = input.nextLine();
System.out.println("Enter Reg Number:");
this.regNum = input.nextLine();
}
}
package library;
import java.util.Scanner;
public class students {
Scanner input = new Scanner(System.in);
student theStudents[] = new student[50];
//books book;
public static int count = 0;
public void addStudent(student s){
for (int i=0; i<count; i++){
if(s.regNum.equalsIgnoreCase(theStudents[i].regNum)){
System.out.println("Student of Reg Num " + s.regNum + " is Already Registered.");
return;
}
}
if (count<=50){
theStudents[count] = s;
count++;
}
}
public void showAllStudents(){
System.out.println("Student Name\t\tReg Number");
for (int i=0; i<count; i++){
System.out.println(theStudents[i].studentName + "\t\t" + theStudents[i].regNum);
}
}
public int isStudent(){
//return index number of student if available
//System.out.println("Enter Student Name:");
//String studentName = input.nextLine();
System.out.println("Enter Reg Number:");
String regNum = input.nextLine();
for (int i=0; i<count; i++){
if (theStudents[i].regNum.equalsIgnoreCase(regNum)){
return i;
}
}
System.out.println("Student is not Registered.");
System.out.println("Get Registered First.");
return -1;
}
public void checkOutBook(books book){
int studentIndex =this.isStudent();
if (studentIndex!=-1){
System.out.println("checking out");
book.showAllBooks();//jjjjjjjjjjjj
book b = book.checkOutBook();
System.out.println("checking out");
if (b!= null){
if (theStudents[studentIndex].booksCount<=3){
System.out.println("adding book");
theStudents[studentIndex].borrowedBooks[theStudents[studentIndex].booksCount] = b;
theStudents[studentIndex].booksCount++;
return;
}
else {
System.out.println("Student Can not Borrow more than 3 Books.");
return;
}
}
System.out.println("Book is not Available.");
}
}
public void checkInBook(books book){
int studentIndex = this.isStudent();
if (studentIndex != -1){
System.out.println("S.No\t\t\tBook Name\t\t\tAuthor Name");
student s = theStudents[studentIndex];
for (int i=0; i<s.booksCount; i++){
System.out.println(s.borrowedBooks[i].sNo+ "\t\t\t" + s.borrowedBooks[i].bookName + "\t\t\t"+
s.borrowedBooks[i].authorName);
}
System.out.println("Enter Serial Number of Book to be Checked In:");
int sNo = input.nextInt();
for (int i=0; i<s.booksCount; i++){
if (sNo == s.borrowedBooks[i].sNo){
book.checkInBook(s.borrowedBooks[i]);
s.borrowedBooks[i]=null;
return;
}
}
System.out.println("Book of Serial No "+sNo+"not Found");
}
}
}
package library;
import java.util.Scanner;
public class Library {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
System.out.println("********************Welcome to the Student Library!********************");
System.out.println(" Please Select From The Following Options: ");
System.out.println("**********************************************************************");
books ob = new books();
students obStudent = new students();
int choice;
int searchChoice;
do{
ob.dispMenu();
choice = input.nextInt();
switch(choice){
case 1:
book b = new book();
ob.addBook(b);
break;
case 2:
ob.upgradeBookQty();
break;
case 3:
System.out.println("Enter 1 to Search with Serial No.");
System.out.println("Enter 2 to Search with Author Name(Full Name).");
searchChoice = input.nextInt();
switch(searchChoice){
case 1:
ob.searchBySno();
break;
case 2:
ob.searchByAuthorName();
}
break;
case 4:
ob.showAllBooks();
break;
case 5:
student s = new student();
obStudent.addStudent(s);
break;
case 6:
obStudent.showAllStudents();
break;
case 7:
obStudent.checkOutBook(ob);
break;
case 8:
obStudent.checkInBook(ob);
break;
default:
System.out.println("CHOICE SHOULD BE BETWEEN 0 TO 8.");
}
}
while (choice!=0);
}
}

