I am new to the java code , now i currently learning the OOP and ADT . i have succesfull implement the addStudent fucntion , but deleteStudent is not

Question

I am implementing the favourite student system, I have created an interface called SortedListInterface that contain some basic operation that a list can do. it is implemented by an ADT called SortedArrayList that defines all the basic operations in the interface. I also created 3 entity class which is User, Admin and Canditate, and Admin and Candidate is inherited from the User class. I wanted to write a simple driver that can Add, Delete, View, and update the student. I have successfully done the Add function into an array, but when I want to implement the delete function it keeps an error or even nothing happens. Here are my Source Code

Interface

package adt;

/**
 *
 * @author Alvin
 *
 */
public interface SortedListInterface<T> {

    public boolean add(T newEntry);

    public boolean remove(T anEntry);

    public boolean contains(T anEntry);

    public T get(int givenPosition);

    public int getNumberOfEntries();

    void doubleArray();

    void makeRoom(int newPosition);

    void removeGap(int givenPosition);

    boolean isArrayFull();

    boolean isEmpty();

}

ADT

package adt;

/**
 *
 * @author Alvin
 * @param <T>
 *
 */
public class SortedArrayList<T extends Comparable<T>> implements SortedListInterface<T> {

    private T[] array;
    private int numberOfEntries;
    private static final int DEFAULT_CAPACITY = 25;

    public SortedArrayList() {
        this(DEFAULT_CAPACITY);
    }

    public SortedArrayList(int initialCapacity) {
        numberOfEntries = 0;
        array = (T[]) new Comparable[initialCapacity];
    }

    @Override
    public boolean add(T newEntry) {
        int i = 0;
        while (i < numberOfEntries && newEntry.compareTo(array[i]) > 0) {
            i++;
        }
        makeRoom(i + 1);
        array[i] = newEntry;
        numberOfEntries++;
        return true;
    }

    @Override
    public boolean remove(T anEntry) {
        if (numberOfEntries == 0) {
            return false;
        } else {
            int index = 0;
            while (index < numberOfEntries && array[index].compareTo(anEntry) < 0) {
                index++;
            }
            if (array[index].equals(anEntry)) {
                removeGap(index + 1);
                numberOfEntries--;
                return true;
            }
        }
        return false;
    }

    public void clear() {
        numberOfEntries = 0;
    }

    @Override
    public boolean contains(T anEntry) {
        boolean found = false;
        for (int index = 0; !found && (index < numberOfEntries); index++) {
            if (anEntry.equals(array[index])) {
                found = true;
            }
        }
        return found;
    }

    @Override
    public int getNumberOfEntries() {
        return numberOfEntries;
    }

    @Override
    public boolean isEmpty() {
        return numberOfEntries == 0;
    }

    @Override
    public String toString() {
        String outputStr = "";
        for (int index = 0; index < numberOfEntries; ++index) {
            outputStr += array[index] + "\n";
        }

        return outputStr;
    }

    @Override
    public boolean isArrayFull() {
        return numberOfEntries == array.length;
    }

    @Override
    public void doubleArray() {
        T[] oldList = array;
        int oldSize = oldList.length;

        array = (T[]) new Object[2 * oldSize];

        for (int index = 0; index < oldSize; index++) {
            array[index] = oldList[index];
        }
    }

    @Override
    public void makeRoom(int newPosition) {
        int newIndex = newPosition - 1;
        int lastIndex = numberOfEntries - 1;

        for (int index = lastIndex; index >= newIndex; index--) {
            array[index + 1] = array[index];
        }
    }

    @Override
    public void removeGap(int givenPosition) {
        int removedIndex = givenPosition - 1;
        int lastIndex = numberOfEntries - 1;

        for (int index = removedIndex; index < lastIndex; index++) {
            array[index] = array[index + 1];
        }
    }

    @Override
    public T get(int givenPosition) {
        T result = array[givenPosition];

        return result;
    }

}

Entity

User

package entity;

/**
 *
 * @author Alvin
 */
public class User implements Comparable<User> {

    private int id;
    private String name;

    public User(int id) {
        this.id = id;

    }

    public User(int id, String name) {
        this.id = id;
        this.name = name;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public int compareTo(User o) {
        return (int) (this.id - o.id);
    }

}

Admin

package entity;

/**
 *
 * @author Alvin
 */
public class Admin extends User {

    private String password;

    public Admin(int id, String name, String password) {
        super(id, name);
        this.password = password;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

    @Override
    public String getName() {
        return super.getName();
    }

    @Override
    public int getId() {
        return super.getId();
    }

    @Override
    public String toString() {
        return "Admin\n" + "ID :" + getId() + "\nName :" + getName() + "\nPassword :" + password;
    }

}

Candidate

package entity;

/**
 *
 * @author Alvin
 */
public class Candidate extends User {

    private int vote = 0;

    public Candidate(int id) {
        super(id);
    }

    public Candidate(int id, String name) {
        super(id, name);
    }

    public Candidate(int id, String name, int vote) {
        super(id, name);
        this.vote = vote;
    }

    public int getVote() {
        return vote;
    }

    public void setVote(int vote) {
        this.vote = vote;
    }

    @Override
    public String getName() {
        return super.getName();
    }

    @Override
    public int getId() {
        return super.getId();
    }

    @Override
    public String toString() {
        return "Candidate\n" + "ID :" + getId() + "\nName :" + getName() + "\nvote :" + vote + "\n";
    }

