How to compare the new student with ArrayList.

[Anne Saizan]

Hello there,
I want to know how to compare the new student that user add, with the one in the arrayList.
If the user add the same studentID , with the one in the ArrayList. It will appear Duplicate Students.
But If the studentId is not exist, then it will add the student into the ArrayList.

Below is my code. hope you guys can help me to fix it.
Thank you.

import java.util.ArrayList;
import java.util.Scanner;

public class StudentList{
	
	private static ArrayList<Student1> students = new ArrayList<Student1>();
	
	public static ArrayList<Student1> getStudents()
	{
		return students;
	}
	
	public static void initStudentList () {
		//method add
		students.add(new Student1("S110", "Ralph Derby", 1987));
		students.add(new Student1("S123", "June Forster", 1974));
		students.add(new Student1("S134", "Susie Karl", 1960));
		students.add(new Student1("S220", "Phillip Gregg", 1958));
		students.add(new Student1("S240", "Henry Wine", 1984));		
	}
	
	
	public static void display()
	{
		
		System.out.println("StudentID      Name        Year");
		for(int index =0; index < students.size(); index ++)
		{
			System.out.println("   "+students.get(index).getID()+"     "+students.get(index).getName()+"    "+students.get(index).getYear());
		}
		
	}
	
	public static void addStudent()
	{
		System.out.println("Add a new student");
		Scanner input = new Scanner(System.in);
		
		System.out.println("Enter your name:");
		String name = input.nextLine();
		System.out.println("Enter your student ID:");
		String studentID = input.nextLine();
		System.out.println("Enter year of birth:");
		int yearofbirth = input.nextInt();
		 
		for (int i = 0; i < students.size(); i++)
		    {
		      if (students.get(i).getID().equals(students.get(i).getID()))
		      {
		    	System.out.println("Duplicate students");
		        return;
		      }
		      else
		      students.add(new Student1(studentID, name, yearofbirth));//add if not in the list already
		    }
		    
		
	}
	
}
	

[Ralph Cook]

What is your question or problem?

You say "help me fix it", but you don't tell us what is wrong. Are we supposed to run the code and try to figure out what is wrong? Does it work, but not as well as you want? Does it detect student id but you want it to detect other things? Do you want a version that will operate in Swing? AWT? Urdu?

The forums work best when you ask a detailed question. If you give us a program and only say "help me fix it", you may not get any responses, not even this one.

rc

[Anne Saizan]

okay ..sorry for that.

when i add the new student that is not exist in ArrayList. it said Duplicate Students.

I want to compare the student that I add with the ArrayList.
If it already exist, it will say Duplicate Students,
If not it will add the student.

[Ralph Cook]

Oh. You are comparing a value from the array list with the same value from the array list. Those will always be equal.

In your comparison, perhaps you meant to compare the student id from the array list with the one that had been input?

rc

[Anne Saizan]

oky thanks.
but if i do it like this. it will loop the new student.
OUTPUT:
Duplicate students
StudentID Name Year
S110 Ralph Derby 1987
S123 June Forster 1974
S134 Susie Karl 1960
S220 Phillip Gregg 1958
S240 Henry Wine 1984
s333 Saya 1234
s333 Saya 1234
s333 Saya 1234
s333 Saya 1234
s333 Saya 1234

for (int i = 0; i < students.size(); i++)
		    {
		      if (students.get(i).getID().equals(studentID))
		      {
		    	System.out.println("Duplicate students");
		        return;
		      }
		      else
		      students.add(new Student1(studentID, name, yearofbirth));//add if not in the list already
		    }
		   

[Ralph Cook]

Your "if - else" is within your loop -- I think what you meant was to loop through all the students, and only IF you do not find a duplicate, THEN enter the new student in. Since the if - else is within, the else is going to be executed as many times as there are students in the loop.

You need to find a way to go through the whole loop and then determine whether a duplicate was found. Then test that to determine whether to put up a message or add the new student.

rc

[Anne Saizan]

yup exactly.
thanks for that.
i will try to figure this out.
thanks

[Ronald Castillo]

anne annetote wrote:yup exactly.
thanks for that.
i will try to figure this out.
thanks

Why not use a Set instead, and override equals() and hashcode(), so it can take care of duplicates for you?

[Anne Saizan]

Thanks for the reply.
Set? how to use it.

[Ronald Castillo]

Anne Saizan wrote:Thanks for the reply.
Set? how to use it.

Yes, another type of Collection that doesn't allow duplicates. But also, they're not identified with an index, meaning you can't use get() and some other functions on it.

For your implementation would be to override equals() and hashCode() on your Student class
Could be as follow:

public boolean equals(Object obj) {
		Student student = (Student) obj;
		return getID().equals(student.getID());
	}
public int hashCode() {
		return getID().hashCode();
	}

Given that, everytime you try to add an element to the set it will perform a check using the hashCode() then the equals() functions to check whether the two given objects are the same. If a duplicate is found, add will return false.

PD: I just gave you an example code, you can use your own implementation as desired

[Campbell Ritchie]

Ronald Castillo wrote: . . .
For your implementation would be to override equals() and hashCode() on your Student class
Could be as follow:

public boolean equals(Object obj) {
		Student student = (Student) obj;
		return getID().equals(student.getID());
	}
public int hashCode() {
		return getID().hashCode();
	}

. . .

That is unreliable; it will throw an Exception for anything but a Student object. The equals() method must accept any type of object, including null as an argument. And it seems odd that you are only using the ID as a test whether the Students are equal. Here is a recent thread where I wrote about equals methods. We have an FAQ and there are other good references, which you can find from this post.

[Ronald Castillo]

That is unreliable; it will throw an Exception for anything but a Student object. The equals() method must accept any type of object, including null as an argument. And it seems odd that you are only using the ID as a test whether the Students are equal. Here is a recent thread where I wrote about equals methods. We have an FAQ and there are other good references, which you can find from this post.

I know the way I did it wasn't perfect but it was just meant to be a simple example. Regarding the ID, it may be unique, since it's why I only compared the ID. Again, this is only a simple example to see how the Set checks for duplicates.

Thanks for the hints ;)