An application for a temporary management with Java

Hello the StackOverflow community! I am recent Java learner with 1 year of experience only. I was asked to design a software that mimics a school DB, storing all names, class, roll, and other details. When asked to, it would display appropriate messages, like calculating performance, deleting records, etc. It is yet an incomplete work but the first part (accepting and storing details) are done. I have spent a lot of time behind this and the only thing I get is a nullPointerError. Sorry, but I have been asked to stick to the basics, so no glitzy code. I have used inheritance. The superclass is "Student".
public class Student {
int roll,age = 0; // Roll to be auto-updated
String cl,name;
// Marks variables now
int m_eng, m_math, m_physics, m_chem, m_bio = 0;
public Student(){
}
public Student(int a, String cla){
age = a;
cl = cla; // Assign values
}
void setMarks(int eng, int math, int phy, int chem, int bio){
m_eng = eng; m_math = math; m_physics = phy; m_chem = chem; m_bio = bio;
}
}
Here's the error:
java.lang.NullPointerException
at Application.accept_data(Application.java:35)
at Application.execute(Application.java:23)
at Application.input(Application.java:16)
at Application.main(Application.java:101)
Here is the code, though:
import java.util.Scanner;
public class Application extends Student {
static int n; static Scanner sc = new Scanner(System.in);
static Student s[];
void input(){
System.out.println("Enter the number of students: ");
n = sc.nextInt();
s = new Student[n]; // Create array for n students
System.out.println("Enter your choice: ");
System.out.println("1. Accept student's details ");
System.out.println("2. Display all records ");
System.out.println("3. Display data student-wise ");
System.out.println("4. Delete record");
System.out.println("5. Display performance status");
System.out.println("6. Exit");
execute();
}
static void execute(){
boolean correct = false;
while (!correct){
int op = sc.nextInt();
switch(op){
case 1: accept_data(); correct = true;
case 2: disp_data();correct = true;
case 3: disp_studentwise();correct = true;
case 4: del_record();correct = true;
case 5: performance();correct = true;
case 6: System.exit(0); correct = true;//Terminate
default: System.out.println("You must enter a choice. Kindly re-enter: ");correct = false;
}
}
}
static void accept_data(){
for (int i = 0; i<s.length; i++){
s[i].roll = i+1; //Autoupdate roll
System.out.println("Enter name: "); s[i].name = sc.nextLine();
System.out.println("Enter age: "); s[i].age = sc.nextInt(); // Refer to object prope.
System.out.println("Enter class: "); s[i].cl = sc.nextLine();
System.out.println("We're heading for marks entry!");
System.out.println("Enter marks in the following order: ENGLISH, MATH, PHYSICS, CHEMISTRY, BIOLOGY");
s[i].setMarks(sc.nextInt(),sc.nextInt(),sc.nextInt(),sc.nextInt(),sc.nextInt());
}
System.out.println("Thanks. Main menu, please enter your choice now: ");
execute();
}
static void disp_data(){
System.out.println("The system will display all stored information of students available.");
for (int i = 0; i<s.length; i++){
if (s[i].roll != -1){
continue; // In case record is deleted, it won't display
}
else {
printrec(i);
}
}
System.out.println("Main menu, please enter your choice: ");
execute();
}
static void disp_studentwise(){
System.out.println("Enter the roll number");
int r = sc.nextInt();
boolean ok = (r>s.length||r<0)?false:true;
while (!ok){
System.out.println("Incorrect roll. Please re-enter: ");
r = sc.nextInt();
if (r>s.length) ok = false;
else ok = true;
}
printrec(r-1);
System.out.println("Main menu, please enter your choice: ");
execute();
}
static void printrec(int n){
int i = n;
System.out.println("For roll number " + s[i].roll + ", details: ");
System.out.println("Name: " + s[i].name); System.out.println("Age: " + s[i].age);
System.out.println("Class: " + s[i].cl);
System.out.println("Subject \t Marks");
System.out.println("English: \t " + s[i].m_eng); // Display record with marks
System.out.println("Maths: \t " + s[i].m_math);
System.out.println("Physics: \t " + s[i].m_physics);
System.out.println("Chemistry: \t " + s[i].m_chem);
System.out.println("Biology: \t " + s[i].m_bio);
}
static void del_record(){
System.out.println("Enter the roll number you want to delete: ");
int rll = sc.nextInt();
for (int i = 0; i<s.length; i++){
if (rll == s[i].roll){
s[i].roll = -1; // Assign non-positive value to refer deleted items
}
}
}
static void performance(){
}
public static void main(String[] args){
Application ob = new Application();
ob.input(); // Start program
}
}
Can anyone point out what's going wrong? Why there's a problem with accepting details of students after pressing for the 1st option? It shows nullPointer on s[i].roll. Keep in mind that roll is autoupdated, and user doesn't intervene there. It serves as a primary key. An explanation would be beneficial, if possible of course, I am eager to learn. Thanks.