}

Driver

package driver;

import adt.*;
import entity.*;
import java.util.Scanner;

/**
 *
 * @author Alvin
 */
public class AdminDriver {

    Scanner input = new Scanner(System.in);
    SortedListInterface<User> stuList = new SortedArrayList<>();

    public static void main(String[] args) {
        AdminDriver am = new AdminDriver();

        am.menu();

    }

    public void addStudent() {

        SortedListInterface<User> adminList = new SortedArrayList<>();

        adminList.add(new Admin(122, "chick**strong text**", "password123"));
        adminList.add(new Admin(111, "chickson", "password123"));

        System.out.println("Enter candidate ID:");
        int adminId = input.nextInt();
        input.nextLine(); // consume newline character

        System.out.println("Enter candidate name:");
        String adminName = input.nextLine();

        stuList.add(new Candidate(adminId, adminName));
        System.out.println("Candidate added successfully.");

    }

    public void deleteStudent() {
        System.out.println("Enter candidate ID:");
        int adminId = input.nextInt();
        input.nextLine(); // consume newline character

        System.out.println("Enter candidate name:");
        String adminName = input.nextLine();

        Candidate candidateToDelete = new Candidate(adminId, adminName);
        boolean success = stuList.remove(candidateToDelete);

        if (success) {
            System.out.println("Candidate removed successfully.");
        } else {
            System.out.println("Candidate not found.");
        }
    }

    public void menu() {

        int choice;

        do {
            System.out.println("\nPlease choose an option:");
            System.out.println("1. Add a new student");
            System.out.println("2. Delete a student");
            System.out.println("3. Update a student's name");
            System.out.println("4. Check if a student exists");
            System.out.println("5. View a student's information");
            System.out.println("6. Generate student report");
            System.out.println("7. Exit");
            System.out.print("Choice: ");

            choice = input.nextInt();

            switch (choice) {
                case 1:
                    addStudent();
                    break;
                case 2:
                    deleteStudent();
                    break;
                case 3:

                    break;
                case 4:

                    break;
                case 5:
                    checkExist();
                    break;
                case 6:
                    System.out.println("Candidate \n\n" + stuList);
                    break;
                case 7:
                    System.out.println("Exiting program...");
                    break;
                default:
                    System.out.println("Invalid choice. Please choose a number from 1 to 7.");
                    break;
            }

        } while (choice != 7);
    }

    public void checkExist() {
        System.out.println("Enter candidate ID:");
        int adminId = input.nextInt();
        input.nextLine(); // consume newline character

        Candidate candidateToCheck = new Candidate(adminId);
        boolean success = stuList.contains(candidateToCheck);

        if (success) {
            System.out.println("Candidate found.");
        } else {
            System.out.println("Candidate not found.");
        }
    }

}

I use

Candidate candidateToDelete = new Candidate(adminId, adminName);
        boolean success = stuList.remove(candidateToDelete);

to pass the value to the remove in the ADT but I not showing anything

I also tried this

System.out.println("Enter candidate ID:");
        int stuId = input.nextInt();
        input.nextLine(); // consume newline character
        stuList.remove(new Candidate(stuId));

it nothing happens also

 System.out.println("Enter candidate ID:");
        int stuId = input.nextInt();
        input.nextLine(); // consume newline character
        stuList.remove(stuId);

the error happen at the stuId it showed int cannot convert to User.

I hope that you can tell me how to solve it and what I did wrong

please only show the relevant information/code. What is the error exactly? is it compile time? runtime? — Stultuske, Apr 21 '23 at 07:36
Please reduce this to a [mcve]. Currently you've posted over 400 lines of code, but left a *very* vague description of the problem: "but when I want to implement the delete function it keeps an error or even nothing happens". Please read https://jonskeet.uk/links/stack-hints and edit your question accordingly. — Jon Skeet, Apr 21 '23 at 07:37
you should start by overriding equals and hashCode in your Candidate class. Now it's doing a referential check, which is not likely to find the element in your list — Stultuske, Apr 21 '23 at 07:37
Please create [minimal, reproducible example](https://stackoverflow.com/help/minimal-reproducible-example). Emphasis is on **minimal**. — Chaosfire, Apr 21 '23 at 07:38
For the remove work work, you will need to add equality testing to `Candidate` - see the javadocs for https://docs.oracle.com/javase/8/docs/api/java/util/ArrayList.html#remove-java.lang.Object- — Scary Wombat, Apr 21 '23 at 07:42

score -1 · Accepted Answer · answered Apr 21 '23 at 07:43

System.out.println("Enter candidate ID:");
        int stuId = input.nextInt();
        input.nextLine(); // consume newline character
        stuList.remove(new Candidate(stuId));

This should resolve your issue of the item not being removed, except that it doesn't find your Candidate in your List.

You didn't provide an equals or hashCode for your class, meaning it's doing a referential check, and both objects won't likely have the same reference.

Add the following to your Candidate class:

@Override
public boolean equals(Object o) {
  if ( o == this ) return true;
  if ( !(o instanceof Candidate) ) return false;
  Candidate tmp = (Candidate)o;
  return this.id == tmp.getId();
}

@Override
public int hashCode() {
  return id;
}

now it will compare your new Candidate instance on the already present id's.

I am new to the java code , now i currently learning the OOP and ADT . i have succesfull implement the addStudent fucntion , but deleteStudent is not

1 Answers1