this :
s = new Student[n]; // Create array for n students
You are just creating an array of 'n' Student objects here ... that doesn't mean that your 'n' Students are initialized ... your array contains only 'null' values ...
you may want in your accept_data method do a :
for (int i = 0; i<s.length; i++){
s[i] = new Student();
s[i].roll = i+1; //Autoupdate roll
....

You are getting an NPE because you create an array of Students in your input method, but you never populate it with Student objects, so in accept_data, you're trying to access the roll field on a non-existent object.
You will need to fill in the array with new Student objects in your input method before you call accept_data.

Comparing user inputted string to an array variable Java

I've included my program below, basically my question is if I input a surname to allow me to find the student I want to delete how can I compare the inputted string to the surname variable in the array?
When I enter the surname and try and compare this to the entries already in the array instead of comparing the surnames it's comparing the inputted string to everything in that instance of it. This is being done in the findStudent method.
I think because i'm passing the entire array here surname contains everything I've entered for that student (surname, forename, exam marks). If I try and just pass the surname parameter i'm getting error messages.
I've searched to see if this was answered already but all I can find is comparing an inputted string to an array that is already defined not an array created from user input.
Any help would be appreciated!
Thanks
import java.util.Scanner;
/**
*
* #author Connor
*/
public class Student {
//declare student variables
private String surname;
private String forename;
private int mark1, mark2, mark3;
private double dblScore;
private static String course = "French";
//constructor
//===========================================================
//
// MODULE : Student
// RETURN TYPE : None
// PARAMETERS : args : String newForename, String newSurname, int newMark1, int newMark2, int newMark3
// DESCRIPTION : Construct Student object
//============================================================
public Student(String newForename, String newSurname, int newMark1, int newMark2, int newMark3) {
this.forename = newForename;
this.surname = newSurname;
this.mark1 = newMark1;
this.mark2 = newMark2;
this.mark3 = newMark3;
this.dblScore = (this.mark1 + this.mark2 + this.mark3) / 3;
}
//===========================================================
//
// MODULE : getCourse
// RETURN TYPE : String course
// PARAMETERS : args :
// DESCRIPTION : get course name
//============================================================
public static String getCourse() {
return course;
}
//===========================================================
//
// MODULE : setCourse
// RETURN TYPE : course
// PARAMETERS : args :
// DESCRIPTION : set course name
//============================================================
public static String setCourse(String newCourse) {
course = newCourse;
return course;
}
//display student details method
//===========================================================
//
// MODULE : displaydetails
// RETURN TYPE : None
// PARAMETERS : args : [DEFAULT]
// DESCRIPTION : write out student details -
// name, course and exam marks
//============================================================
public void displaydetails() {
System.out.println(" ");
System.out.println("Student details - ");
System.out.println("Name - " + surname + " " + forename);
System.out.println("Course - " + course);
System.out.println("exam scores - " + mark1 + " " + mark2 + " " + mark3);
System.out.println("Overall score - " + dblScore);
System.out.println(" ");
}
}
class StudentMenu {
//===========================================================
//
// MODULE : add student
// RETURN TYPE : None
// PARAMETERS : args : studentArray, numStudents
// DESCRIPTION : requests user input to add student to array.
// Upto 6 students can be entered
//============================================================
public static void addStudent(Student[] studentArray, int numStudents) {
String newSurname, newForename;
int newMark1, newMark2, newMark3;
Scanner input = new Scanner(System.in);
System.out.println("1. Add a student");
System.out.println(" ");
System.out.println("Student surname - ");
newSurname = input.next();
input.nextLine();
System.out.println("Student forename - ");
newForename = input.next();
input.nextLine();
System.out.println("First exam mark - ");
newMark1 = input.nextInt();
input.nextLine();
System.out.println("Second exam mark - ");
newMark2 = input.nextInt();
input.nextLine();
System.out.println("Third exam mark - ");
newMark3 = input.nextInt();
input.nextLine();
studentArray[numStudents] = new Student(newSurname, newForename, newMark1, newMark2, newMark3);
}
//===========================================================
//
// MODULE : find student
// RETURN TYPE : int position
// PARAMETERS : args : surname, delSurname, position
// DESCRIPTION : finds the position of the selected student
// in the array
//============================================================
public static void findStudent(Student[] surname,String delSurname, int position, int totalStudents) {
int index;
for (index = 0; index < totalStudents; index++) {
// if (surname[index].equals(delSurname)) {
if (surname[index].equals(delSurname)) {
position = index;
} else {
index++;
}
}
}
//===========================================================
//
// MODULE : delete student
// RETURN TYPE : None
// PARAMETERS : args : myClass, position, NUM_STUDENTS
// DESCRIPTION : deletes chosen student from array. Moves
// all other entries down by one
//============================================================
public static void deleteStudent(Student[] myClass, int position, int NUM_STUDENTS) {
int index = 0;
for (index = position + 1; index < NUM_STUDENTS; index++) {
myClass[index - 1] = myClass[index];
}
}
//===========================================================
//
// MODULE : main method
// RETURN TYPE : None
// PARAMETERS : args : myClass, position, NUM_STUDENTS
// DESCRIPTION : runs menu program allowing user input
// to add/modify/delete student information
//============================================================
public static void main(String[] args) {
//declare scanner
Scanner input = new Scanner(System.in);
//declare variables
boolean menu = true;
int option;
int totalStudents = 0;
final int NUM_STUDENTS = 6;
int position = 0;
String delSurname, newCourse;
//array
Student[] myClass;
myClass = new Student[NUM_STUDENTS];
//main method
//menu
while (menu != false) {
System.out.println("1. Add a student");
System.out.println("2. Delete student");
System.out.println("3. Display all students");
System.out.println("4. Change course details");
System.out.println("5. Search for student");
System.out.println("6. Exit program");
// enter choice
System.out.print("Please enter selection - ");
option = input.nextInt();
System.out.println(" ");
// calling methods using switch
if ((option > 6) || (option < 1)) {
System.out.println("Invalid selection made - reenter. Between options 1-6 only.");
} else {
switch (option) {
case 1:
//option 1 - add student
addStudent(myClass, totalStudents);
totalStudents++;
break;
case 2:
//option 2 - delete student
System.out.println("Student surname to delete - ");
delSurname = input.next();
input.nextLine();
findStudent(myClass,delSurname, position, totalStudents);
if (position >= 0 && position < NUM_STUDENTS) {
deleteStudent(myClass, position, NUM_STUDENTS);
}
totalStudents--;
break;
case 3:
//option 3 - display details
Student.getCourse();
for (int index = 0; index < totalStudents; index++) {
myClass[index].displaydetails();
}
break;
case 4:
//option 4 - change course
Student.getCourse();
System.out.println();
System.out.println("Enter new course details - ");
newCourse = input.next();
input.nextLine();
Student.setCourse(newCourse);
break;
case 5:
//option 5 - search for student by name
System.out.println("Student surname to display - ");
delSurname = input.next();
input.nextLine();
findStudent(myClass,delSurname, position, totalStudents);
if (position >= 0 && position < NUM_STUDENTS) {
myClass[position].displaydetails();
}
break;
case 6:
//option 6 - exit program
menu = false;
break;
}
}
}
}
}

you'll have to check if surname[index].surname.euqals(delSurname) because in your surname array you have Students and not the strings
Right now your code checks if an instance of the class Student is equal to an instance of the class String, which cannot.

Java: sorting/arranging arrays based on user input

How can I sort this array based on user input? While using a constructor, I am returning values to the create the output. What I'd like to do is after receiving how the user would like to arrange his/her inputs, I'd like to perform something like an Arrays.sort(books[x].getBook());
But this does not work. Is there a way to arrange the returned values for each input? The following is the error I receive upon using the code below within each if statement, although getBooks() is very well in existence:
Error log
LibraryBookSort.java:56: error: cannot find symbol
Arrays.sort(books[x].getBooks());
^
symbol: method getBooks()
location: class LibraryBook
1 error
Code
public class LibraryBookSort {
public static void main(String[] args) {
LibraryBook[] books = new LibraryBook[5];
for (int x = 0; x < 5; x++) {
books[x] = new LibraryBook();
}
Scanner input = new Scanner(System.in);
for (int x = 0; x < 5; x++) {
// Get title
System.out.print("Enter the title of a book: ");
String title = input.nextLine();
books[x].setBook(title);
// Get author
System.out.print("Enter the author of this book: ");
String author = input.nextLine();
books[x].setAuthor(title);
// Get page count
System.out.print("Enter the number of pages for this book: ");
int pages = input.nextInt();
books[x].setPages(pages);
input.nextLine();
}
System.out.println("How would you like to organize your values?");
System.out.println("Sort by title > Enter 1: ");
System.out.println("Sort by author's last name > Enter 2: ");
System.out.print("Sort by page count > Enter 3: ");
int sortBy = input.nextInt();
// Sort by title
if(sortBy == 1) {
//????;
}
// Sort by author
else if(sortBy == 2) {
//????;
}
// Sort by page count
else if(sortBy == 3) {
//????;
}
// Print sorted array >> Use of constructor
for(int x = 0; x < 5; x++) {
System.out.println();
System.out.println("Book ");
System.out.println("Title: " + books[x].getBook());
System.out.println("Author: " + books[x].getAuthor());
System.out.println("Page Count: " + books[x].getPages());
}}}
The following will arrange my inputs using a comparator. However, the question remains, is there another approach to sorting this data outside of using a comparator?
class TitleComparator implements Comparator {
public int compare(Object book1, Object book2) {
String title1 = ((LibraryBook) book1).getBook();
String title2 = ((LibraryBook) book2).getBook();
return title1.compareTo(title2);
}}

I tried to add like this and it is working. Store the books inside the collection. So that you can make use of Collections.sort function and Comparator.
LibraryBookSort.java
import java.util.ArrayList;
import java.util.Collections;
import java.util.Scanner;
public class LibraryBookSort {
private static ArrayList<LibraryBook> list;
public static void main(String[] args) {
LibraryBook[] books = new LibraryBook[5];
for (int x = 0; x < 5; x++) {
books[x] = new LibraryBook();
}
Scanner input = new Scanner(System.in);
list = new ArrayList<LibraryBook>();
for (int x = 0; x < 5; x++) {
// Get title
System.out.print("Enter the title of a book: ");
String title = input.nextLine();
books[x].setBook(title);
// Get author
System.out.print("Enter the author of this book: ");
String author = input.nextLine();
books[x].setAuthor(title);
// Get page count
System.out.print("Enter the number of pages for this book: ");
int pages = input.nextInt();
books[x].setPages(pages);
list.add(books[x]);
input.nextLine();
}
System.out.println("How would you like to organize your values?");
System.out.println("Sort by title > Enter 1: ");
System.out.println("Sort by author's last name > Enter 2: ");
System.out.print("Sort by page count > Enter 3: ");
int sortBy = input.nextInt();
// Sort by title
if(sortBy == 1) {
Collections.sort(list,new TitleComparator());
}
// Sort by author
else if(sortBy == 2) {
//????;
}
// Sort by page count
else if(sortBy == 3) {
//????;
}
for(LibraryBook st: list){
System.out.println(st.title+" "+st.author+" "+st.pages);
}
}}
Also added another class TitleComparator.java.
import java.util.Comparator;
public class TitleComparator implements Comparator <LibraryBook>{
#Override
public int compare(LibraryBook arg0, LibraryBook arg1) {
return arg0.title.compareTo(arg1.title);
}
}
This is for sort books based on the title. You can also add PageComparator and AuthorComparator for the other two options.